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SYSTEMATIC ANATOMY

 Chapter 2 Central Nervous System

Section 1  Spinal Cord

The spinal cord is the least modified por­tion of the embryonic neural tube and the only part of the adult nervous system in which the primitive segmental arrangement clearly is preserved.

. External Features of Spinal Cord

The spinal cord is an elongated, cylindrical mass of nerve tissue, slightly flattened dorsoventrally, located in the upper two thirds of the spinal canal of the vertebral column. Protection for the cord is provided not only by the vertebrae and their ligaments but also by the meninges and a cushion of cerebrospinal fluid (Figs-2-1). The cord is normally 42-45 cm long in adults with its maximum transverse diameter being 10-12 mm. It is continuous with the brain stem at the foramen magnum and terminates as the conus medullaris at the level of the inferior border of first lumbar vertebra. The conus medullaris is conical-shaped. It tapers rather abruptly into a slender filament called the filum terminale consisting of pia mater and neuroglial elements( Fig.-2-1,2)which is invested by the spinal dural mater below the second segment of the sacrum and attaches to the dorsum of the coccyx.

 

Fig-2-1. Spinal Cord and Brain of a Newborn Child (Posterior View)

2. Enlargements

In the thoracic region, the spinal cord is almost cir­cular in cross-section. In the lower cervical and lumbosacral regions, the greatly increased nervous nurons in number, which is associated with the limbs, is reflected in swellings or enlargements in the spinal cord, particularly in the cervical region. The cervical enlargement includes segments C4-T1, with most of the corresponding spinal nerves forming the brachial plexuses for the nerve supply of the upper limbs. Segments L2-S3 are included in the lumbar(or lumbosacral) enlargement, and the corresponding nerves constitute most of the lumbosacral plexuses for the innervation of the lower limbs( Fig.-2-1,2).

                    

Fig-2-2. Schematic illustration of the relationships between the vertebral column, and the spinal cord segment.

2. External Longitudinal Fissures and Sulci

A transverse section of the spinal cord shows a deep anterior(or ventral) median fissure and a shallow posterior (or dorsal) median sulcus, which incompletely divide the cord into symmetric right and left halves. A shallow posterolateral sulcus lies a short distance lateral to the posterior median sulcus,into which a single linear array of the dorsal rootlets of spinal nerve attached. The anterolateral sulcus marks the site of emergence of ventral rootlets. Since the ven­tral rootlets emerge less tidily, often several deep rather than in a single linear array, the anterolateral sulcus may be more difficult to distinguish. In cervical and upper thoracic spinal segments, the posterior intermediate sulcus indents the spinal cord between the posterior median and posterolateral sulci (Fig.-2-3,4).

       Fig-2-3. Dorsal view of the cervical enlargement of the spinal cord

          

       Fig-2-4. Diagrammatic view of a lumbar tap

3. Segments of Spinal Cord

The part of spinal cord associated with the emergence of a pair of spinal nerves is called a segment of spinal cord. The spinal cord is divided into approximately 31 segments—8 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 1 coccygeal segments (Co)— that correspond to 31 pairs of the dorsal and ventral roots of spinal nerves. Individual segments vary in length; they are about twice as long in the midthoracic region as in the cervical or upper lumbar area. There are no sharp boundaries between segments within the cord itself.

4. Vertebral Levels of Spinal Cord Segments (Fig.-2-2) 

In the third month of intra-uterine life, the spinal cord fills the length of the vertebral canal, but from then on the vertebral column grows more rapidly than the spinal cord. At birth, the cord extends as far as the third lumbar vertebra. It reaches its adult level gradually, as growth proceeds, ending at the lower border level of the body of the first lumbar vertebra. The mismatch between levels of the cord segments and vertebral levels is of clinical importance in locating the level of a lesion of the cord and in approaching it surgically.

The approximate vertebral body levels of the spinal cord segments are as follows:

Table  Anatomic relationship of spinal cord with vertebral body in adults

Spinal cord segments

Vertebral bodis

C1-4

C1-4

C5-T4

C4-T3

C5-8

T3-6

T9-12

T6-9

L1-5

T10-12

S1-Co1

L1

 

A dorsal and ventral root unite to form a spinal nerves, which emerge from vertebral canal via the corresponding intervertebral foramina. Since the spinal cord is shorter than the vertebral column, the lower the nerve root, the greater the distance between its origin in the segment of the spinal cord and its point of exit from the vertebral canal. Thus, the lumbar and sacral roots descend for a considerable distance in the subarachnoid space before reaching their respec­tive intervertebral foramina. The large number of lumbosacral roots surrounding the filum terminale is known as the cauda equina(Fig-2-1,2,).

Clinical Note: Lumbar Puncture

It may be necessary to insert a needle into the subarachnoid space to obtain a sample of cerebrospinal fluid for analysis or for other reasons. A spinal lumbar puncture is the preferred method: The needle is inserted between the spines of the third and fourth lumbar vertebrae without risk of damaging the spinal cord

(Fig.-2-5).Lumbar Puncture

. Internal Structure of the Spinal Cord

The spinal cord consists of central gray and peripheral white matters. The central canal is a tube that pierces the center of the gray matter(Fig-2-6).

Fig-2-6.(A) Transverse section of seventh cervical segment, Weigert stain for myelin.

 

Fig-2-6.(B) Transverse section of second thoracic segment, Weigert stain for myelin.

) Columns of Gray Matter and Central canal

A transverse section of the spinal cord shows an H-shaped internal mass of gray matter surrounded by white matter. The gray matter is made up of two symmetric portions joined across the midline by a narrow transverse connection, the gray commissure. It encompasses the central canal, which is lined with ependymal cells and filled with cerebrospinal fluid . It opens upward into the inferior portion of the fourth ventricle. In adults, the canal usually disappears except at cervical levels, though it extends the length of the spinal cord during development.

Each half of the gray matter has a posterior gray column(in transverse section, called the posterior horn) which reaches almost to the posterolateral sulcus . A anterior gray column (in transverse section, the anterior horn) extends anteriorly. There is an  intermediate zone between the dorsal and ventral horns. Thoracic and upper two or three lumbar spinal seg­ments are characterized near the base of the anterior horn by a small, pointed lateral horn(column).

The form and quantity of the gray matter vary at different levels of the spinal cord. The proportion of gray to white is greatest in the lumbosacral and cervical enlargements. In the thoracic region, both the dorsal and ventral horns are narrow, and there is a lateral horn.

1.  Laminar Architecture of Gray Matter

Based upon the cytoarchitecture of the neuronal cell bodies, a cross section of the gray matter is classified by Rexed into ten laminae, termed Rexed's laminae Rexed(Fig-2-7).

1) Laminais a thin layer that caps the dor­sal horn and contains the posteromarginal nucleus. It receives some of the incoming dorsal root fibers, and contributes a small proportion of the axons of the contralateral spinothalamic tract.

2) Lamina ,also known as the substantia gelatinosa (of Rolando), is made up of Golgi typeneurons, receiving fibers that carry pain and temperature sensations. This lamina is the main processing center for nociceptive (noxious) stimuli in the spinal cord.

3) Laminae and contain the nucleus proprius and occupy a large region of the dorsal horn. This nucleus contributes axons to the spinothalamic tract and receives virtually all sensory modalities carried by the dorsal root.

4) Lamina occupies the neck of the posterior hornand is divisible into a lateral third and medial two-thirds. The lateral third part is mixed with bundles of fibres: hence the term reticular formation, which should not be confused with the reticular formation of the brain stem.

5) Lamina is present in the spinal cord enlarge­ments and particularly absent in the fourth thoracic through the second lumbar segments.

Lamina and receive most of the terminals of proprioceptive primary afferents and profuse corticospinal projections from the motor and sensory cortex and subcortical levels, suggesting intimate involvement in the regulation of movement.

6) Lamina includes much of the intermediate zone , as well as much of the space within the ventral horn. It contains prominent neurons of the thoracic nucleus and intermediomedial and intermediolateral columns at their spinal levels.

The thoracic nucleusalso called nucleus dorsalis, or Clarke's column Clarke is me­dial and ventral to the base of the dorsal horn in segments T1-L3 or L4, giving the uncrossed fibers to dorsal spinocerebellar tract.

The intermediolateral cell column occupies the lateral horn of the cord in segments T1-L2 or L3. This column consists of the cell bodies of the preganglionic neurons of the sympathetic nervous system. The sacral autonomic nucleus is an equiva­lent column of cells in the lateral part of lamina in segments S2-S4. It consists of the cell bodies of the preganglionic neurons of the sacral division of the parasympathetic nervous system.

The intermediomedial cell column is present just lateral to lamina X throughout the length of the cord. It receives pri­mary afferent fibers and may be involved in visceral reflexes.

Lamina in its lateral part has extensive ascending and descend­ing connections with the midbrain and cerebellum  and is thus involved in the regulation of posture and movement. Its medial part has numerous propriospinalreflex connections with the adjacent grey matter and segments concerned both with movement and autonomic functions.

7) Lamina spans the base of the thoracic anterior horn but is restricted in limb enlargements to the medial aspect of the anterior horn . It contains mainly the propriospinal neurons,or intersegmental neurons. It is a site of termination of some descending fibers, including many of those of the vestibulospinal and reticulospinal tracts. The neurons project both ipsilaterally and contralaterally at the same and nearby segmental levels to laminae and .

8) Lamina consists of several distinct groups of motoneurons of the anterior horn. They comprise large α and small γ motoneurons that innervate the extrafusal and intrafusal muscle fibers, respectively. In thoracic regions of the spinal cord several islands of motoneurons occupy the ventral part of the an­terior gray horn, but in the cord enlarge­ments greatly increased numbers of motoneurons form two larger columns: The medial motor neuron column and lateral motor neuron column, they contains the lower motor neurons innervating axial muscles (ie, muscles of the trunk and proximal parts of the limbs)and the distal muscles of the arm and leg respectively . In general, flexor muscles are innervated by motor neurons located centrally in the anterior horn, close to the central canal, whereas extensor muscles are innervated by motor neurons located more peripherally.

 In ad­dition, lamina IX contains numerous small interneurons whose axons extend up and down the spinal cord in the fasciculus proprius. By virtue of collateral axonal branches that arise near the cell body, these cells also serve as local circuit neurons in the anterior horn, some of which were thought to be inhibitory in nature.

9) Lamina surrounds the central canal, including the posterior and anterior gray commissures.

 

Fig-2-7. Cytoarchitectonic laminae of the gray matter in C7, T5, and S2 spinal cord segments.

 

) White Matter

The white matter consists largely of nerve fibres, many of which being

longitudinal and grouped into white funiculi. Each half of the spinal cord has anterior ,posterior and lateral funiculi around the spinal gray columns. The posterior funiculus lies between the posterior median sulcus and the posterolateral sulcus. The lateral funiculus lies between the posterolateral sulcus and the anterolateral sulcus. The anterior funiculuslies between the anterolateral sulcus and the anterior median fissure (Fig-2-6). Anterior to the grey commissure is the ventral white commissure. It connects the white matter on both sides.

The white matter of the spinal cord is com­posed of myelinated and unmyelinated nerve fibers. As a convenient simplification, those nerve fibres are assigned to five groups: afferent fibres from neurons in posterior root ganglia entering by dorsal roots and extending variable spinal distances; long ascending fibres, derived from spinal neurons conducting afferent impulses to supraspinal levels; long descending fibres from supraspinal sources to synapse with spinal neurons;fibres effecting intrasegmental and intersegmental connections; fibres from motor neurons in anterior and lateral grey columns issuing in anterior nerve roots.

Fibres in all except the last category form longitudinaltracts. Arrangements are not, however, so simple. Many fibres proceed in oblique and even hori­zontal directions, particularly across the midline in grey and white commissures. Many others are commissural intrasegmental connections linking the neurons in grey columns to contralateral neurons. Most, if not all, fibres entering by the dorsal root divide into ascending and descending branches, all with collaterals extending into the gray matter. Fiber bun­dles having the same origin, course and termination are known as tracts or fasciculi. The ascending and descending tracts fibers of the spinal cord are organized into more or less distinct bundles which occupy partic­ular areas in the white matter. In general, long tracts tend to be located peripherally, while shorter propriospinal tracts tend to be situated adjacent to the gray matter(Fig-2-8, 10).

 

 

Fig-2-8. A diagram showing the principal ascending tracts in the spinal cord.

 

Fig-2-9. Schematic diagram of principal ascending pathways in the spinal cord.

 

 

1. Organization and dispersion of the dorsal roots

As central processes of spinal ganglion cells approach the dorsal root entry zone between the apex of the posterior horn and the posterolateral sulcus of the cord, the axons become segregated into two divisions within each rootlet.

The lateral division contains most of the unmyelinated, or group C axons and some thin group A myelinated axons, conveying impulses related to pain, thermal and light tactile sense. These ax­ons enter the dorsolateral tract (of Lissauer). It occupies the lateral part of the dorsal root en­try zone(Fig-2-9). The tract is formed in part by fibres of the lateral bundles of dorsal roots which bifurcate into ascending and descending branches. The branches travel one or two segments projecting collaterals to and around the cells of the posterior grey column. The tract also contains many propriospinal fibres. Many fibres are short axons of small neurons of the substantia gelatinosa which then re-enter the posterior grey column.

The medial division consists of the larger, thick myelinated A fibers

of peripheral nerve. Such large fibers mediate discriminatory sensory modalities involving touch, texture, form, kinesthesia, and a modality termed proprioception. These enter the spinal white matter medial to the dorsal horn, where, like those of the lateral division, they divide into ascending and descending branches. The descending branches run caudally within the dorsal funiculi for varying distances, some to nearby segments and others almost the whole length of the cord, to terminate eventually in the postrior horn. The ascending branches of afferent fibers entering the dorsal funiculus are also of differing lengths, with many reaching the gracile and cuneate nuclei in the medulla. At the other ex­treme, many axons from the medial divi­sion of the dorsal root enter the gray matter at their own segmental levels. These fibers are conspicuous in lamina of the dorsal horn. Primary afferent axons conveying signals from muscle spindles  have some branches that terminate on motor neurons and are involved in the stretch reflex.

 

2. Fasciculus Proprius

Fasciculi proprii, or propriospinal tracts are mainly concentrated around the margins of the grey matter(Fig-2-8,10). Propriospinal neurons are confined to the spinal cord. Thus, their ascending or descending fibres, both crossed and uncrossed, begin and end within the spinal grey matter; they constitute the pro­priospinal or intersegmental tracts. The fibres interconnect with the local cells within the same segment and/or with other cells in more distant segments of the spinal cord. The majority of spinal neurons are propriospinal neurons which are mostly located in laminae -. The propriospinal system plays an important role in spinal functions as exemplified by the distribution of descending pathways to specific subgroups of propriospinal neurons which in turn relay to motor neurons and other spinal cells.

Fig-2-10. Schematic diagram of principal descending pathways in the spinal cord.

3.  Ascending tracts

1) Fasciculus gracilis and fasciculus cuneatus(Fig-2-68).

These tracts convey well-localized sensations of fine touch, vibration, two-point discrimination, and proprioception(position sense) from the skin and joints, they ascend, without crossing, in the posterior funiculus of the spinal cord to the lower brain stem. The fasciculus gracilis courses next to the posteromedian septum, it conducts input from the lower half of the body, with fibers that arise from the lowest, lies in most medial part. The fasciculus cuneatus lies between the fasciculus gracilis and the dorsal gray column; it transmits input from the upper half of the body, with fibers from the lower (thoracic) segments more medial than the higher (cervical) ones. Thus, the posterior funiculus contains fibers from all segments of the ipsilateral half of the body arranged in an orderly fashion from medial to lateral, such an arrangement is called somatotopic organization(Fig-2-11). These two fasciculi end, respectively, in the gracile and cuneate nuclei in the lower medulla. As a useful approximation, the gracile fasciculus and nucleus may be said to deal with sensations from the lower limb, and the cuneate fasciculus and nucleus may be said to deal with sensations from the upper limb.

Fig-2-11. Somatotopic organization in the spinal cord.

2) Spinothalamic tracts

Cell bodies of small and intermediate size in the posterior root ganglia have central processes that constitute the lateral divi­sions of the dorsal rootlets. These fibers conduct impulses from pain and tempera­ture receptors. The fibers for light touch and pressure enter the posterior gray horn through the medial division of the dorsal rootlets, the pain and tempera­ture fibers enter the dorsolateral tract (of Lissauer) , then synapse with posterior horn neurons. Cells largely in laminaeand - give the fibers acrossing the median line in the anterior white commissure to form the spinothalamic tracts, which actually consist of two adjacent pathways: The anterior spinothalamic tract is located in the anterior funiculus of the spinal cord and continuous laterally with the lateral spinothalamic tract, which is sited in the lateral funiculus lying medial to the anterior spinocerebellar tract(Fig-2-89). On the basis of clinical evidence, the anterior spinothalamic tract conveys impulses sub­serving crude tactile(or light touch) and pressure modalitiesand the lateral tract subserves pain and temperature sensibilities.  Although there is a small ipsilateral projection, the majority of spinothalamic tracts formed by the crossing fibers in the anterior white commissure and then project rostrally to the thalamus. The decussation is completed in the segment above the entrance of the dorsal root fibres. The spinothalamic tracts, like the dorsal column system, show somatotopic organization (Fig-2-11). In particular, sensation from sacral parts of the body is carried in lateral parts of the spinothalamic tracts, whereas impulses originating in cervical regions are carried by fibers in medial parts of the spinothalamic tracts.

 

3) Spinocerebellar tracts(Fig-2-8)

(1) posterior spinocerebellar tract   It originates from the ipsilateral dorsal nucleus and ascends in the peripheral part of the lateral funiculus. It transmits subconscious proprioceptive impulse from lower limb and lower trunk to the cerebellumvia the infe­rior cerebellar peduncle.

 (2) anterior spinocerebellar tract  It is believed to relay internal feedback signals to the cerebellum that reflect the amount of neural activity in descending motor pathways. It occupies a crescentic area in the periphery of the lateral funiculus, anterior to the posterior spinocerebellar tract. The anterior spinocerebellar pathway, similar to the posterior spinocerebellar pathway, transmits information about the lower limb and lower trunk. It originates from neurons that are spread throughout the intermediate zone, many of these neurons are located along the gray matter border. The axons forming the anterior spinocerebellar tract mostly decussate in the spinal cord, with a few remaining ipsilateral, and ascend in the lateral funiculi, coursing along the superior cerebellar peduncles to the cerebellum.

4. Descending tracts(Fig-2-10)

 1) Corticospinal tract

Corticospinal tract is the most important descending pro­jection pathway in humans for controlling voluntary movements. It arise from the cells in precentral motor cortex , the premotor area, the postcentral gyrus, and adjacent pa­rietal cortex. These corticofugal fibers converge and pass downward through the internal capsuleand the brain stem, at the pyramidal decussation of medulla, about 75%-90% of the corticospinal fibres cross the median plane to continue as the lateral corticospinal tract; the rest of the uncrossed fibres continue as the anterior corticospinal tract. The lateral tract also contains some uncrossed corticospinal fibres, which form the anterolateral corticospinal tract( of Barnes) .

(1) Lateral corticospinal tract

It descends in the lateral funiculus throughout most of the length of the spinal cord, progressively diminishing to end at about the fourth sacral spinal segment. This pathway terminates primarily in the lateral portions of the intermediate zone and anterior horn of the cervical and lumbosacral enlargements to control distal limb muscles of the hand and foot. The somatotopic organization of the lateral corticospinal tract is such that fibers destined for the lowest levels of the spinal cord are the most laterally placed(Fig.-2-11).

(2) Anterior corticospinal tract

It descends in the ipsilateral anterior funiculus and extends only to the upper thoracic cord. This pathway terminates in the medial portions of the intermediate zone and anterior horn where axial and girdle motor neurons are located. Many anterior corticospinal tract axons have branches that decussate in the spinal cord. As a consequence, the anterior corticospinal tract on one side influences axial and girdle muscles bilaterally; whereas the fibres of the lateral tracts mostly end on ipsilateral cord neurons. The anterior corti­cospinal tract projects to the cervical and upper thoracic spinal cord and thus may be preferentially involved in the control of the neck, shoulder, and upper trunk muscles.

(3) Anterolateral corticospinal tract(of Barnes)

Fibers of this tract descend as uncrossed fibers within the lateral corticospinal tract. They provide synaptic input (probably via polysynaptic circuits) to lower motor neurons controlling axial (ie, trunk and proximal limb) muscles involved in maintaining body posture.

Finally, a small percentage of the fibers of the corticospinal tract project to the posterior gray column and function as modifiers of afferent (sensory) information, allowing the brain to suppress, or filter, certain incoming stimuli and pay attention to others; these fibers may also modify local reflex activity within the spinal cord.

2) Rubrospinal tract (Fig.-2-10)

The axons forming the rubrospinal tract originate from the red nucleus in the midbrain and decussates to the opposite,then descend to spinal cord . The tract lies anterior to the lateral corticospinal tract and terminates primarily in the lateral portions of the intermediate zone and ventral horn of the cervical cord. It plays a role in controlling limb muscles and regulating voluntary movement.

Evidence from animal studies shows that rubrospinal fibres facilitate flexor muscles and inhibit extensor ones. Despite its small size, the rubrospinal tract may be clinically impor­tant because it is thought to subserve some residual motor function after damage of the lateral corticospinal tract.

3) Vestibulospinal tract (Fig-2-10)

It arises from the lateral vestibular nucleus,and descends ipsilaterally in the anterolateral funiculus to all levels of the spinal cord. It projects to the medial portions of the intermediate zone and ventral horn. This pathway is important in maintaining balance.

Data from stimulation of vestibular nuclei show that axons of the lateral vestibulospinal tract excite, through mono- and polysynaptic connections, motor neurons of extensor muscles of the neck, back and limbs, but inhibit the motor neurons of flexor limb muscles, via inhibitory interneurons.

4) Reticulospinal tracts (Fig-2-10)

Neurons in the pontine and medullary reticular formation give rise to the pontine and medullary reticulospinal tracts. The pon­tine reticulospinal tract  descends in the ventral funiculus of the spinal cord, whereas the medullary reticulospinal tract descends in the ventrolateral quadrant of the lateral funiculus. Both of the tracts descend predom­inantly in the ipsilateral side for the entire length of the spinal cord. Fibers of the reticulospinal tracts project to the medial portions of the intermediate zone and anterior horn. The tracts play a role in controlling axial and girdle muscles and regulating posture. They are thought to help control relatively automatic movements, such as maintaining posture or walking over even terrain.

5) Medial longitudinal fasciculus(MLF)

Caudal extension of the MLF in the spinal cord is primarily composed of the medial vestibulospinal tract. It is also known as the descending medial longitudinal fasciculus. Neurons giving rise to the medial vestibulospinal fibres are found mainly in the medial vestibular nucleus. This tract descends in the anterior funiculus, with almost all fibers being ipsilateral. Fibres of the tract project mainly to the cervical cord segments, ending in the medial portions of the intermediate zone and anterior horn. It plays a role in controlling head position.

6) Tectospinal tract

It originates primarily from neurons in the superior colliculus of the contralateral midbrain and descend in the medial part of the anterior funiculus of the spinal cord. Fibres of the tract project only to the upper cervical cord segments, ending in the medial portions of the intermediate zone and anterior horn. It therefore is believed to participate primarily in the control of neck, shoulder, and upper trunk muscles. Because the superior colliculus also plays a key role in controlling eye movements, it is likely that the tectospinal tract is important for coor­dinating head movements with eye movements. It causes head turning in response to sud­den visual or auditory stimuli.

Both the MLF and the tectospinal tract influence neurons that inner­vate the muscles of the neck, including those supplied by the accessory nerve, af­fecting movements of the head as required for fixation of gaze and maintaining equilibrium, respectively.

. Functions of spinal cord

Although the spinal cord constitutes only 2% of the central nervous system, its functions are tremendously important since it: One of the principal functions of the spinal cord is to convey afferent impulses, which initiates from the somatic and visceral receptors to the brain and to conduct efferent impulses from the brain to the effectors. The second principal function is related to the reflexes.  

1  Stretch reflex

The simplest reflex is the stretch reflex, a reflex in which muscles contract in response to a stretching force applied to them. The stretch reflex has a two-neuron or monosynaptic reflex arc (Fig-1-7). It consists of four steps: Muscle spindles detect stretch of the muscle; Sensory neurons conduct action potentials to the spinal cord; Sensory neurons synapse with alpha motor neurons; Stimulation of the alpha motor neurons causes the muscle to contract and resist being stretched.

Slight stretching of a muscle stimulates the sensory endings in neuromuscular spindles, and the resultant excitation reaches the spinal cord by way of primary sensory neurons that have large group A axons. The proximal branches of these axons in the dorsal funiculus give off collateral branches that excite alpha motor neurons , causing the stretched muscle to contract. This is an important postural reflex. The neuromuscular spindles are delicate moni­tors of change in the length of the muscle, and the stretch reflex alters tension in such a way as to maintain a constant length.

2  Knee-jerk reflex or patellar reflex

It is a classic example of the stretch reflex. Clinicians use this reflex to determine whether the higher CNS centers that normally influence this reflex are functional. When the patellar ligament is tapped, the tendons and muscles of the quadriceps femoris muscle group are stretched. The muscle spindle fibers within these muscles are also stretched, and the stretch reflex is activated. Consequently, contraction of the muscles extends the leg, thus producing the characteristic knee-jerk response.

A diminished or absent tendon jerk indicates disease affecting either the afferent or the efferent neurons of the stretch reflex. Exaggerated jerks indicate loss of inhibition of motor neurons by activity in descending tracts from the brain.

3  Gamma reflex loop

The stretch reflex arc forms part of the gamma reflex loop, which is one of the mechanisms by which descending motor pathways control muscle tension. Corticospinal, reticulospinal, and vestibulospinal fibers excite gamma motor neurons, causing contraction of intrafusal muscle fibers and an increase in the rate of firing from sensory endings in the neuromuscular spindles. Through the monosynaptic reflex arc of the stretch reflex, the sensory impulses are conveyed to larger numbers of alpha motor neurons that supply the main muscle mass. Gamma motor neurons are responsible for regulating the sensitivity of the muscle spindles. The activity of the muscle spindles help control and coordinate muscular activ­ity, such as muscle tension, and muscle length. The activity of the gamma reflex loop is important in maintaining posture and in coordinating muscular activity.

 

4  Flexor reflex

The function of the flexor, or withdrawal, reflexis to remove a limb or other body part from a painful stimulus. At least three neurons are involved, so this is a polysynaptic reflex (Fig.-2-12). It consists of four steps: Pain receptors detect a painful stimulus; Sensory neurons conduct action potentials to the spinal cord; Sensory neurons synapse with excitatory interneurons that synapse with alpha motor neurons; Excitation of the alpha motor neurons results in contraction of the flexor muscles and withdrawal of the limb from the painful stimulus.

The Pain receptors are free nerve endings that respond to potentially injurious stimuli. Action potentials from painful stimuli are conducted by sensory neurons through the dorsal root to the spinal cord, where they synapse with excitatory interneurons, which in turn synapse with alpha motor neurons. The alpha motor neurons stimulate muscles, usually flexor muscles, that remove the limb from the source of the painful stimulus. Collateral branches of the sensory neurons synapse with interneurons that send ascending fibers to the brain, providing conscious awareness of the painful stimuli. The collateral branches also synapse with excitatory interneurons that cross over to the opposite side of the spinal cord to stimulate extension of the contralateral limb, supporting body weight during the withdrawal reflex..

 

Fig-2-12 Schematic diagram of a flexor reflex arc.

  Clinic Notes

In localizing spinal cord lesions, the student should ask the following questions:

Which tracts are involved?

On which side?

At what level does the abnormality begin (ie, is there a sensory level, below which sensation is impaired)? Is motor function impaired below a specific myotomal level?

What sensory modalities are involved (all modalities, suggesting involvement of the lateral and dorsal columns; vibration and position sense, suggesting dorsal column dysfunction; or dissociated loss of sensibility for pain and temperature, suggesting a lesion involving the spinothalamic fibers, possibly in the central part of the cord where they cross)?

 (1) Spinal cord transection

The cord may be damaged by penetrating wounds (caused by stabbing or gunfire) or by spinal fracture or dislocation(especially from road traffic accidents). Complete transection results in loss of all sensibility and voluntary movement below the lesion. During an initial period of spinal shock, lasting from a few days to several weeks, all somatic and visceral reflex activity is abolished. On return of reflex activity, there is spasticity of muscles and exaggerated tendon reflexes. The lower limbs assume positions of flexion because the vestibulospinal tract (which stimulates extensors) is one of the transected descending pathways. Bladder and bowel functions are no longer under voluntary control.

(2) Spinal cord hemisection

Complete hemisection of the cord produces a Brown-Séquard syndrome as a result of bullet or stab wounds, syringomyelia, spinal cord tumor, hematomyelia, etc.  

The main signs and symptoms include:

Ipsilateral spastic paralysis of the muscles below the level of the lesion;

Ipsilateral loss of proprioceptive, vibratory, and two-point discrimination sense below the level of the lesion;

Contralateral loss of pain and temperature sense below the lesion.

The motor deficits of distal limb are all expressed on the ipsilateral side because the lateral descending motor pathways, which target distal limb muscle motor neurons,  decussate in the brain stem. Because the medial descending pathways terminate bilaterally, axial motor function is usually not seriously affected with unilateral spinal cord lesions.

Axons in the dorsal columns are uncrossed in the spinal cord; hence deficits in touch and limb position sense are present ipsilateral to the spinal cord lesion. In contrast, the axons of the ventrolateral system decussate in the spinal cord. Therefore, pain and tem­perature senses are impaired on the side of the body that is contralateral to the lesion.

(3) Ventral horn lesion

It produces the signs of LMN lesions described as above. Typical causes include poliomyelitis, in which a virus selectively attacks ventral horn cells or equivalent neurons in the brain stem, with no sensory modalities being involved.

(4)Lesions in the central portion of the spinal cord

A small central lesion of the spinal cord occurs, for example, in syringomyelia, which is one type of the degenerative diseases of the CNS. In this pathological process, a cavity(or syrinx) forms in the central portion of the spinal cord. In the early stages of this condi­tion, decussating fibers for pain and temperature in the ventral white commissure are damaged selectively; this results in a loss of pain and temperature sense. Because of the location of the cavity, the axons of the dorsal columns are spared, and therefore touch and limb posi­tion senses are unaffected(dissociated anesthesia). Syringomyelia interrupts decus­sating axons from both sides of the body; hence, the sensory loss that occurs is usually bilaterally symmetrical. Cavity formation occurs most commonly in the cervical and upper thoracic spinal segments, thereby producing a cape-like distribution of sensory loss on the arms, shoulder, and upper trunk(Fig.-2-13).

 

            

Fig-2-13. Distribution of loss of pain and temperature sense over body in syringomyelia involving the cervicothoracic portion of the spinal cord.

                Huang Yaode : Shanghai Second Medical University

 

 

Section 2   Brain Stem

 

Brain stem is composed of the medulla oblongata, pons and midbrain, which connects the cerebrum, cerebellum with the spinal cord.

I   External features(Fig. VI-2-14,15)

1Medulla oblongata  The medulla oblongata is the most caudal part of the brain stem. Its ventral surface rests upon the basilar portion of the occipital bone, while its dorsal surface is in large part covered by the cerebellum. It is somewhat piriform, its broad (superior) end merging into the pons and its narrow lower end continuous with the spinal cord. The spinal central canal is prolonged into its lower half, expanding above as the fourth ventricle. The medulla is hence divided into a closed part containing the central canal and an open part containing the lower half of the fourth ventricle. Its anterior and posterior surfaces have median fissures.

           

Fig. Fig. VI-2-14  The ventral surface of the brain stem

The fissure and sulci presented in the spinal cord extend upwards in the medulla oblongata. The anterior median fissure extends along the entire length of medulla oblongata, while the posterior median sulcus just extends to the closed part of the medulla oblongata. A series of rootlets of cranial nerves enter or leave through the anterior or the posterolateral sulcus.

On each side of the anterior median fissure is an oblongated elevation, the pyramid. It is composed of a strong bundle of nerve fibers, which originates from the cerebral cortex Near the lower extremity of the medulla oblongata a great number of fibers leave the pyramids in successive bundles, and decussate with those of the opposite side in anterior median fissure. These interdigitating bundles of the fibers are known as the decussation of pyramid.

Posterolateral to the pyramid is an oval elevation, called olive The underlying groups of nerve cells form the inferior olivary nucleus.

On each side of the posterior median sulcus the fasciculus gracilis and the fasciculus cuneatus continue upwards in situ. At the lower angle of the fourth ventricle, the fasciculus gracilis ends in an elongated swelling, the gracile tubercle. Lateral and adjacent to it, there is another swelling, the cuneate tubercle, in which the fasciculus cuneatus ends. Rostral to the gracile and cuneate tubercles a thick rounded ridge is the inferior cerebellar peduncle.

Four pairs of cranial nerves emerge from the surface of the medulla oblongata. Between the pyramid and olive the fibers of the hypoglossal nerve emerge in linear series from the anterolateral sulcus; between the olive and inferior cerebellar peduncle there are a series of rootlets along the posterolateral sulcus, which make up the glossopharyngeal nerve, vagus nerve and cranial roots of accessory nerve.

 

Fig. VI-2-15  The dorsal surface of the brain stem

 

2.  Pons  The pons interposes between the midbrain and the medulla oblongata, and is characterized by the huge ventral swelling. It rests upon the dorsum sellae of the sphenoid bone and the adjacent basilar part of the occipital bone. The pons is covered by the cerebellum dorsally. The ventral surface of the pons is markedly convex from side to side and shows many broad transverse bands of nerve fibers across the median p1ane. The transverse fibers converge on each side into a compact mass, which forms the middle cerebellar peduncle and finally enter the corresponding hemisphere of the cerebellum. A marked shallow median groove on the ventral surface of the pons is called the basilar sulcus  in which the basilar artery(基底动脉) is lodged.

The dorsal part of the pons is opened to form the upper half of the f1oor of the fourth ventricle, along the lateral borders of which there are two prominent and rather large strands of nerve fiber called the superior cerebellar peduncles. The interval between the two superior cerebellar peduncles is bridged by a thin lamina of white matter, the superior(or anterior) medullary velum. The fibers of the trochlear nerve decussate in this structure, and then emerge from it.

Four pairs of the cranial nerves make their exits in the ventral surface of the pons. In the bulbopontine sulcus there emerge the abducent nerve, facial nerve and vestibulo-cochlear nerves ranged mediolaterally. The trigeminal nerve, consisting of a smaller superomedial motor root, and a large infero1ateral sensory root, emerges from the junction of the basilar part of the pons and middle cerebellar pedunc1e.

 Rhomboid fossa  The floor of the fourth ventricle is rhomboidal in shape, named rhomboid fossa Fig. VI-2-15,16 . It is formed by the posterior surface of both the pons and the open part of medulla oblongata. The boundaries of the rhomboid fossa are the superior cerebellar peduncles. The inferior cerebellar peduncles, and the cuneate and gracile tubercles from above downwards. Of the four angles of the rhomboid fossa, two are lateral1y placed; the caudal angle of rhomboid fossa is continuous with the central canal in the closed part of medulla oblongata, the rostral angle with the mesencephalic aqueduct of the midbrain.

The rhomboid fossa is divided into symmetrical halves by a median sulcus, a ridge on each side of this sulcus is termed medial eminence. The medial eminence is bounded laterally by a sulcus, called sulcus limitans. About the middle of the rhomboid fossa a few fine bundles of fibers may be seen through the ependyma. They emerge from the median line and pass laterally towards the inferior cerebellar peduncle, called the striae medullares, which divides the rhomboid fossa into pontine and medullary parts. Adjacent to the top of the sulcus limitans there is a depression that presents bluish-grey in colour in fresh specimens, called the locus ceruleus which owes its colour to a group of underlying pigmented cells. At the middle of the medial eminence above the striate medullares a rounded swelling is referred to as facial colliculus. Below the striae medullares on each side of the median sulcus there are two triangular areas. Of these the most medial one is called the hypoglossal triangle. the lateral one is the vagal triangle. There is a triangular field lateral to the sulcus limitans called the vestibular area.

 

Fig. VI-2-16 The choroid tissue of the fourth ventricle

1. Midbrain 

The midbrain is the shortest brain stem segment. It connects the pons and cerebellum with the cerebrum. The ventral surface of the midbrain is a pair of longitudinal columns of nerve fibers, the cerebral peduncles (crus cerebri). On transverse section, it is semilunar in shape and consists of the descending fibers from the cerebral cortex. The pyramidal tract occupies the middle 3/5, the frontopontine tract which arises in the frontal lobe occupies the medial l/5, and the corticopontine tract from the temporal, parietal and occipital lobes occupies the lateral l/5 of the crus cerebri. A deep depression bounded by the cerebral peduncles is known as interpeduncular fossa. The oculomotor nerve emerges from a groove on the medial side of each peduncle, On the dorsal surface of the midbrain there are four rounded eminences, the superior and inferior colliculi or corpora quadrigemina. From the 1ateral aspect of each colliculus there is a ridge termed the brachium, ascending in a ventrolatera1 direction. The brachium of superior colliculus passes inferior to the lateral geniculate body. The brachium of inferior colliculus ascends ventrally from the inferior colliculus to the medial geniculate body The mesencephalic (or cerebral) aqueduct passes through the midbrain and serves to connect the third ventricle above with the fourth ventricle below.

 

II. Internal structure 

The internal structure of brain stem comprises the cranial nerve nuclei, non-cranial nerve nuclei, ascending and descending pathways and reticular formation.

1.  Nuclei of cranial nerve  Almost all the cranial nerve nuclei are located in the brain stem except the nuclei of olfactory and optic nerve, they are evaginations of the brain itself. The functional composition of the lower 10 pairs of the cranial nerve can best be analyzed by referring the development of the nuclei (Fig.VI-2-17,18).

 

Fig. VI-2-18  The nuc1ei of cranial nerves in the brain stern (dorsal view)

1) General somatic motor nuclei  The General somatic motor nuclei innervate striated muscles that are derived from somites and are involved in the movement of the tongue and eyeballs. They are located closely to the median plane and include oculomotor, trochlear, abducent and hypoglossal nuclei from midbrain to medulla oblongata.

(l) The oculomotor nucleus  The oculomotor nucleus is located ventromedially to the central grey matter at the level of the superior colliculus. It consists of the large motor nerve cells, their efferent fibers supply most of the extraocular muscles except the lateral rectus and the superior obliques. Each extraocular muscle innervated by the ipsilateral oculomotor nerve has its own sub group cells in the nucleus, the oculomotor nucleus receives the fibers of the bilataral corticonuclear tracts.

(2) The trochlear nucleus  The trochlear nucleus lies in the ventral region of the central grey matter close to the midline intimately related to medial longitudinal fasciculus. Its outgoing fibers pass laterallv and dorsally around the central grey matter. They reach the cranial end of the superior medullary velum, decussating with those of the opposite side and become the unique cranial nerve which emerges from the dorsal surface of the brain stem.

(3) The abducent nucleus  The abducent nucleus lies deeply in the facial colliculus. The fibers of abducent nerve course ventrally and inferiorly, passing through the tegmentum and basilar part of the pons to emerge between its lower border and the pyramid.

(4) The hypoglossal nucleus  The hypoglossal nucleus lies under the hypoglossal triangle and extends down to the closed part of medulla oblongata on each side of the median plane. The hypoglossal nucleus consists of a group of multipolar neurons, and gives origin to nerve fibers of the hypoglossal nerve. These nerve fibers run ventrolaterally along the lateral border of the medial lemniscus, finally emerging between the pyramid and the olive to become rootlets of hypoglossal nerve. A relatively small lesion in the ventral part of the medulla ob1ongata may therefore impair both the corticospina1 tract and the hypoglossal nerve causing a characteristic crossed paralysis. The muscles of the tongue are paralyzed on the same side as the lesion, but the muscles of the limbs are paralyzed on the opposite side of the lesion.

 

2) Special visceral motor nuclei  The Special visceral motor nuclei innervate muscles that are derived from the brachial arches and include motor nucleus of trigeminal nerve, facial nerve nucleus, nucleus ambiguus and spinal accessory nucleus.

(1) Motor nucleus of trigeminal nerve  The motor nucleus of trigeminal nerve is a group of motor neurons, which lies in the reticular formation of the pons, deep to the lateral part of the f1oor of the fourth ventricle. The motor root of trigeminal nerve arises from this nucleus.

(2) Nucleus of facial nerve  The nucleus of facial nerve is a group of large motor cells, situated at the ventrolateral part of the reticular formation, ventromedial to the spinal tract and nucleus of trigeminal nerve. The upper portion of the nucleus, which innervates the frontal belly of occipitofrontalis and the orbicularis oculi, receives fibers from the corticonuclear tract of both sides, while the lower portion, which innervates the muscles of the lower part of the face, receives the fibers from those of the opposite side only. The efferent fibers of the facial nucleus pursue a remarkable course,.At first they incline dorsomedially towards the rhomboid fossa, then course upwards on the medial side of the nucleus of abducent nerve. The fibers of facial nerve wind the upper pole of the nucleus of abducent nerve to form the genu of facial nerve. Finally, they turn ventrolateral1y and descend through the reticular formation between their own nucleus and the spinal nucleus of trigeminal nerve to the surface of the brain stem where they emerge.

(3) Nucleus ambiguus  The nucleus ambiguousis placed deeply in the reticular formation. This nucleus provides the specia1 visceral efferent fibers for the glossopharyngeal, vagus and accessory nerves. The rootlets of these nerves leave the medulla oblongata by passing through the posterolateral sulcus.

(4) Nucleus of accessory nerve  The nucleus of accessory nerve is located in grey matter of the lower part of the medulla oblongata, and gives rise to efferent fibers to supply the sternocleidomastoid and trapezius. In corresponding to the spina1 roots of the accessory nerve, the spinal nucleus of accessory nerve may extend downwards as low as the 1evel of the fifth cervical segment of the spinal cord.

3)  General visceral motor nuclei  General visceral motor nuclei are parasyrnpathetic preganglionic neurons that provide autonomic innervations of the smooth muscles and glands in the head, neck and torso. They include accessory oculomotor nucleus, superior salivatory nucleus, inferior salivatory nuc1eus and dorsal nucleus of the vagus nerve.

(1)Accessory nucleus of oculomotor nerve  The accessory nucleus of oculomotor n.(Edinger-Westphal nucleus) is situated dorsally to the somatic efferent nucleus of the oculomotor nucleus and consists of the smaller motor nerve cells, their efferent fibers travel in the oculomotor nerve to relay in the ciliary ganglion, subsequently in the short ciliary nerves to innervate the ciliary muscle and the sphincter pupillae.

(2) Superior salivatory nucleus  The superior salivatory nucleus sends preganglionic parasympathetic fibers into the facial nerve, which controls the secretion of the lacrimal, submandibular and sublingual glands.

(3) Inferior salivatory nucleus  The inferior salivato1y nucleus is near to the rostral end of the dorsal nucleus of vagus, and sends preganglionic parasympathetic fibers into the glossopharyngeal nerve.

(4) Dorsal nucleus of vagus nerve  The dorsal nuc1eus of vagus nerve lies under the vagus triangle and gives rise to the preganglionic parasympathetic fibers which innervate the thoracic visceral organs and a large number of abdomina1 visceral organs via the vagus nerve.

4) General and special visceral sensory nucleus  The general and special visceral sensory nucleus consist of just one nucleus- the nucleus of solitary tract, which is an elongated sensory nucleus in the medulla. This nucleus receives visceral afferent fibers consisting of the primary afferent fibers from the facial. glossopharyngeal and vagus nerves, which enter the brain stem by passing through the posterolateral sulcus. In fact, the nucleus of solitary tract is shared by these three cranial nerves. Some authors consider the upper end of this nucleus as gustatory nucleus.

5) General somatic sensory nuclei  The general somatic sensory nuclei receive and relay sensory stimuli from the skin and mucosa of most of the head and divide into three nuclei: spinal nucleus of trigeminal nerve , pontine nucleus of trigeminal nerve and mesencephalic nucleus of trigeminal nerve. The spinal nucleus of trigeminal nerve is directly continuous with the substantia gelatinosa of the dorsal grey column of the spinal cord. It lies on the medial side of the spinal tract of trigeminal nerve, which is descending fibers of the sensory root of trigeminal nerve and is concerned with the mediation of pain and the thermal sensibilities of the trigeminal area. The pontine nucleus of trigeminal nerve lies on the lateral side of the motor nucleus, it extends inferiorly and continues with the spinal nuc1eus of trigemina1 nerve. The pontine nucleus and the spinal nucleus receive the primary afferent fibers of the trigeminal nerve. Functionally, the pontine nucleus relates to the conduction of the tactile and pressure impulses; the spinal nucleus, the pain and thermal impulses from the skin, mucous membrane, cornea, conjunctiva and the meninges through the branches of the trigeminal nerve. The mesencephalic nucleus of trigeminal nerve is a group of unipolar primary sensolv neurons located in the lateral margin of the f1oor of fourth ventricle. It extends upwards to enter the lateral portion of the periaqueductal grey matter of the midbrain, is concerned with the conduction of the proprioceptive impulse from the trigeminal nerve (Fig.VI-2-19).

 

Fig.VI-2-19 The trigeminal nucleus and the connection with central nervous system

6) Special somatic sensory nuclei  The special somatic sensory nuclei include the four vestibular and two coch1ear nuclei that receive stimuli via vestibulocochlear nerve. The vestibular nucleus complex is made up of the medial, inferior, lateral and superior vestibular nuclei. Dorsolateral to the sulcus limitans, the inferior nucleus, the lower part of the medial nucleus and lateral nucleus can be observed in the medulla oblongata. The remainder may only be observed in the pons. The major afferent fibers of complex originate from the vestibular ganglion and the cerebellum; the efferent fibers of the complex project extensively to the spinal cord through the vestibulospinal tract, to the cerebellum through the vestibulocrerebellar tract, to the nuclei of the oculomotor, trochlear, abducent, accessory nerve and the cervical anterior grey column cells through the medial longitudinal fasciculus. The cochlear nuclei conspicuously lies in the section cut through the bulbopontine sulcus. The dorsal cochlear nucleus is on the dorsolateral aspect of the inferior cerebellar peduncle, the ventral cochlear nucleus is on the ventrolateral aspect of the peduncle. These two nuclei receive afferent fibers from the cochlear ganglion through the cochlear nerve that enter the cerebellopontine angle(Fig. VI-2-22,23).

2. The non-cranial nerve nuclei

l) Nucleus gracile and nucleus cuneate  The gracile nucleusand cuneate nucleus are located in the lower medulla oblongata, deep to the gracile tubercle and cuneate tubercle respectively. Uncrossed gracile and cuneate fascicular fibers synapse in their respective nuclei at different levels. Axons from the nuclei emerge as internal arcuate fibers, at first curving ventrolaterally around the central grey matter and then ventromedially between trigeminal spinal tract and the central grey matter and decussate, constituting an ascending contralateral tract, the medial lemniscus (Fig. VI-2-20,21).

                       Fig. VI-2-20

 

 2) Inferior olivary nuclear complex  The inferior olivary nuclear complex is composed of the inferior olivary nucleus, the medial and dorsal accessory olivary nuclei. The inferior olivary nucleus, corresponding to the surface elevation of the olive, is the largest component of the inferior olivary nuclear complex. It is formed by a large hollow, mass of grey matter with crumpled walls and a hilum facing medially. A fibrous capsule, composed of afferent fibers, surrounds the nucleus. In addition to the spinoolivary tract, afferent connections of the nucleus come from the cerebrum, thalamus, basal nuclei, red nucleus and central grey matter of the midbrain. Some of these afferent fibers are said to travel in a bundle called central tegmental tract. The efferent fibers, olivocerebellar fibers, emerge from the hilum and run medially, intersecting the fibers of the medial lemnisci, the same named fibers and the spinal tract of trigeminal nerve of the opposite side to enter the inferior cerebellar peduncle (Fig. VI-2-22).

3) Superior olivary nucleus  The superior olivary nucleus is located in the reticular formation of caudal pons, dorsolateral to the trapezoid body. It receives the fibers from the cochlear nuclei and is involved in some acoustic ref1ex mechanism(Fig. VI-2-24).

Fig. VI-2-25

4) Pontine nuclei  The pontine nuclei comprise all the masses of nerve cells, which are scattered throughout the basilar part of the pons, constituting cell stations on the pathway from the cerebral cortex to the cerebellum(Fig. VI-2-24,25).

5) Nucleus ceruleus  The nucleus ceruleus consists of a group of pigmented nerve cells located in the lateral margin of the f1oor of fourth ventricle, connects with the extensive area of the CNS and seems to be involved in neural mechanisms concerned with somatic motor, visceral activity control and biological rhythms.

6) Inferior colliculus  The inferior colliculus is a conspicuous mass of grey matter in the tectum, it is ovoid in shape and surrounded by a capsule of nerve fibers. The chief afferent pathway to the inferior colliculus is the lateral lemniscus. The efferent fibers of the inferior colliculus nucleus pass through the brachium of inferior colliculus, accompanying some fibers of lateral lemniscus without relay in the nucleus of inferior colliculus, to enter the nucleus of medial geniculate body. As in other sensory pathways, there is a descending projection from the auditory cortex to the inferior colliculus via the medial geniculate nucleus. The nucleus of inferior colliculus also gives a few efferent fibers to take part in the tectospinal and tectotegmenta1 tracts via the superior colliculus. The inferior colliculi are ref1ex centers for auditory responses and concerned in the ability to localize the source of sounds (Fig.VI-2-26).

Fig.VI-2-26

Fig.VI-2-26

 

7) Superior colliculus  The superior colliculus consists of laminated architecture and receives input from the retina associated with the contralateral visual field and the visual cortex of the ipsilateral side. It sends efferent fibers to the thalamus, lateral geniculate nucleus, nuclei in the brain stem and the anterior horn cells of the spinal cord. A concrete pathway concerned with superior colliculus is the tectospinal tract. It arises from the deep layers of the superior colliculus and decussates near the median raphe ventral to the medial longitudinal fascicules to reach the cervical spinal segments. In man, the superior colliculi serve as ref1ex center correlating movement of the head and eyes used to localize and follow, visual stimuli. It may also respond to auditory stimuli by the spinotectal fibers(.

8)Red nucleus  The red nucleus is an ovoid mass situated in the center of the tegmentum and dorsomedial to the substantia nigra. Its pinkish colour, apparent in fresh specimen, is due to its rich vascularity and the presence of an iron-containing pigment, which occurs in much of the cells in the nucleus.

The efferent fibers decussate in the ventral tegmental decussation to constitute the rubrospinal tract, some of the efferent fibers constitute the rubrobulbar tract. The afferent fibers projecting to the red nucleus are derived from the cerebellar nuclei via the superior cerebellar peduncle and from the cerebral cortex via the corticorubral fibers .

9) Substantia nigra  The substantia nigrais a lamina of grey matter containing numerous, deeply pigmented nerve cells. It extends throughout the whole length of the midbrain and the subthalamic region. Pigmented cells are found in the most mammals, but the intensity of the pigmenation reaches maximum in man. However, the pigmented cells do not appear in the substantia nigra until the fourth or fifth year of life in man. The afferent fibers arise mainly from the caudate nucleus and putamen of the basal ganglia of the cerebrum, the other afferent fibers are derived from the spinal cord, subthalamic nucleus and cerebral cortex. The efferent fibers passing to the striatum of the cerebrum convey dopamine to the striatum (Fig.VI-2-26).

10)  Pretectal area  The pretectal area is situated at the junction of the midbrain and the diencephalon. It receives fibers from the occipital and preoccipital cortex and from the lateral root of the optic tract. Its efferent fibers pass to accessory oculomotor nucleus of both sides.

3. Long ascending and descending pathways

1)  Long ascending pathways

(l) Medial lemniscus  The fibers from fasciculi gracilis and cuneatus relay in the gracile and cuneate nuclei. The efferent fibers emerge from the ventral aspects of these nuclei, go forwards, laterally round the central grey matter and then bend medially to reach the median plane, where they decussate with the corresponding fibers of the opposite side(Fig.VI-2-21). Thereafter, they turn upwards and ascend on the opposite side close to the median raphe constituting the medial lemniscus. The decussation of medial lemniseus appears in the area dorsal to the pyramidal tract and in front of the central grey matter. The medial lemniscus gradua1ly takes on a more flattened profile and assumes a transverse orientation with a medial lateral axis at the junction between the basilar part and the tegmentum of pons. In the upper part of the pons, the lemnisci are arranged as a transverse band composed, from the medial to the 1ateral side, of the medial, trigeminal, spinal and lateral lemnisci. They are arranged in the lateral side of the decussation of the superior cerebellar peduncle at the level of inferior colliculus, of the red nucleus at the level of superior colliculus. The medial lemniscus is the important tract for conducting proprioceptive impulses(Fig. VI-2-22,25).

(2) Spinothalamic lemniscus  The spinothalamic lemniscus is composed of anterior and posterior spinothalamic tracts. It lies dorsally to the inferior olivary nucleus and is separated from the surface of the medulla oblongata by the anterior spinocerebellar tract. As it ascends through the upper part of the medu1la oblongata, the spinal lemniscus c1osely relates to the nucleus ambiguus. It is closed to the medial lemniscus in the pons and midbrain. Most of fibers terminate in the ventral posterolateral nucleus of thalamus. This tract conducts the impulses of pain, thermal and tactile sensations from opposite side of the body.

(3)  Lateral lemniscus  The efferent fibers of the cochlear nuclei run medially and rostrally in the ventral part of the tegmentum intersecting with the vertical fibers of medial lemniscus to form a conspicuous transverse bundle of fibers termed the trapezoid body(Fig.VI-2-24). Then the fibers cross the median raphe and decussate with the corresponding fibers of the opposite side. At the ventrolateral portion of the tegmentum they turn sharply just dorsolateral to the superior olivary nucleus in a longitudinal course to form the principal ascending auditory pathway, the lateral lemniscus. In reaching the midbrain some of the fibers of lateral lemniscus end in the inferior colliculus, others traverse the brachium of the inferior colliculus to reach the nucleus of medial geniculate body. Because the lateral lemniscus contains the uncrossed efferent fibers arisen from the cochlear nuclei of the same side.,unilateral damage of the lateral lemniscus at the levels rostral to the cochlear nuclei does not cause deafness of either ear.

(4) Trigeminal lemniscus  The axons of the spinal and pontine nucleus of trigeminal nerve successively cross the median plane to form the trigeminal lemniscus, which conducts the tactile, pressure, pain and thermal impulses ascending with the medial lemniscus to the thalamus and terminating in the ventral posterolateral nucleus of the thalamus.

(5) Medial longitudinal fasciculus  The medial longitudinal fasciculus is formed by a small compact tract of nerve fibers, situated closely to the median plane and ventral to the hypoglossal nucleus. It is continuous upward throughout the pons and the midbrain and downwards in the anterior funiculus of the spinal cord. This fasciculus is the main pathway that connects the vestibular and cochlear nuclei with the nuclei controlling the extraocular muscles and the cervical anterior grey matter, principally those innervating the muscles of neck. Its chief function is to ensure the coordinate movement of the eves and head in response to stimulation of the vestibu1ocochlear nerve (Figs. VI-2-22)

2) Long descending pathways

(1) Pyramidal tract (Fig.VI-2-20,21)  The pyramidal tract includes the corticospinal tract and the corticonuclear tract. Both continue downwards from the cerebral peduncle, entering the basilar part of the pons. They become dispersed into numerous smaller bundles from a compact collection of fibers, separated from one another by the pontine nuclei and transverse fibers of the pons. The corticonuclear tract terminates successively in the motor nuclei of the cranial nerve located in the brain stem. The corticospinal tract descends through the whole length of the pons and enters the pyramids of the medulla oblongata, where they converge into compact tracts again. In the lower part of the pyramid, about three­quarters of nerve fibers cross the median plane and continue down the spinal cord in the lateral funiculus as the lateral corticospinal tract, The uncrossed fibers retain their ventromedial position and descend in the anterior funicu1us of the spinal cord as the anterior corticospinal tract.

(2) Tectospinal tract  The tectospinal tract arises from the tectum of the midbran, descends on the ventral to the medial longitudinal fasciculus, passing through the pons and medulla oblongata to end in the anterior grey column of the spinal cord(Fig. VI-2-22)

4.  Reticular formation of brain stem

The reticular formation of brain stem have long been rccognized that outside the more conspicuous fiber bundles and nuclei of the brain stem, there is an extensive field of intermingled grey and white matter collectively termed the reticular formation. According to the traditional view, the reticular formation forms the central core throughout the brain stem;  appears in the upper cervical cord, lying laterally between postertor and anterior horns; in the medulla oblongata it lies among the olivary nucleus, inferior cerebellar peduncle, f1oor of fourth ventricle and medial lemniscus; in the pons and midbrain it fills most of the tegmentum embedding various nuclei and tracts.

The fibers contributing to the reticular formation interlace each other to form widespread network. The neurons contributing to the reticular formation can make up nuclei, such as the rapheal nuclei throughout the brain stem, the lateral reticular nucleus and gigantocellular reticular nucleus of the medulla ob1ongata, the rostral and caudal pontine reticular nuclei of the pons and the mesencephalic reticular nucleus etc.

The afferent projections of the reticular formation are the direct corticoreticular fibers and the collaterals of the corticospinal tract, the spinoreticular fibers and the collaterals of the long as cending tracts such as the lemnisci, the vagal pathway and the optic patbway via the tectoreticularfibers, and the fibers deriving directly or indirectly from the diencephalon to the corpus striatum.

The efferent connections of the reticular formation are the reticulospinal fibers to the spinalautonomic and locomotor control centers, by short descending paihways to sensory and locomotornuclei of the cranial nerves, and ascending reticular fibers to the nonspecific nuclei of the diencephalon and through lvhich to the corpus striatum and to the cerebral cortex, including most regions of neocortex and many areas of the 1imbic system.

The reticular fomation is an important integration center for the vital activity. Its major functions may sum up as follows:

1) Somatic motor control  The reticular formation exerts controlling influences on activity patterns of the skeletal muscles ranging from relatively simple ref1ex loops, coarse to fine locomotion and to complicated patterns associated with emotional expression involving the complexities of speech, gestures and fluctuations of facial expression.

2) Activation of the behavioral arousal  It is essential for the maintenance of the conscious state of cerebral cortex. In man, lesion of the brain stem, which results in damages to the reticular formation often, produces disturbances of consciousness, which range from f1eeting unconsciousness to deep and sustained coma.

3) Visceromotor control  Cardiovascular readjustments and the activity of nonstriated muscles and many glandular cells in the thoracicoabdominal viscera are, in most cases, under controlling inf1uences of postganglionic autonomic neurons. Their preganglionic neuronal somata and dendrite are either directly, or through the intermediary of local interneurons, partially controlled by reticulobulbar and descending reticulospinal fibers. Based mainly on the physiological investigations a series of vital functional centers such as the "cardiovascular controlling center", "respiratory center", "vomiting center"etc. have been established.

III.  Transverse sections of brain stem

1. Transverse section at the level of the pyramidal decussation

The outline of this section is somewhat as same as that of the cranial end of the spinal cord ( Fig. VI-2-20 ).The decussated fiber bundles of pyramidal tract interrupt the anterior median fissure and form the decussation of the pyramid. The substantia gelatinosa of the posterior grey horn of the spinal cord is replaced by the spinal nucleus of trigeminal nerve. The gracile nucleus and cuneate nucleus begin to be visible within the fasciculus gracilis and fasciculus cuneatus respectively. Lateral to the crossed fiber bundles are the reticular formation and long tracts.

 

 

Fig.VI-2-20 The transverse section through the medulla oblongata at

the level of the pyramidal decussation

2. Transverse section at the level of decussation of medial lemniscus  A pair of strong pyramidal tracts occupies the ventral part of the medulla oblongata (Fig. VI-2-21 ). Closely dorsal to the pyramidal tracts is the decussation of medial lemnisci, a characteristic sign in this section. Some nuclei of the cranial nerve. such as the hypoglossal nucleus, dorsal nucleus of vagus nerve and the nucleus of solitary tract, are presented within the ventral part of the central grey matter. The spinal nucleus and tract of trigeminal nerve are in original position.

 

 

Fig.VI-2-21 The transverse section through thc medulla oblongata at the

level of decussalion of medial lemniscus

3. Transverse section at the mid-olivary level  The central canal has been opened to become the lower portion of fourth ventricle (Fig. VI-2-22). The median sulcus and the sulcus limitans seen on the rhomboid fossa can be identified in this section. The hypoglossal nucleus is situated ventromedial to the sulcus limitans; the vestibular nucleus is located lateral to sulcus limitans; the dorsal nucleus of vagus, the solitary nucleus and tract are placed between the hypoglossal and vestibular nuclei. The spinal tract and nucleus of trigeminal nerve are still in the relative position and close to ventra1 aspect of the inferior cerebellar peduncle. The pyramidal tract occupies the most ventra1 position; dorsal to it and close to median raphe are the medial lemniscus, tectospinal tract and medial longitudinal fasciculus successively. A1l of the inferior olivary nucleus complex may be observed in the section, but the inferior olivary nucleus is the most conspicuous one among them. The rootlets of the hypog1ossal nerve traverse between the pyramidal tract and the inferior olivary nucleus and emerge from the surface of the medulla oblongata to constitute the hypoglossal nerve. An extensive area between the inferior olivary nucleus and the interior cerebellar peduncle is the reticular formation in which the nucleus ambiguus is embedded.

 

Fig. VI-2-22  Tht transverse section through the medulla oblongata at the mid-olivary level

4. Transverse section through the rostral end of the olive  The hypoglossal nucleus and dorsal nucleus of vagus nerve can no longer be visible in the grey matter of the floor of fourth ventricle (Fig. VI-2-23). The pyramidal tract, the inferior olivary nucleus, the spinal tract and nucleus of trigeminal nerve show little alteration in position; the medial lemniscus, tectospinal tract and medial longitudinal fasciculus are close to the median raphe. The inferior cerebellar peduncle is perfectly formed, the afferent fibers of the vestibulocochlear nerve and the ventral cochlear nucleus are revealed on its ventrolateral aspect, the dorsal cochlear nuc1eus on its dorsolateral aspect.

 

5. Transverse section at the level of facial colliculus  The trapezoid body intermingles with the medial lemniscus and separates the basilar part from the tegmentum of pons (Fig. VI-2-24). The clusters of nerve cells presented in the basilar part of the pons are the pontine nuclei. The transverse fibers arise from the cells of pontine nuclei and are collected as the pontocerebellar fibers, which continue transversely to become middle cerebellar peduncle. The longitudinal bundles of fibers belong to the pyramidal tract and corticopontine tract. In the tegmentum, the nucleus of abducent nerve and the internal genu of facial nerve underlie the facial colliculus. Lateral to the sulcus limitans are the vestibular nucleus and, more deeply, the spinal nucleus and tract of trigeminal nerve. The nucleus of facial nerve is a group of large motor cells, situated at the ventrolateral part of the reticular formation, ventromedial to the spinal tract and nucleus of trigeminal nerve. The reticular formation appears in an extensive area bounded by the medial lemniscus and the floor of fourth ventricle.

 

 

6. Transverse section at the level of trigeminal nerve  The structures of basilar part are the same as those seen in the prior section (Fig. VI-2-25 ). The superior, and middle cerebellar peduncles are arranged in the lateral wall of the fourth ventricle. The pontine nucleus and motor nucleus of trigeminal nerve are the prominent structures in the dorsolateral portion of the tegmentum.

 

Fig. VI-2-25  Transverse section through the pons at the level of trigeminal nerve

7. Transverse section through the inferior colliculus  The periaqueductal (central) grey matter is placed around the cerebral aqueduct, the nucleus of trochlear nerve is situated on its ventromedial part, the mesencephalic nucleus of trigeminal nerve in its lateral margin. Dorsolateral to the central grey matter are a pair of nuclei of inferior collicu1i. A strong decussation of superior cerebellar peduncle appears ventral to the central grey matter (Fig. VI-2-26). The substantia nigra separates the tegmentum from the crus cerebri, the latter made up of compact longitudinal bundles of fibers.

 

Fig. VI-2-26 The transverse section of the midbrain through the inferior colliculus

8. Transverse section through the superior colliculus  As compared with the prior scction, the nucleus of oculomotor nerve corresponds to the nucleus of trochlear nerve in position, the a1ternating laminae of grey matter and white matter appear in the depth of the superior colliculus, a pair of round red nuclei is located dorsomedial to the substantia nigra .  The oculomotor nerve passes through the medial edge of the red nucleus. The medial lemniscus, spinothalamic tract and trigeminothalamic tract are situated dorsolateral to the red nucleus.

 

 

  Clinic Notes

 The brain stem is an anatomically compact, functionally diverse, and clinically important structure. Even a single, relatively small lesion nearly always damages several nuclei. ref1ex centers, tracts or pathways. Such lesions are often vascular in nature, but tumors, trauma, and degeneration or demyelinating processes can also injure the brain stem.

1.Medial medullary syndrome usually involves the pyramid, part or all of the medial lemniscus, and nerve XII. If it is unilateral, it is also called alternating hypoglossal hemiplegia; it means that the cranial nerve XII paralysis is on the same side as the lesion, but the body paralysis is on the opposite side. The area involved is supplied by the anterior spinal artery or by medial branches of the vertebral artery.

2.Basal pontine syndrome can involve both the corticospinal tract and cranial nerve (VI, VII or V) in the affected region. Depending on the extent and level of the lesion. The syndrome is called alternating abducent (VI), facial (VII), or trigeminal hemiplegia (V) . If the lesion extent is large, it may include the medial lemniscus.

3.Peduncle syndrome (Weber syndrome) in the basal midbrain can involve nerve III and portions of the cerebral peduncle, which cause a cranial nerve III palsy on the side of the lesion and contralateral hemiparesis. The artery supply of this region is by the posterior perforators and branches of the posterior cerebral artery.

 

            Section 3  Cerebellum

The cerebellum is the second-largest portion of the brain and occupies the inferior and posterior aspects of the cranial cavity. Specifically, it is posterior to the medulla and pons and inferior to the occipital lobes of the cerebrum. It is separated from the cerebrum by the transverse fissure and by an extension of the cranial dura mater called the tentorium cerebelli which supports the occipital lobes of the cerebral hemispheres.

The cerebellum has two so-called  cerebellar hemispheres joined by a sagittal, transversely narrow median vermis.

Cerebellar surface is called the cortex which is separated into by numerous curved, transverse fissures, giving it a laminated appearance and separating its folia. Consists of gray matter in a series of slender, parallel ridges called folia. Beneath the gray matter are white matter tracts that resemble branches of a tree. Deep within the white matter are masses of gray matter, which are called the cerebellar nuclei or central nuclei.

The tonsils of cerebellum are two elevated masses on the inferior surface of the hemispheral portion just behind the flocculonodular lobe and nearby the foramen magnum (Fig. VI.-2-28).

Fig.VI-28 The cerebellum

The cerebellum is attached to the brain stem by three paired bundles of fibers (tracts) called inferior, middle and superior cerebellar peduncles.

 

 

I. Lobes of the cerebellum  The cerebellum can be primarily divided into a flocculonodular lobe and the corpus cerebelli, the latter having anterior and posterior lobe : The flocculonodular lobe is predominantly vestibular in its connections and constitutes the oldest part of the cerebellum, so it is called the vestibulocerebellum or archicerebellum; The anterior lobe and the rostral part of the inferior vermis are predominantly spinocerebellar in its connections and is phylogenetically the next part to appear, it is also called the spinocerebellum or paleocerebellum; The posterior lobe is predominantly corticopontocerebellar in its connections and constitutes the pontocerebellum or neocerebellum.

II. Cerebellar cortex  The cerebellar cortex is uniformly structured in all parts and three layers are evident in histologic sections. From the surface to the white matter of the folium, these are the molecular layer, the Purkinje cell layer, and the granular layer. There are two types of afferent fibers to the cortex. Mossy fibers terminate in synaptic contact with cells of the granular layer, through which they affect the Purkinje cells, whereas climbing fibers enter the molecular layer and wind around the dendrites of Purkinje cells. The only fibers leaving the cortex are axons of Purkinje cells, these fibers terminate in central nuclei of the cerebellum, with the exception of some fibers from the cortex of the flocculonodular lobe that proceed to the brain stem.

III.  Central nuclei of cerebellum  Three grey masses are embedded in the cerebellar white core on each side. Most lateral and largest is the nucleus dentatus, medial to which are the smaller nuclei emboliformis and globosus, the most medial being the nucleus fastigial from which most fibers run directly to the brain stem through the inferior cerebellar peduncle, ending in the vestibular nuclei of both sides and in the reticular formation of medulla oblongata. The dentate nucleus is the largest one and lies most laterally, which receives the fibers from the cerebellar cortex. The efferent fibers derived from the dentate nucleus form the major part of the superior or cerebellar peduncle and reach the red nucleus and the thalamus of contralateral side(Fig. VI.-2-29 ).

 

Fig.VI-2-29  The cerebellar nuclei

IV. White Core and Cerebellar Peduncles  

The white core of cerebellum consists mainly of the intrinsic and projection fibers, intrinsic fibres connect cerebellar regions. Association fibres connect the cortical folia, including the vermis; they do not decussate and most of them are short but some longer.

Projection fibres, connecting the cerebellum with many other regions, form three large peduncles on each side, issuing from the ventral cerebellar notch. Superior peduncles connect the cerebellum to the midbrain, middle peduncles to the pons and inferior peduncles to the medulla( Fig. VI-2-15,).

l. Iinferior cerebellar peduncle (restiform body)  It is composed of the fibers from the inferior olivary complex (olivocerebellar tract ), the posterior spinocerebllar tract, the reticular formation in the medulla oblongata, the vestibular nuclei and vestibular nerve. The restiform body also contains the efferent fibers which proceed from the flocculonodular lobe and fastigial nucleus to the vestibular nuclei and the reticular formation in the medulla oblongata and pons.

2. Middle cerebellar peduncle (brachium pontis)  It continues from the dorsolateral region of the pons and is composed almost exclusively of the pontocerebellar fibers .

3. Superior cerebellar peduncle (brachium conjunctivum)  It connects the cerebellum with the midbrain and thalamus. It consists mainly of the efferent fibers from the globose, emboliform and dentate nuclei (the dentatorubral tract and the dentatothalamic tract) . It also contains afferent fibers, such as the anterior spinocerebellar tract.

V. Functions

Functionally, the cerebellum is an area of the brain concerned with coordinating subconscious contractions of skeletal muscles (the cerebellar peduncles are the fiber tracts that channel information into and out of the cerebellum) . The cerebellum constantly receives input signals from proprioceptors in muscles, tendons, and joints, receptors for equilibrium, and visual receptors of the eyes. Such input permits the cerebellum to collect information on the physical status of the body with regard to posture, equilibrium, and all movements at joints. In addition, when other motor area of the brain, such as the motor cortex of the cerebrum and basal nuclei, send signals to skeletal muscles, they also send a duplicate set of signals to the cerebellum. The cerebellum compares this input information regarding the actual status of the body with the intended movement determined by motor areas of the brain (cerebrum and basal ganglia), If the intent of these motor areas is not being attained by the skeletal muscles, the cerebellum detects the variation and sends feedback signals to the motor areas or either to stimulate or inhibit the activity of skeletal muscles. This interaction produces smooth, coordinated movements of our body’s skeletal muscles.

The cerebellum also functions in maintaining equilibrium(平衡) and controlling posture(姿势), for example, receptors for equilibrium send nerve impulses to the cerebellum, informing it of body position. When the direction of movement changes, the cerebellum sends corrective signals to the motor cortex of the cerebrum.The motor cortex then sends signals over motor tracts to somatic motor neurons to skeletal muscles to reposition the body.

Another function of the cerebel1um is related to predicting the future position of a body part during a particular movement. Just before moving a part of the body to reach its intended position, the cerebellum sends signals over motor tracts to somatic motor neurons to skeletal muscles to slow the moving part and stop it at a specific point. This function of the cerebellum is used in actions such as walking.

There is some evidence that the cerebellum may play a role in a person’s emotional development, modulating sensation of anger and pleasure, allowing normal emotional expression and interpretation.

 

Section 4.  Diencephalon

 

The diencephalons, being median, has right and left halves between the medulla oblongata and the hemispheres of the cerebrum, being almost entirely surrounded by the hemispheres of the cerebrum, exposes only the ventral surface of the diencephalons to view in a diamond-shaped area containing hypothalamic structures. It consists of the dorsal thalamus, hypothalamus, epithalamus, subthalamus, and metathalamus (Figs. VI.-2-30).

 

Fig.VI.-2-30  The dorsal view of diencephalons

I  Dorsal thalamus

The dorsal thalamus or thalamus(Figs. VI.-2-31) is an ovoid structure above the midbrain that measures about 3 cm in length and constitutes four fifths of the diencephalons. It consists of paired ovoid masses of most gray matter organized into nuclei that form the lateral walls of the third ventricle. The masses are frequently joined by a bridge of gray matter that crosses the third ventricle, called the intermediate mass or interthalamic adhesion. Each mass is deeply embedded in a cerebral hemisphere and is bounded laterally by the internal capsule.

The thalamus is chiefly grey matter but its superior and lateral aspects are covered respectively by the white stratum zonale and external medullary lamina. It is incompletely divided into major parts by the vertical, white, internal medullary lamina, which splits in a Y-shaped manner, dividing the thalamus into anterior (rostral), medial and lateral nuclear group. The anterior nuclear group is enclosed by a bifurcation of the lamina and forms a rostromedial swelling known as the anterior tubercle. The medial nuclear group contains the large dorsomedial nucleus. The lateral nuclear group consists of ventral and dorsal tiers of nuclei which have been identified because of differing fiber connections. Five nuclei are recognized in the ventral tier, i. e. the medial and lateral geniculate nuclei, the ventral posterior, ventral lateral and ventral anterior nuclei. The dorsal tier consists of the pulvinar, lateral posterior nucleus and lateral dorsal nucleus. In the central part of the thalamus, the internal medul1ary lamina partially encloses the intralaminar nuclei, including the we1l-developed centromedian nucleus, The midline nuclei lie in the periventricualr grey matter of the thalamus and in the interthalamic adhesion. The thalamus also contains a intra1aminar nuclear group, median nucleus, and a reticular nucleus in its reticular formation.

On the basis of phylogeny, connections with other parts of the brain, and function, the thalamic nuclei may be classified into the following nucle.

l. Reticular nucleus  They receive collateral branches of thalamocortical and corticothalamic fibers.

2. Midline and intralaminar nuclei  They receive afferents from the reticualr formation of the brain stem and project mainly to other parts of the diencephalon.

3. Specific thalamic nuclei  They comprise the ventral tier of the lateral nuclear mass and send fibers to sensory and motor areas of cortex. The ventral posterior nucleus (general sensations) are specific sensory nuclei. The ventral lateral nucleus and the ventral anterior nucleus are specific motor nuclei, in a sense that they receive data from the cerebellum, corpus striatum, and substantia nigra, and project fibers to motor areas of cortex in the front lobe.

The ventral posterior nucleus functions as a thalamic relay nucleus for general sensations. The lateral and anterior spinothalamic tracts, the medial lemniscus, and the trigeminothalamic tracts al1 terminate in this nucleus. The nucleus receives the fibers from the rostral part of the nucleus of solitary tract and the vestibular nuclear complex.

There is a detailed topographic projection of the opposite half of the body on the ventra1 posterior nucleus, The lower 1imb is represented in the dorsolateral part of the nucleus, the upper limb in an intermediate position, and the head most medially, The medial region receiving sensory data from the head is usually referred to as the ventral posteromedial nucleus (VPM), and larger lateral portion for the remainder of the body as the ventral posterolateral nucleus (VPL). Nerve fibers leave the lateral aspect of the ventral posterior nucleus in large numbers, traverse the internal capsule and medullary center of the cerebral hemisphere, and end in the general sensory area of cortex in the parietal lobe.

4.  Nonspecific thalamic nuclei  They have reciprocal connections with association areas of cerebral cortex. This group includes the dorsomedial nucleus and the dorsal tier of the lateral nuclear mass, i. e. , the pulvinar, lateral posterior nucleus, and lateral dorsal nucleus. The anterior nucleus is included conventionally under this heading ,even though it is a thalamic component of the limbic system of the brain.

 

Fig. VI-2-31  A stereogram of nuclei of dorsal thalamus

Clinic Notes

Thalamic syndrome Aside from the motor aspects of ventral lateral and ventral anterior nuclei, the thalamus is sensory part of the brain and contributes to emotional responses to sensory experience. The thalamic syndrome is essentially a disturbance of these aspects of thalamic functions, subsequent to a lesion (usually vascular in origin )involving the thalamus or its connections. The symptoms vary usually raised on the opposite side of the body ,but when the threshold is reached the sensations are exaggerated, perverted and exceptionally disagreeable. For example, the prick of a pin may be felt as a severe burning sensation, and even music that is ordinarily pleasing may be disagreeable. There is spontaneous painin some instances, which may become inteactable to analgesics.

. Epithalamus

The epithalamic structures occupy the posterior diencephalic roof and adjacent areas of the third ventricular walls. They include right and left habenular nuclei, each deep to its habenular trigone and receiving a complex stria medullaris thalani; also included is the median pineal gland and habenular and posterior commissures crossing in anterior and posterior laminae of the pineal peduncle. The pineal gland, an endocrine gland in mammals, is attached to the posterior commissure on the midline (Fig. VI.-2-30).

III  Subthalamus 

The subthalamus is situated immediately ventral to the dorsal thalamus, in part lateral to the hypothalamus, and emerges caudally with the tegmentum of midbrain. The region includes the rostral extension of the red nucleus and substantia nigra, the prominent subthalamic nucleus (nucleus of Luys). The latter, located beneath the dorsal thalamus and medial to the internal capsule, has the shape of a thick biconvex lens. Caudally the medial part of the nucleus overlies the rostral portion of the substantia nigra. It is a component of the extrapyramidal system (Fig. VI-2-32).

IV. Metathalamus

The metathalamus is located posterolateral to the thalamus. It includes lateral geniculate body and medial geniculate body (Fig. VI-2-15,31),they are specific relay nuclei.

The lateral geniculate body receives the fibers of optic tract ,then gives rise to the optic radiationwhich prejects to the visual area of cerebra.

The medial geniculate body receives the acoustic fibers passing through the brachium inferior colliculus,and  then gives origin of the acoustic radiation prejected to the auditory area of cerebra.

V. Hypothalamus

The hypothalamus, occupying only a small part of the brain weighed about 4g, has a functional importance that is quite out of proportion to its size. The hypothalamus surrounds the third ventricle ventral to the hypothalamic sulci. From front to back they are: the optic chiasma, the tuber cinereum, tuberal eminences and the infundibular stalk, the mamillary bodies, the posterior perforated substance, included here for convenience.

Tuber cinereum, between the mamillary bodies and the optic chiasma, is a convex mass of grey matter; its ventral aspect is not completely smooth but presents a series of eminences with intervening grooves of varying depth. Behind the optic chiasma a median, conical, hollow infundibulum depends ventrally to the solid posterior lobe of the hypophysis cerebri.

 Mamillary bodies are smooth, hemispherical and about the size of two small peas, side by side in the interpeduncular fossa’s floor, anterior to the posterior perforated substance, each enclosed in white fascicles largely from the fornix. Internally are a number of nuclei.

Posterior perforated substance, a small grey depression, lies posteriorly in the interveal between the diverging crura cerebri, pierced by small apertures for central branches of posterior cerebral arteries. Deep within it is the interpeduncular nucleus, small in man and homologous with a more extensive complex in submammalian forms. It receives looped terminals of the fasciculus retroflexus of both sides and has other connections with the mesencephalic reticular formation and mamillary bodies.

Electrical stimulation of the hypothalamus in experimental animals shows that some regions give parasympathetic, and others sympathetic responses. Parasympathetic responses are most regularly elicited by stimulation of the anterior hypothalamus, notably the preoptic area and anterior nucleus.

 

Fig. VI.-2-32  The coronal section of diencephalon

The responses include slowing of the heart rate, vasodilation, lowering of blood pressure, salivation, increased peristalsis in the gastrointestinal tract and contraction of the urinary bladder. Sympathetic responses, most readily elicited by stimulation in the region of the posterior and lateral nuclei, include cardiac acceleration, elevation of blood pressure, cessation of peristalsis in gastrointestinal tract, dilatation of the pupils and hyperglycemia.

Relations of the hypothalamus with the pituitary gland (hypophysis), neurohypophysial hormones are elaborated, and hormone productions by the adenohypophysis are controlled by chemical Substances synthesized in hypothalamic cells. The nervous system, through the neurosecretory function of hypothalamic cells, has therefore an intimate relation with the entire endocrine system.

 

Fig .VI.-2-33  The representation of hypothalamic nuclei

VI. Third ventricle 

The diencephalic part of the ventricular system consists of the narrow third ventricle. The anterior wall of this ventricle is formed by the lamina terminalis, the anterior commissure crosses the midline in the dorsal part of the lamina terminalis. The rather extensive lateral wall is marked by the hypothalamic sulcus, separating the thalamus from the hypothalamus. A massa intermedia (interthalamic adhesion)bridges the ventricle in 70% of brain. The floor of the third ventricle is bounded with the optic chiasma, infundibulm, tuber cinereum,and mammillary bodids .The membranous roof of the third ventricle is attached along the striae terminalis, and a pair of choroids plexuses is suspended from the roof. Cerebrospinal fluid enters the third ventricle from each lateral ventricle through the interventricular foramen (foramen of Monro). The fluid leaves the third ventricle by way of the mesencephalic aqueduct, through which it reaches the fourth ventricle. From the latter it passes through the apertures on the ventricle into the subarachnoid space surrounding the brain and spinal cord.

Wu Aiqun  Gao Yan The Second Military Medical University, ShangHai  

 

 

Section 4  Telencephalon

 

The telencephalon, also called cerebrum, consists mainly of two large cerebral hemispheres. Two hemispheres are almost completely separated by a deep median cleft, the cerebral longitudinal fussure. At the bottom of this fissure a large bundle of transverse fibers, the corpus callosum, crosses between the two hemispheres. The cerebrum and the cerebellum are completely separated by the cerebral transverse fissure.

Each cerebral hemisphere is composed of an external stratum of grey matter(neurons), termed the cortex, and an internal white matter (neuronal processes), the medullary substance. Within the hemisphere, there are several masses of grey matter deeply situated known as the basal nuclei and a cavity, the lateral ventricle.

I. External features 

 Each cerebral hemisphere presents superolateral (dorsolateral), medial and inferior surfaces .The superolateral surface is adapted to the concavity of its half of the cranial vault. The medial surface is flat and vertical. The inferior (basal) surface is irregular and divided into orbital and tentorial regions. The anterior and posterior hemispheric extremity is called the frontal pole and the occipital pole respectively. About 4cm anterior to the occipital pole on the inferolateral border is the preoccipital incisure(notch).

The complicated folding of the surface of the cerebral hemispheres substantially increases the surface area and therefore the volume of cerebral cortex about three times. The upfolds are called gyri and the downfolds are referred to as sulci or fissures. Although most gyri are constant, others vary in their dimensions and minor details, not only in different individual but in the hemispheres of one brain.

There are three constant sulci (or fissures) for demarcation on the surface of the hemisphere. The lateral sulcusor fissure of Sylvius begins as a deep furrow on the inferior surface of the hemisphere and runs posteriorly and upward onto the dorsolateral surface. The central sulcus or sulcus of Roando starts in the superior border of the hemisphere about 1 cm behind the midpoint between the frontal and occipital poles, it then slopes downward and forward, stopping just short of the lateral sulcus. The parietooccipital sulcus, like the lateral sulcus, is a deep groove which appears early in the development of the fetal brain; it lies on the medial surface and separates the parietal and occipital lobes as its name indicates (Fig. VI-2-34)..

 

Fig. VI-2-34 The superolateral surface of cerebral hemisphere

The hemisphere can be divided into four lobes and an insula by these sulci and an imaginary line joining the upper end of the parieto-occipital sulcus to the preoccipial notch, viz imaginary parieto-occipital line The frontal lobe is the area in front of the central sulcus and above the lateral sulcus. The parietal lobe is the area bounded by the central sulcus, the lateral sulcus and the imaginary line. The area posterior to the imaginary line and the parieto-occipital sulcus is the occipital lobe. The temporal lobe occupies the area inferior to the lateral sulcus and in front of the imaginary line.

The insula(insular lobe) lies deeply in the floor of the latera1 sulcus, and is overlapped by portions of the frontal, parietal and temporal lobes (Fig. VI-2-35).

1.  Gyri and sulci on the superolaleral surface of the hemisphere

In the frontal lobe, the precentral sulcus (often considered as two or more parts) is in front of and parallel to the central sulcus, these sulci demarcate the precentral gyrus. The remaining surface of the frontal lobe is divided into superior, middle and inferior frontal gyri by superior and inferior frontal sulci, which are roughly perpendicular to the precentral sulcus and forward.

 

Fig. VI-2-35The insula

In the parietal lobe, the postcentral sulcus is behind and parallel to the central sulcus, these sulci bound the postcentral gyrus. The intraparietal sulcus extends posteriorly from the middle of the postcentral sulcus, and divides the part behind the postcentral gyrus into superior and inferior parietal lobules. Those portions of the inferior parietal lobule that surround the upturned ends of the lateral sulcus and the superior temporal sulcus are called the supra-marginal gyrus  and the angular gyrus  respectively.

In the temporal lobe, the superior and inferior temporal sulci divide the lateral suface of the temporal lobe into superior, middle and inferior temporal gyri. The upper surface of the superior temporal gyrus forms two gyri in the f1oor of the lateral sulcus, known as transverse temporal gyri.

In the occipital lobe, the calcarine sulcus on the medial surface of the hemisphere continues for a short distance over the occipital pole . The other sulci and gyri of this lobe are not constant.

2.  Gyri and sulci on the medial and inferior surfaces of the hemisphere (Fig. VI-2-36)

 

Fig. VI.-2-36 The medial surface of cerebral hemisphere

On the medial surface, the cingulate gyrus is separated from the corpus callosum by the callosal sulcus. The cingulate sulcus intervenes between the cingulate gyrus and the extension of the superior frontal gyrus on the medial surface of the hemisphere. After it gives off the paracentral sulcus, it branches into the marginal sulcus (ramus) and subparietal sulcus. The region bounded by the paracentral and marginal sulci is paracentral lobule. It is notched by the central sulcus, and formed by the precentral and postcentral gyri which extend from the superotaleral surface onto the medial surface of the hemisphere. The parieto-occipital sulcus and calcarine sulcus are two obvious sulci on the posterior part of the medial surface. The former is described previously. The calcarine sulcus continues for a short distance over the occipital pole and joins the parieto-occipital sulcus in a Y-shaped formation. The cortex on both sides of the calcarine sulcus represents the striate(visual) cortex

On the inferior surface of the hemisphere (Fig. VI-2-37) a gyrus extends from the occipital pole almost to the temporal pole. The posterior part of this gyrus consists of the lingual gyrus, the anterior part forms the parahippocampal gyrus which hooks sharply backward as the uncus.

On the orbital surface of the frontal lobe, the olfactory bulb and olfactory tract conceal most of the olfactory sulcus. The gyrus rectus is medial to the olfactory sulcus, and the large area lateral to the olfactory sulcus consists of irregular orbital gyri.

 

Fig. VI-2-37 The inferior surface of cerebrum

II. Limbic system

 On the medial surface of the cerebral hemisphere, a large arcuate convolution, formed primarily by the cingulate, parahippocampal gyri, uncus and the hippocampal formation surrounding the upper brain stemconstitutes the limbic lobe. The limbic system is used to include the limbic lobe as we11 as associated subcortical structures including amygdaloid body, septal nuclei, mammillary body, anterior nucleus of thalamus, olfactory bulb.Fig. VI-2-38.

 

Fig VI.-2-38 Selected components of the limbic system and surrounding structures

 

 

The hippocampal formation(Fig. VI-2-39) includes the dentate gyrus, the hippocampus proper[hippocampus, Ammons horn], the subicular complex [prosubiculum, subiculum, presubiculum, parasubiculum] and the entorhinal cortex. Dentate gyrus is a cerebral gyrus between the hippocampus and parahippocampal gyrus. Amygdaloid body  includes several groups of neurons located at the tail end of the caudate nucleus. Septal nuclei are located within the septal area which is formed by the regions under the corpus callosum and the paraterminal gyrus . Olfactory bulbs are flattened bodies formed by the olfactory nerves and neurons, resting on the cribriform plate. Bundles of interconnecting myelinated axons interconnect various components of the limbic system, which include the fornix, stria terminalis, stria medullaris, medial forebrain bundle and mammillothalamic tract.

              

 

 

                

 

(1)Hippocampus, the posteior and inferior                     (2)Hippocampus and related structure

horn of the right lateral ventricle                           (seen in a coronal section of the floor

(posted from above)                                     of the inferior horn of the lateral ventricle)

Fig. VI-2-39 Hippocamus and hippocampal formation

The limbic system functions in the emotional aspects of behavior related to survival. The hippocampus, together with portions of the cerebrum, also functions in memory. Memory impairment results from lesions in the limbic system. People with such damage forget recent events and cannot commit anything to memory. How the limbic system functions in memory is not clear. Although behavior is a function of the entire nervous system, the limbic system controls most of its involuntary aspects. Experiments on the limbic system of monkeys and other animals indicate that the amygdaloid nucleus assumes a major role in controlling the overall pattern of behavior.

Other experiments have shown that the limbic system is associated with pleasure and pain. When certain areas of the limbic system or the hypothalamus, thalamus, and midbrain are stimulated in animals, their reactions indicate that they are experiencing intense punishment. When other areas are stimulated, the animals reactions indicate that they are experiencing extreme pleasure. Stimulating other areas of the limbic system results in an opposite behavioral pattern: docility, tameness, and affection. Because the limbic system assumes a primary function in emotions such as pain, pleasure, anger, rage, fear, sorrow, sexual feelings, docility, and affection, it is sometimes called the “visceralor emotional brain”.

The limbic system is concerned with: emotional expression and genesis, together with visceral response to the emotions;②survival of individual and species;③cognitive processes involved in memory. The limbic system is also known as the visceral brain because of its substantial influence on visceral functions through the autonomic nervous system.

III. Cerebral cortex

1. Histology of the cerebral cortex  Each cerebral hemisphere has a mantle of grey matter, the cortex or pallium , which makes up about 40% of the weight of the human brain. The cerebral cortex is often divided into an older (original part), consisting of the archicortex and paleocortex, such as hippocampus and entorhinal cortex, and a newer development, neocortex, which may be equated with systems of sensory and mortor activity. The cortex has a characteristic structure, consisting of nerve cells (neurons) and nerve fibers arranged in layers. Generally, the neocertex can be divided into six layers, from the external surface to the deep layer in proper order, including the molecular (plexiform), external granular, pyramidal, internal granular, ganglionic and polymorphic (multiform) lamina (Fig. VI-2-40). The details of these layers differ from one region to another, From neocortex to archicortex, they change their structure from a modified six-layered to a four or three-layered cortex, for instance, the hippocampus is a primitive trilaminar cortex with molecular , pyramidal, and polymorphic layers from the original external surface to the ventricular (Fig. VI-2-39).

 

 

Fig. VI-2-40 Representation of the layers of the human cerebral cortex

The cerebral cortex has been divided into cytoarchitectural areas, based on differences in the thickness of individual layers, neuronal morphology in the layers, and the details of nerve fiber lamination. Brodmann’s map, consists of approximately 52 areas, remains the most widely used map of cortical cytoarchitectural areas (Fig. VI-2-41).

 

 

 

Fig. VI-2-41The subdivision of the cerebral cortex

2. Functional localization of the cerebral cortex  Clinicopathologic studies and animal experiments conducted over more than a century have provided information concerning functional specialization in different regions of the cerebral cortex (Fig. VI-2-42). Three main sensory areas are for general sensation(somatosensory), vision and hearing, to which may be added gustatory and vestibular areas. There are also motor areas(somatomotor) from which contraction of the skeletal musculature can be elicited by electrical stimulation. The remainder of the neocortex falls under the general heading of association cortex, which may be closely related functionally with the sensory areas or have an essential bearing on even more complex levels of behavior and the intellect.

It is convenient to consider the neocortex as consisting of the following two main parts: cortex of the parietal, and temporal lobes, and cortex of the frontal lobe. The former is concerned with the reception and conceptual elaboration of sensory data, whereas the latter is concerned with motor responses and judgement, foresight, and moods associated with behavior. Because of the complex nature of all aspects of the cerebral cortex, descriptions of functional areas and the terminology used vary greatly from one author to another. This is especially true of the association cortex situated adjacent to sensory areas and of areas from which motor responses are elicited by electrical stimulation. The following summary of this highly complicated subject is in accordance with one of several interpretations, especially with respect to the areas for which different modes of treatment are most pronounced.

 

                                

 

 

Fig VI-2-41

 

 

 

Fig. VI-2-42 The main functional areas (centers) of the cerebral cortex

 

(1) Primary somesthetic area (Fig. VI-2-42)  It occupies the postcentral gyrus on the superolateral surface of the hemisphere and the posterior part of the paracental lobule on the medial surface. It consists of areas 3,1 and 2 of Brodmann’s map. It is possible to elicit motor responses by stimulating the somesthetic area, as well as sensory responses from the motor area in the precentral gyrus. The connections and functions of the two areas, therefore, overlap to some extent and they should be considered as a sensorimotor strip surrounding the central sulcus.

The ventral posterior nucleus of the thalamus is the main source of afferent fibers for the general sensory area. The fibers traverse the internal capsule and medullary center, conveying data for the various modalities of general sensation. Each point of the somesthetic area receives sensations from specific parts of the body. The contralateral half of the body is represented as inverted (Fig .VI-2-43). The pharyngeal region, tongue, and jaws are represented in the most ventral (lowest) part of the somesthetic area, followed by the face, hand, arm, trunk, and thigh. The area for the remainder of the leg and the perineum is in the extension of the somesthetic cortex on the posterior part of the functional importance of the part and its need for sensitivity. Thus the area for the face, especially the lips, is disproportionately large, and a large area is signed to the hand, particularly the thumb and index fingers.

In addition to the main or primary somesthetic area already discussed, the existence of a secondary somesthetic area has been demonstrated in primates, including man. This small area is situated in the dorsal and extends to the insula. The parts of the body are represented bilaterally, although contralateral representation predominates.

(2) Primary somatomotor area It is located in the precentral gyrus, including the anterior wall of the central sulcus and the anterior part of the paracentral lobule on the medial surface of the hemisphere. The motor cortex (area 4 of Brodmann ) is 4.5mm in thickness, in which giant pyramidal cells of Betz are present in the fifth layer.

 

 

 

Fig .VI-2-44 motor cortex

 

The main sources of input to area 4 are the premotor cortex (area 6), the somesthetic cortex, and the ventral lateral and ventral anterior thalamic nuclei. Although area 4 contributes fibers to extrapyramidal motor pathways, the efferent that gives it a special significance are those included in the pyramidal motor system (corticonuclear and corticospinal tracts). About 40% of these fibers arise in area 4, the remains originate in area 6 of the frontal lobe and in the parietal lobe, including the somesthetic area.

Electrical stimulation of the motor area elicits contraction of muscles predominantly on the opposite side of the body. Although cortical control of the skeletal musculature is mainly contralateral, there is significant ipsilateral control of most of the muscles of the head and of the axial muscles of the body. The body is represented in the motor area as inverted, the pattern being similar to that described for the somesthetic cortex (Fig VI-2-44). The sequence from below upward is pharynx, larynx, tongue and face; the region for muscles of the head constituting about one-third of the whole of area 4. Continuing dorsally, there is a small region for muscles of the neck, followed by a large area for muscles of the hand, this being consistent with the importance of manual dexterity in man. Next in order are areas for the arm, shoulder, trunk, and thigh, continuing with an area on the medial surface of the hemisphere for the remainder of the leg, the anal and visceral sphincters concerned with the voluntary control over the defecation and micturition.

(3) Acoustic area(auditory area)  It is located in the transverse temporal gyri corresponding to areas 4l and 42 of Brodmann. The medial geniculate nucleus of the thalamus is the principal source of fibers ending in the acoustic cortex, these fibers constitute the acoustic radiation in the medullary center. There is a spatial representation in the acoustic area with respect to the pitch of sounds, impulses for high frequencies on the posteromedial part.

(4)  Visual area It surrounds the ca1carine sulcus on the medial surface of the occipital lobe, corresponding to area 17 of Brodmann. The visual area is also called the striate area(纹状区). The chief source of afferent fibers to area l7 is the lateral geniculate nucleus of thalamus by way of the geniculocalcarine tract.

Through a synaptic relay in the lateral geniculate nucleus, the visua1 cortex receives data from the temporal half of the ipsilateral retina and the nasal half of the contralateral retina. The left half of the field of vision is therefore represented in the visual area of the right hemisphere and vice versa.

(5) Gnostic area (areas 5,7, 39, and 40) A common integrative area located among the somesthetic, visual, and auditory association areas. The gnostic area receives nerve impulses from these areas as well as from the taste and smell areas, the thalamus, and lower portions of the brain stem. It integrates sensory interpretations from the association areas and impulses from other areas so that a common thought can be formed from the various sensory inputs. It then transmits signals to other parts of the brain to cause the appropriate response to the sensory signal.

(6) Taste area (gustatory area) It is located in the dorsal wall of the lateral sulcus with an extension to the insula, corresponding to area 43 of Brodmann. This location is similar to that of the secondary somatic sensory area, and it places the taste area adjacent to the general sensory cortex for the tongue and pharynx.

(7) Secondary and supplementary motor areas They have been identified by cortical stimulation in primates, including man, although they are not known to be of importance clinically.

The secondary motor area is ventral to the sensorimotor strip in the dorsal wall of the lateral sulcus, overlapping the secondary somesthetic area. The supplementary motor area is on the medial surface of the hemisphere anterior to the paracentral lobule. In both areas, contraction of muscles in different parts of the body can be elicited by electrical stimulation.

 (8)  Premotor area Which coincides with Brodmann’s area 6 and is situated in front of the motor area on the superolateral and medial surfaces of the hemisphere. The cytorarchitecture of area 6 is like that of the area 4, except that Betz cel1s are absent. In addition to connections with other cortical areas, the premotor cortex receives fibers from the ventral anterior and ventral lateral thalamic nuclei. Area 6 may be concerned in part with learned motor activity of a complex and sequential nature. The term aphaxia refers to the result of a cerebral lesion, characterized by impairment in the performance of learned movements even though there is no voluntary motor paralysis.

(9) Prefrontal cortex It corresponds to areas 9, l0 and l2 of Brodmann, and is well developed only in primates and especially so in man. It has extensive connections through fasciculi in the medullary center with the parietal, temporal and occipital lobes, thus gaining access to contemporary sensory experience and the repository of data derived from past experience. There are also reciprocal connections with the dorsomedial thalamic nucleus, forming a system which determines affective reactions to situations presently encountered on a background of past experience. The prefrontal cortices also monitor behavior and exercise control based on such higher mental faculties as judgement and foresight.

(10) Language areas( Fig. VI-2-42) They have been identified by the study of patients in whom these areas were damaged by occlusion of blood vessels. The most reliable information is based on long term studies of patients with deficits in the use of language whose brains were subjected to careful postmortem examination. Two cortical areas of special importance in language have been demonstrated. One such area in the temporal and parietal 1obes is concerned with sensory aspects of language, and a second area, in the inferior frontal gyrus (Broca area), functions in motor aspects of language (speech). The two areas are in communication through the superior longitudinal fasciculus in the medullary center. They are situated in the left hemisphere with few exceptions, and this hemisphere is therefore the dominant hemisphere as a rule with respect to language.

The translation of speech or written words into thought involves sensory areas (auditory, visual and Gnostic).

l) Sensory language area Observations made during cortical stimulation in conscious patients led to the concept of a sensory (or “ideational”) language area in the temporal and parietal lobes. This area to a considerable extent occupies the posterior part of the superior and middle temporal gyri and most of the inferior parietal lobule. In relation to the Brodmann map, it includes portions of areas 21, 22 and 37 in the temporal lobe, and most of areas 39and 40 in the inferior parietal lobule. However, clinicopathologic studies of patients with speech disorders have shown quite clearly that the lesion responsible for a sensory or receptive type of language defect typically involves the auditory speech area or Wernicke’s area. A child first experiences language by hearing others talk and only later learns to read and write, it is perhaps for this reason that Wernicke’s area has a special role in sensory aspects of language. In any case, association cortex in and around Wernicke’s area receives pertinent data to language from the sensory areas and synthesizes these data into a comprehensive totality of the sensory aspects of language.

2) Visual language area(reading area)  The angular gyrus is the area for visual word-images. The damage of the area will cause the Alexia.

3) Motor language area or Broca area  It occupies the posterior portion of the inferior frontal gyrus, corresponding to areas 44 and 45 of Brodmann. Its inf1uence on the lower one-third of the motor cortex for muscles of the head is essential for the appropriate and coordinated action of the diverse muscles used in speech.

4) Writing area  It is the posterior portion of the middle frontal gyrus. The damage of the area will cause the agraphia,

A lesion involving the language areas or their connections results in aphasia, of which there are several types, depending on the location of the lesion. Sensory aphasia(Wernicke aphasia), in which comprehension of language, naming of objects, and repetition of a sentence spoken by the examiner are all defective, is caused by a lesion in the sensory language area, more specifically in Wenlicke’s area. Infarcts, which isolate the sensory tempora1 cortex, may cause nominal aphasia, characterized by f1uent but circumlocutory speech due to word-finding difficulties. Alexia refers to loss of ability to read and occurs with or without other aspects of aphasia. “Pure alexia”, may result from a lesion involving the occipital lobe of the dominant hemisphere and the splenium of the corpus callosum. Motor aphasia (Broca aphasia), caused by a lesion in Broca area of the frontal lobe, is characterized by hesitant and distorted speech with relatively good comprehension. A lesion involving writing area may cause agraphia. Although the patients can move their hand voluntarily, they can not write to express their ideas.

The middle part of the superior and middle fronta1 gyri are synergetic motor area of eyeball , corresponding to area 8 of Brodmann. This area controls voluntary scanning movement of the eye.

. Internal structures

 Each cerebral hemisphere includes a large volume of white matter constituting the basal nuclei, the medullary center and the lateral ventricle.

1. Basal nuclei(ganglia They are several masses of grey matter situated in the central portion of the cerebral hemisphere(Figs. Fig VI-2-45,47). The basal nuclei consist of corpus striatum , claustrumand amygdaloid nucleus

 

Fig. VI-2-45 A diagram of corpus striatum and dorsal thalamus

(the lower two are the horizontal sections of the upper one)

(l) Corpus striatum  It consists of the caudate nucleus and the lentiform nucleus, the latter being divided into the putamen and the globus pallidus. The caudate nucleus remains in the wall of the lateral ventricle and grows around with it in a C-shaped course. It consists of an anterior portion or head, which tapers into a slender tail extending backward and then forward into the temporal lobe and terminates at the amygdaloid nucleus. The lentiform nucleus (lenticular nucleus) is wedge-shaped. The narrow part facing medially is the globus pallidus.

The caudate nucleus and putamen are new structures phylogenetically, which have a common embryological origin, identical histological appearances, and similar connections, so both together are called the neostriatum, or simply the striatum. While, the globus pallidus is referred to as the paleostriatum.

(2) Claustrum  It is a thin sheet of grey matter lying in the medullary substance of the hemisphere between the insula cortex and the putamen of the lentiform nucleus, from which it is separated by the fibers of the external capsule.

(3) Amygdaloid nucleus  It lies at the end of the tail of the caudate nucleus in the temporal lobe and is a component of the olfactory and limbic systems.

The term, “basal nuclei” is now used clinically to refer to those structures whose damage causes extrapyramidal syndrome. Thus, it means the combination of caudate nucleus, putamen, globus pallidus, subthalamic nucleus, substantia nigra and some other structure. Parkinsonism(is the most common and probably the best-known disease involving the basal nuclei. The symptoms are quite variable in relative severity and onset, but they usually include tremor, rigidity  and difficulty in moving The tremor is a resting tremor, characteristically involving the hands in a “pill-rolling” movement; it diminishes during voluntary movement and increases during emotional stress.

2. Medullary center It is composed of three kinds of fibers depending on the nature of their connections.

(l) Association fibers  They connect and transmit nerve impulses between gyri in the same hemisphere (Fig. VI-2-46). The longer fibers constitute the superior and inferior longitudinal fasciculus to provide the communication between four lobes of the cortex, the cingulum to connect the parahippocampal gyrus with the cingulate gyrus, the uncinate fasciculus to connect the temporal pole with the frontal lobe. The short association fibers are known as the short arcuate fibers which connect the adjacent gyri.

 

Fig VI-2-46 The association fibers of cerebral hemisphere

(2) Commissural fibers  They connect and transmit impulses from the gyri in one hemisphere to the corresponding gyri in the opposite cerebral hemisphere. Three important groups of these fibers constitute the corpus callosum, the remainder are included in two very small bundles, the anterior and posterior commissures. The corpus callosum can be divided into 4 parts: the rostrum, genu, trunk(body) and splenium from forward backward (Fig. VI-2-36).

(3) Projection fibers They connect the cerebral cortex with the subcortical structures including lower levels in brain and spinal cord, which are corticopetal and corticofugal fibers, and form ascending and descending tracts that transmit impulses from other part of the brain and spinal cord. The fibers fan out as the corona radiata in the medullary center.

The internal capsuleFigs. VI-2-47, 48) is a concentrated structure of the projection fibers located between the caudate nucleus, centiform and thalamus. In horizontal cerebral section the internal capsule is seen as a broad white band, with a lateral concavity adapted to the convex medial aspect of the lentifom nucleus. It consists of an anterior limb, genu and posterior limb, which have topographic relationships with adjacent grey masses. The anterior limb is bunded by the lentiform nucleus and the head of the caudate nucleus, and the genu is medial to the apex of the lentiform nucleus. The posterior limb includes the following parts: the thalamolentiform part is between the lentiform nucleus and the thalamus, the retrolentiform part consists of fibers occupying the region behind the lentiform nucleus, and the sublentiform part includes those fibers that pass beneath the posterior part of the lentiform nucleus.

 

Fig VI-2-47 Two horizontal section of cerebrum

 

 

Fig. VI-2-48 A diagram of internal capsule

Many of the projection fibers establish reciprocal connections between the thalamus and the cerebral cortex, called the corona radiata (Fig. VI-2-49). The anterior thalamic radiation, which is included in the anterior limb of the interna1 capsule, consists mainly of fibers connecting the dorsomedial thalamic nucleus and the prefrontal cortex. The middle thalamic radiation is a component of the posterior limb of the internal capsule, includes the projection from the ventral posterior thalamic nucleus to the somesthetic area in the parietal lobe, ie, thalamo-cortical tract. The posterior thalamic radiation contained in the retrolentiform part establishes connections between the thalamus and cortex of the occipital lobe. The geniculocalcarine tract (optic radiation) ending in the visual cortex is a particularly important component of this radiation. The inferior thalamic radiation consists of fibers directed horizontally in the sublentiform part of the posterior limb of the internal capsule, connecting thalamic nuclei with cortex of he temporal lobe. Most of the fibers are included in the acoustic radiation, which originates in the medial geniculate nucleus and terminates in the acoustic area.

 

Fig. VI-2-49 Lateral ventricle and corona radiata

The remaining projection fibers are corticofugal and incorporated in tracts concerned with motor functions, the corticonuclear (corticobulbar) and corticospinal tracts, which together constitute the pyramidal system, originate in the motor and premotor areas in the frontal lobe and in the parietal lobe. The fibers converge as they traverse the corona radiata and enter the posterior limb of the internal capsule. From the beginning of the present century, it was believed that the corticonuclear tract occupied the genu of the internal capsule and that the corticospinal tract was situated in the posterior limb immediately behind the genu. As evidence accumulated in recent years, both from electrical stimulation of the internal capsule and the tracing of fiber degeneration following lesions of the capsule, it was shown clearly that the conventional view is incorrect. In the human brain the pyramidal fibers are in fact situated in the posterior one-third of the thalamolentiform part of the posterior limb. In the region thus defined corticobulbar fibers are most anterior, followed in sequcnce by corticospinal fibers related to the upper limb, trunk and lower limb. However, there is considerable overlapping of fibers for the major regions of the body.

Corticopontine tract originate in widespread areas of cortex, but in greatest numbers in the frontal and parietal lobes. Fibers of the frontopontine tract traverse the anterior limb of the internal capsule and probably the genu and adjacent part of the posterior limb as well. The temporopontine tract is inappropriately named because the most fibers of it originate in the parietal lobe and traverse the retrolentiform part of the internal capsule.

The extrapyramidal motor fibers terminate in various subcortical centers depending on the destination, called corticostriate, corticorubral, corticonigral, corticoreticular and corticoolivary fibers. These extrapyramidal fibers orginate in widespread areas of the cerebral cortex, and they are therefore situated in various parts of the corona radiata and the internal capsule. The corticostriate and corticoreticular fibers, ending in the putamen and the reticular formation, are included in the external capsule as wel1 as the internal capsule. Extrapyramidal fibers destined for the red nucleus, substantia nigra and inferior olivary complex come mainly from the sensorimotor strip and adjoining cortex. Although their passage through the corona radiata and internal capsule has not been traced precisely many of these fibers are probably closely related to the pyramidal motor fibers topographically.

The basal nuclei are interconnected by many fibers. They are also connected to the cerebral cortex, thalamus, and hypothalamus(Fig. VI-2-50). The caudate nucleus and the putamen control large automatic movements of skeletal muscles, such as swinging the arms while walking. Such gross movements are also consciously controlled by the cerebral cortex. The globus pallidus is concerned with the regulation of muscle tone required for specific body movements.

 

Fig. VI-2-50The association of fibers of corpus striatum

An infarction in the posterior part of the thalamolentiform part of the internal capsule, spreading into the retrolentiform part, results in serious neurologic deficits. These include the defects of an upper motor neuron lesion because of interruption of pyramidal and extrapyramidal pathways, the general sensory deficits due to involvement of the thalamocortical projection to the somesthetic area, and a visual field defect because of interruption of geniculocaloarine fibers. Stated differently, the consequences of such a lesion are voluntary motor paralysis and impairment of general sensation on the opposite side of the body and blindness in the opposite visual field, the details vary according to the size and precise location of the lesion.

3. Lateral ventricles  They are roughly C-shaped cavities lined by ependymal epithelium, one in each cerebral hemisphere, and filled with cerebrospinal f1uid (figs. VI-2-49, 51, 52) . Each lateral ventricle consists of a body(,central part) in the region of the parietal lobe from which anterior, posterior and inferior horns (cornua) () extend into the frontal, occipital and temporal lobes, respectively. The lateral ventricles communicate with the third ventricle through the interventricular foramen (foramen of Monro). The body of the lateral ventricle, situated mainly in the parietal lobe, has a flat roof formed by the corpus callosum, its floor includes part of the dorsal surface of the thalamus. The anterior horn extends forward in the frontal lobe and the inferior horn extends to within 3-4 cm of the temporal pole(the anterior extremity of the temporal lobe). The floor of the inferior horn includes a particularly important structure, the hippocampus. The hippocampus may be visualized as a medial extension of the parahippocampus gyrus in the external surface.

 

 

Fig. VI-2-51 The superior view of lateral ventricles

 

 

Fig. VI-2-52 Projective diagram of the ventricular system of brain.

Portions of lateral ventricle contain choroids plexus(Fig. VI-2-51), a highly vascularized fringe of pia matter and ependyma, formed by an invagination of pia mater covered layer of ependyma on the medial surface of the cerebral hemisphere. This plexus presents in the body and the inferior horn of the lateral ventricle and is the main structure for production of cerebral spinal fluid (CSF).