The Cerebellum II: Cellular and Lobular Arrangement

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lobesThe lobules and fissures of the cerebellum are more easily understood if it is imagined that the surface of the cerebellum has been flattened as shown opposite. Using this representation, many of the areas of the cerebellum can be quickly and easily drawn schematically, and their relationship to other cerebellar structures understood.

                                           The medial vermal cerebellum has been subdivided into lobes running down the middle. These are, from dorsal to ventral, the lingula, culmen, declive, folium, tuber, pyramid, uvula, and nodule. The various lobular subdivisions and the main lobes and fissures are also distinguished using this view.

                                            Most of the cerebellar cortex is buried in the folia, and only about 15% is visible. In section, the cerebellar cortex is seen to be uniformly structured throughout, with three clearly defined layers that contain five different types of neurons. The cerebellar cortical layers are, from the surface inwards, the molecular, Purkinje cell (sometimes called piriform), and the granular layers. The medullary layer lies beneath the granular layer.

                                          The molecular layer is relatively sparsely populated with two types of nerve cells: basket cells and outer stellate cells. The axons and dendrites of the outer stellate cells do not leave the molecular layer, and neither do the dendrites of basket cells. These processes run roughly horizontally in the layer, transverse to the long axis of the depth of the folia or infolding. The basket cell bodies are close to those of the Purkinje cells in the next layer, and project fibers that form basket shapes around the cell bodies of the Purkinje cells. Below this layer is the relatively narrow Purkinje cell layer. The Purkinje cells are large Golgi type I neurons; their cell bodies lie in rows along the folia, and their axons project to the intracerebellar nuclei. Some of these Purkinje axons in the archicerebellum project to the brainstem vestibular nuclei. Purkinje dendrites proliferate densely, transverse to the plane of the folia. Immediately below is the relatively wide granular layer, whose cells are very tightly packed and send axons up into the molecular layer, where they branch in Tshapes and run as parallel fibers along the horizontal axis of the folia. Each Purkinje dendritic tree may form synapses with up to half a million parallel fibers that have projected up from the granular layer. Also in the granular layer is a relatively small population of inhibitory Golgi neurons, which project their dendritic trees up into the molecular layer. One Golgi cell may synapse with a row of ten to twelve Purkinje cells, and it appears that Golgi cells do not overlap with respect to the innervation of the Purkinje cells.                       

                                        TCerebellum here are two main types of afferent input to the cerebellum, and both are excitatory. Each Purkinje cell is supplied by one climbing fiber from the contralateral inferior olive . The phylogenetically more ancient archicerebellum and paleocerebellum are served by the correspondingly older accessory olivary nuclear cells. The neocerebellum is supplied with fibers by the newer inferior olive. The second afferent input is through the mossy fibers from many different sources, including the pontine nuclei. These fibers diverge extensively, and one mossy fiber may serve several folia. The mossy fiber axons form multiple rosettes, which synapse with several granular cell dendrites. Inhibitory Golgi axons synapse in these rosettes. It follows, therefore, that since mossy fiber rosettes synapse with granular fibers, which in turn synapse with Purkinje cells, that one mossy fiber can indirectly affect electrical activity in very many Purkinje cells.


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The hindbrain or rhombencephalon consists of the medulla (myelencephalon), pons (metencephalon), and the cerebellum as its largest structure. The cerebellum consists of two hemispheres joined medially by a relatively narrow vermis, sits in the posterior cranial fossa of the skull beneath the tentorium cerebelli, and is separated from the medulla and pons by the fourth ventricle. The cerebellar cortex has many curved transverse fissures in the form of narrow infoldings called folia. Structurally, the cerebellum is covered by a cortex of gray matter with a medulla of white matter, which holds four intrinsic pairs of nuclei (see below). Observation of the superior surface shows two deep transverse fissures, the primary and the posterior superior fissures. Viewed from the ventral surface, the cerebellum is  divided approximately into superior and  inferior halves by the horizontal fissure. Three pairs of cerebellar peduncles connect the cerebellum to the three lower brain segments. The inferior, middle, and superior cerebellar peduncles connect it to the medulla, pons, and midbrain, respectively.

The superior vermis lies between the hemispheres as a longitudinal ridge; it is more clearly differentiated visually from the hemispheres on the ventral surface, where it is divided by fissures into the nodule, uvula, and pyramid. A stalk extends from the nodule on each side to the flocculus, which forms the flocculonodular lobe. The tonsil is a lobule that lies over the inferior vermis. The inferior medullary velum is exposed if the tonsil is removed.

From an embryological and functional viewpoint, the cerebellum can be divided into three main parts. (i) The archicerebellum, or flocculonodular node, is made of the pairs of flocculi and their peduncular connections. The flocculonodular node is the most ancient part of the cerebellum, present in fish as well as humans, and is connected with the vestibular nuclei and system. It is connected particularly with the dentate nucleus, one of the intrinsic medullary cerebellar nuclei. (ii) The paleocerebellum, or anterior lobe of the cerebellum, lies dorsal to the primary fissure. The lobe also includes the pyramid and uvula of the inferior vermis. The anterior lobe receives inputs via the spinocerebellar tract, originating in stretch receptors, and is the lobe most involved in the control of involuntary muscle tone. This lobe is connected principally to the globose and emboliform nuclei, which project to the red nucleus, and thence to the central tegmental, rubroreticular, rubsospinal, and rubrobulbar efferent pathways. The paleocerebellum evolved in terrestrial vertebrates, which need to use limbs to support the body against the pull of gravity; therefore its connections are mainly spinal, and its functions are concerned with such stereotyped movements such as posture, locomotion, and muscle tone. (iii) The neocerebellum, which, as its name implies, is the phylogenetically newest part of the cerebellum, communicates with the thalamus and motor cortex. This lobe is made up of virtually all the posterior lobe, except for the pyramid and uvula of the vermis. The neocerebellum modulates non-stereotyped, learned behavior such as the learning of manual skills.Cerebellum

Fourth Ventricle

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Fourth VentricleThe fourth ventricle is an expansion of the central canal of the medulla oblongata. It is a roughly tent-shaped cavity filled with cerebrospinal fluid (CSF), situated beneath the cerebellum, above the pons and above the rostral half of the medulla. Laterally, the ventricle is bounded by the superior and inferior cerebellar peduncles.

                                The roof or dorsal surface of the fourth ventricle consists of a sheet of white nonnervous tissue called the inferior medullary velum. A layer of pia mater covers the inner lining or ependyma. Situated at the caudal part of the roof is an opening, the median eminence or foramen of Magendie, which connects the subarachnoid space and the interior of the ventricle. The ventricle communicates with the subarachnoid space also through two lateral openings called the foramen of Luschka . Situated caudally above the ventricular roof is a double layer of pia mater, called the tela choroidea, which lies between the cerebellum and the ventricular roof. The tela choroidea is highly vascularized, and its blood vessels project through the roof of the caudal part of the ventricle to form the choroid plexus. The choroid plexus together with others situated in the lateral and third ventricles produce the CSF.

                                           The floor or rhomboid fossa of the fourth ventricle is formed by the rostral half of the medulla and the dorsal surface of the pons, and is divided longitudinally into symmetrical halves by the median sulcus. The floor is raised because of the nucleus and the fiber tracts that run beneath it in the pons and medulla. Thus, there is a slight swelling in the floor, the facial colliculus, caused by the fibers leaving the motor nucleus of the facial nerve as they arch over the abducens nucleus.  Although not shown here, there are other nuclei situated immediately belowthe floor of the fourth ventricle. These include the vagal and hypoglossal nuclei, together with their fiber connections.

                            The ventricle is filled with fluid, and if it is overfilled, as can occur through abnormal production of CSF, hydrocephalus may result. This is a clinically significant increase in the volume of CSF, due to the obstruction of the foramina of the roof of the fourth ventricle, or to displacement of the medulla by a tumor, or adhesions of tissues through meningitis, or through the presence of a congenital septum. The increase in fluid increases pressure on the  nuclei and fiber tracts immediately below the floor of the fourth ventricle, and can  result in autonomic and motor disturbances such as cardiac, respiratory, and vasomotor problems.

                                                Tumors may arise in the ependymal lining of the fourth ventricle, or in the pons, or the vermis of the cerebellum and may spread to the fourth ventricle. Alternatively tumors of ependymal origin may invade the cerebellum, causing locomotor disturbances.brain ventricles


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The pons (metencephalon) lies beneath (anterior to) the cerebellum and is around 2.6 cm in length. The pons has been arbitrarily divided into the dorsal or posterior tegmentum, and a basal or anterior part, sometimes referred to as the pons proper.

Transection of the caudal pons at the level of the facial colliculi shows the fourth ventricle prominently, as well as  the middle cerebellar peduncles. (The term colliculus refers to the visible swellings caused by the mass of the nucleus.) The superior and inferior cerebellar peduncles and the nuclei and spinal tracts of several cranial nerves are also visible. The medial lemniscus runs at the base of the tegmentum, and above it the area occupied by the reticular formation is now much larger than that of the medulla. The trapezoid body consists of fibers from the cochlear nuclei and the nuclei of the trapezoid nucleus in the pons; these convey, for example, auditory information arriving in the pons. Ascending and descending fiber tracts, such as the corticospinal tractcourse through the pons.

The basal (anterior or ventral) portion of the pons consists of transverse and longitudinal bundles of fibers. The fibers constitute, mainly, a massive relay system from the cerebral cortex to the contralateral cerebellar cortex.

Dorsolateral to the reticular formation, lying in the floor of the fourth ventricle are the vestibular nuclei, which receive afferent inputs concerning equilibrium and balance and which are then well placed to be relayed to the cerebellum. The cerebellum in turn sends afferents from Purkinje cells to the vestibular nucleus; these are inhibitory, and release the neurotransmitter !-aminobutyric acid (GABA). The vestibular nuclei project efferent fibers to the middle ear.

The motor nucleus of the facial nerve innervates facial muscles, and its functionis clearly manifested when the facial nerve is damaged. This results in partial paralysis of the facial muscles (Bell’s palsy), and possibly autonomic disturbances. Transverse section through the pons higher up (rostrally) reveals similar structural features, except that the motor and sensory nuclei of the trigeminal nerve are now clearly visible. The principal sensory nucleus of the trigeminal nerve lies lateral to the motor nucleus, and its sensory incoming fibers lie laterally to the efferent fibers of the trigeminal nerve, which leave the trigeminal motor nucleus. The superior cerebellar peduncle is now more prominent, as is the lateral lemniscus, which runs dorsolateral to the medial lemniscus.

Damage to the pons results, typically, in muscle paralysis or weakness of structures innervated by cranial nerves. For example, a childhood tumor of the pons called astrocytoma of the pons, is the most prevalent  brainstem tumor, and causes a number of symptoms that reflect the paralysis of the ipsilateral cranial nerve; thus there may be weakness (hemiparesis) of facial muscles due to damage to the facial nucleus. The pons may be damaged by hemorrhage of the cerebellar arteries or of the basilar artery, and, depending on whether the damage is unilateral or bilateral, will result in facial paralysis and contralateral paralysis of lower limbs, through damage to corticospinal fibers which traverse the ventral pons.mid pons

Transverse Section of Medulla Oblongata

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Higher transection of the medulla oblongata at the level of the middle of the olivary nuclei clearly shows the fourth ventricle, the roof of which is formed by the choroid plexus in the inferior medullary velum at the base of the cerebellum. The floor of the ventricles is pushed up by the hypoglossal and dorsal vagal nuclei. The reticular formation, a network of nerve cells in the brain stem, is nowclearly visible, as are the major fiber tracts.

                                      The pyramids, medial lemnisci, and tectospinal tract lie medially in section. The tectospinal tract carries descending fibers from the tectum, which is the roof of the midbrain, consisting of superior and inferior colliculi. Also prominent is the inferior vestibular nucleus, which lies just medial to the inferior cerebellar peduncle.

                                             The most prominent feature of the transverse section at this level is the convoluted inferior olivary nucleus, which has a massive input to the cerebellum through the olivocerebellar tract which constitutes most of the inferior cerebellar peduncle. If it could be dissected entirely, the inferior olive would resemble a collapsed purse or bag. Axons of olivary cells leave the nucleus and decussate to the other side of the medulla and sweep up into the peduncle. The fibers radiate to virtually all parts of the cerebellum and many have an excitable effect on cerebellar Purkinje cells. The inferior olivary complex has been divided into the principal, medial accessory and dorsal accessory olivary nuclei, based mainly on their cerebellar connections. For example, the fibers arising from the medial portion of the principal nucleus and those from the accessory nuclei terminate mainly in the vermis of the cerebellum.

                                            The olive receives descending corticoolivary fibers from the occipital, parietal, and temporal cortex, which terminate bilaterallymainly in the principal olivary nucleus. The principal olive also receives rubro-olivary fibers from the red nucleus, and fibers in the central tegmental tract from the periaqueductal gray matter in the midbrain, some of which also terminate in the medial accessory nuclei. The dorsal and medial accessory olives receive ascending fibers in the spino-olivary tract, which runs up the cord in the anterior (ventral) funiculus of the white matter.

                                           There are other nuclei at this level. The nucleus ambiguus is a longitudinal column of nerve cells within the reticular formation, extending through the medulla from the medial lemniscus to the midrostral portion of the inferior olive. The cells are multipolar motoneurons, and the efferents from this nucleus arch upward to join efferents from the dorsal vagal nucleus and from the nucleus of the tractus solitarius. Efferents from the rostral part of the nucleus ambiguus become visceral efferents of the lossopharyngeal nerve,which innervate the stylopharyngeus muscle. The more caudal portion of the nucleus gives rise to fibers of the spinal accessory nerve.The nucleus of the tractus solitarius gives rise to fibers, which, among other destinations, target the hypothalamic nuclei which release the peptide vasopressin. The reticular formation contains several important raphe nuclei which extend in the pons, and which project 5-HT neuronal processes to the midbrain, diencephalon and cerebral cortex. These central gray projections appear to mediate rhythmic processes such as arousal.

Medulla Oblongata

Medulla Oblongata

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Medulla Oblongata

The spinal cord becomes the medulla oblongata, which also contains white and gray matter, but the arrangement hanges, due to the embryonic expansion of the central canal to form the hindbrain vesicle, which will become the fourth ventricle. Development of the ventricle pushes dorsally situated structures more dorsolaterally. The transition is clearly seen in transverse section. The spinal cord becomes the medulla, which initially resembles the upper cervical segments. The substantia gelatinosa is now much larger in size and has become the spinal nucleus of the trigeminal nerve. In transverse section, descending fibers of the spinal trigeminal tract can be seen immediately dorsolateral to the nucleus. There is an increase in the amount of gray matter surrounding the central canal.

                                       At low medullary level, the most prominent sign of transition to medulla is the appearance of the decussation at the pyramids. This is where the descending corticospinal motor tracts cross over. These fibers cross ventral (anterior) to the central gray matter and project dorsolaterally across the base of the ventral horn of the medulla. The pyramidal decussation almost eliminates the spinal anterior median fissure. (In the human, approximately 90% of the descending corticospinal fibers decussate and descend the cord in the lateral corticospinal tract, while about 10% do not cross, and descend in the uncrossed lateral and ventral corticospinal tracts.) The decussation explains the contralateral control of body movements by the motor cortex. At this level can also be seen the tracts of the gracile and cuneate fasciculi, which are the CNS projections of the cells of the spinal ganglia, and the lower ends of the gracile and cuneate nuclei where they terminate. At this level are also the cut fibers of the ascending ventral (anteriorand lateral spinocerebellar tracts, which carry information from the sense organs in tendons and muscle spindles, the inferior olivary nucleus, and the spinal root of the accessory nerve.

                                       Transaction at a higher level of the medulla (B) reveals another prominent decussation, that of the medial emniscus. This is where fiber tracts from the ascending gracile and cuneate nuclei cross the midline of the medulla on their way up to higher centers. The nuclei are complex and arranged to correspond topographically with the body areas from which the ascending 0fibers come. Ascending fibers from the nuclei curve round the central gray matter and decussate to form the medial lemniscus. At this level, the spinal nucleus of the trigeminal nerve, which innervates the head region, is prominent, and immediately dorsolateral to it are the fibers of the descending trigeminal nerve. At both levels, the ascending spinocerebellar and spinothalamic tracts are both visible, and in B the medial accessory olivary nucleus lies medial to these tracts.

                                              In summary, the transition from spinal cord to medulla is marked by (i) the expansion of the central canal; (ii) decussation at the pyramids; (iii) formation of the medial lemniscus through the decussation of ascending fibers arising from the cuneate and gracile nuclei; (iv) dorsolateral displacement of the dorsal horn of gray matter; (v) appearance of cranial nerve nuclei and various relay nuclei projecting to the cerebellum.

Medulla Oblongata

Dorsal View

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The dorsal surface of the brain stem, and particularly that of the medulla and pons, is obscured by the cerebellum. When this is removed, the bilateral swellings caused by the ascending cuneate and gracile fasciculi can be seen, as well as the corresponding tubercles, which are the swellings caused by their nuclei. Dorsal to the olives are the inferior cerebellar peduncles, which climb to the lateral aspect of the fourth ventricle and then swing into the cerebellum between the middle and superior cerebellar peduncles. The inferior cerebellar peduncle receives fibers in the stria medullaris, a tract from the hypothalamic arcuate nucleus. The stria medullaris fibers pass dorsally through the midline of the medulla and cross the floor of the fourth ventricle.

                                             The floor of the fourth ventricle (also called the rhomboid fossa) is in part the dorsal surface of the pons; the dorsal surface of the pons (also called the tegmentum of the pons) forms the rostral half of the floor of the ventricle, and is divided longitudinally by a medial sulcus into two symmetrical halves. The ventricle is broadin the middle and narrows caudally to the obex, the most caudal end of the fourth ventricle, and rostrally towards the aqueduct of the midbrain. Caudally, the ventricle narrows into two triangles or trigones. Beneath the medial area of the ventricle are several motor nuclei; the rostral ends of both the vagal and hypoglossal nuclei lie beneath these trigones. There is a swelling at the lower end of the medial eminence, the facial colliculus, which is formed by fibers from the motor nucleus of the facial nerve. The roof of the fourth ventricle is tent-shaped and extends upwards towards the cerebellum. The roof is formed rostrally by the superior cerebellar peduncles and by a sheathcalled the superior medullary velum. The rest of the roof consists of another sheath, the inferior medullary velum, which is often found adhering to the underside of the cerebellum. The sheath may be incomplete, creating a gap called the median aperture of the fourth ventricle or the foramen of Magendie, which constitutes the main communication between the ventricular system and the subarachnoid space. The lateral walls of the fourth ventriclesare provided mainly by the inferior cerebellar peduncles. There are recesses in the lateral walls, which extend around the medulla, and these open ventrally as the foramina of Luschka, through which cerebrospinal fluid can enter the subarachnoid space.

                                            The dorsal surface of the midbrain is defined by four rounded swellings: the superior and inferior colliculi (the corpora quadrigemina). The colliculi make up the roof or tectum, and define the length of the dorsal surface, round 1.5 cm. The inferior colliculus is mainly a relay nucleus in the transmission of auditory impulses en route to the thalamus and cerebral cortex. The superior colliculus mediates control of voluntary eye movements and the head in response to visual and other forms of stimuli. The lateral surface of the midbrain is formed principally by the cerebral peduncle. Parts of the epithalamus (see, the habenular nuclei and the stria medullaris are seen ostral to the midbrain. The third ventricle of the diencephalon and the pineal body are also shown.


Brain Stem

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Brain Stem

The brain stem consists of the medulla oblongata (or medulla), the pons and the midbrain. The three brain areas each contain cranial nerve nuclei, and the fourth ventricle lies partly in the pons and partly in the medulla. The brain stem may occasionally be referred to as the ‘bulb’ in such terms as the ‘corticobulbar’ tract.

The medulla is around 3 cm long in adult humans and widens rostrally. It is continuous with the spinal cord from just belowthe foramen magnum, at the level of the upper rootlet of the first cranial nerve, and extends through to the lower  (caudal) border of the pons. The medulla lies on the basilar part of the occipital bone, and is obscured from view by the cerebellum. Externally, the spinal cord and medulla appear to merge imperceptibly, but internal examination reveals extensive reorganization of white and gray matter at the junction. In the medulla the central canal widens into the fourth ventricle.

From the ventral aspect, the central median fissure appears as a central groove, which is a continuation of that of the spinal cord. The progress of the fissure is interrupted by the decussation (crossing over) of the fiber tracts of the corticospinal tract, where they cross over at the pyramid of the medulla to form the lateral corticospinal tract. Lateral to the pyramids on each side is the olive, made up of a convoluted mass of gray matter called the inferior olivary nucleus. The olive is separated from the pyramids by the rootlets of the hypoglossal nerve (XII). Rootlets of the vagus (X) and the cranial accessory (XI) nerves arise lateral to the olive, the latter two being united with the spinal accessory nerve (XI). The facial (VII) and vestibulocochlear (VIII) nerves arise at the border between the lateral medulla and the pons.

The pons is about 2.5 cm in length. Its name is Latin for ‘bridge’, since it appears to connect the cerebellar emispheres though this is not actually the case. Ventrally, the pons is a sort of relay station, where cerebral cortex fibers terminate ipsilaterally on pontine nuclei, whose axons become the contralateral middle cerebellar peduncles. Thus the ventral (or basal) pons is a sort of massive synaptic junction that connects each cerebral hemisphere with the contralateral cerebellar hemisphere. Functionally, this system maximizes efficiency of voluntary movement.

The ventral surface of the midbrain extends rostrally from the pons to the mamillary bodies, which mark the caudal  border of the diencephalon. On either side are prominent swellings called the crus cerebri (basis pedunculi). These are made up of the fiber tracts of the descending pyramidal motor system, and fibers from the cortex to the pons (corticopontine fibers). Although not shown here, the midbrain is penetrated by several small blood vessels in the floor of the interpeduncular  fossa, and the area has been named the posterior perforated substance because of these blood vessels. The oculomotor nerve (III) to the eye leaves the brain through the cavernous venous sinus from each side of the interpeduncular fossa. The optic chiasm and optic nerves, together with the diencephalic tuber cinereum are exposed on the ventral surface. brain stem

Gray Matter

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Gray Matter

Gray Matter

The gray matter of the cord is butterfly shaped, with the so-called dorsal (posterior) horns forming the upper wings of the butterfly shape. These are linked by a thin gray commissure in which lies the central canal. In the thoracic and upper lumbar segments the gray matter extends on both sides to form lateral horns. The lower wings of the butterfly shape are formed by the ventral (anterior) horns of the gray matter. (The size of the gray matter is greatest at egments that innervate the most skeletal muscle. These are the cervical and lumbosacral, which innervate upper and lower limb muscles, respectively.)

Structurally, the gray matter is composed of neuronal cell nuclei, their processes, neuroglia  and blood vessels. The overall arrangement of the gray matter of the cord was systematized by Rexed, who proposed the generally accepted laminar arrangement, commonly referred to as the cytoarchitectonic organization of the spinal cord. The gray matter is divided arbitrarily into nine visually distinct laminae, labeled I through IX, and an area X, which surrounds the central canal. Most laminae are present throughout the cord, but VI, for example, is apparently absent from T4 to L2.



Lamina I is at the apex of the dorsal horn, and contains the posterior marginal nucleus. These cells respond to thermal and other noxious stimuli, and receive axosomatic connections from lamina II. Near the apex, in lamina II, is the substantia gelatinosa, which is found throughout the length of the cord, and which receives touch, temperature and pain afferents, as well as inputs from descending fibers. Both I and II are rich in substance P, considered to be an excitatory neurotransmitter of pain impulses, in opioid receptors and the enkephalin.



Ventral to the substantia gelatinosa, extending through III and IV, is the largest dorsal horn nucleus, the nucleus proprius, which also exists at all cord levels. This receives inputs concerning movement, position, vibration and two-point discrimination from the dorsal white column. The nucleus reticularis is present in the broad lamina V, which is divided into medial and lateral zones, except in thoracic segments. Lamina VI, seen only at cord enlargements, receives group I muscle afferents in its medial zone, and descending spinal terminations in its lateral zone. Lamina VII contains the nucleus dorsalis of Clark (Clark’s column), a group of relatively large multipolar or oval nerve cells that extends from C8 through L3 or L4. Most of the cells respond to stimulation of muscle and tendon spindles. Layer VIII is a zone of heterogeneous cells most prominent from T1 through L2 or L3, associated with autonomic function.

Lamina IX is situated in the anterior or ventral horn of the gray matter, and contains clusters of large, motor nerve cells. The larger cells send out a efferent motoneuron axons, which innervate the extrafusal skeletal muscle fibers, while smaller cells send out g motoneuron axons, which innervate the intrafusal spindle fibers.



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The nervous system consists of two main divisions: TheCentral Nervous System (CNS), consisting of brain and spinal cord,and the peripheral nervous system, consisting of cranial and spinal nerves, and their associated gangliaThree membranes surround both spinal cord and brain: dura mater, arachnoid mater, and pia mater. The dura mater is a tough, fibrous coat that encloses the spinal column and cauda equina, which is abundle of nerve roots from the lumbar, sacral and coccygeal spinal nerves. The dura mater runs rostrally and is continuous beyond the foramen magnum with the dural meninges, which cover the brain. Caudally, the dura ends on the filum terminale at the level of the lower end of the second sacral vertebra. The dura is separated from the walls of the vertebral canal by the extradural space, which contains the internal vertebral venous plexus. The dura extends along the nerve roots and is continuous with the connective tissue that surrounds the spinal nerves. The inner surface of the dura is in direct contact with the arachnoid mater.

                                 The arachnoid mater is a relatively fragile, impermeable layer that covers the spinal cord, the brain and spinal nerve roots, and is separated from the pia by the wide subarachnoid space, which is filled with cerebrospinal fluid. The pia mater is a highly vascularized membrane closely apposed to the spinal cord. It thickens on each side between the nerve roots to form lateral supports, anchored to the arachnoid, which suspend the spinal cord securely in the center of the dural sheath.

                               The spinal cord is an approximately cylindrical column, continuous with the medulla oblongata, that extends in adults from the foramen magnum to the lower border of the first lumbar vertebra. Structurally, the cord contains central gray matter, roughly H-shaped, consisting of the anterior and posterior horns and joined by a thin commissure containing the central canal, which is connected to the fourth ventricle. The gray matter is surrounded by white matter, which consists mainly of ascending and descending tracts, and has been divided arbitrarily into anterior, lateral, and posterior columns. The individual tracts will be dealt with in more detail later.

                               In the peripheral nervous system, there 12 pairs of cranial nerves, which leave the brain through foramina (apertures) in the skull, and 31 pairs of spinal nerves, which leave the spinal cord through vertebral foramina. There are eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal pair of spinal nerves. The spinal nerves are linked to the cord by dorsal (posterior) nerve roots, which carry afferent
nerves into the CNS, and ventral (anterior) nerve roots, which carry efferent nerves away from the CNS. Afferent fibers are also called sensory fibers, and their cell bodies are situated in the swellings or ganglia on the dorsal roots.