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The Huilan Nervous Systen: An Andlomical Viewoint, Sinh Ediliol,Mlnay L. Bd and John A. Kiemm. J.B. Lippincott Compant Philadelphia, @ 1993. Ten .lons oeurory. tion l0: ER, sof rrk, ctal t t- lor- sG r3br- :nlund u,ea u92 ois ;ci te 2, ls ls ,Y )- The hemispheres, vermis, flocculus, nodule, and tonsil are major landmarks of the cerebellar corte:<. Afferent fibers end in the three-layered cerebellar cortex. The Purkinje cells have axons that end in the cerebellar nuclei. The fastigial, interposed, and dentate nuclei receive branches of all cerebellar afferent fibers and the output of the cor'tex. These nuclei contain the cerebellar efferent neurons. The superior cerebellar peduncle contains cerebellar efferent fibers and the ventral spinocerebellar tract. The middle cerebellar peduncle consists of fibers from the contralateral pontine nuclei, and the inferior cerebellar peduncle contains olivocerebellar and dorsal spinocerebellar fibers and the vestibulocerebellar and fastigiobulbar connections' The vestibular system is connected ipsilaterally with the vestibulocerebellum, which comprises the flocculonodular lobe and the fastigial nucleus. This nucleus projects to the ipsilateral vestibular nuclei and to the reticular formation. hoprioceptive signals are carried ipsilaterally to the spinocerebellum, which consists of vermis, paravermal zones, and interposed nuclei' These nuclei project to the contralateral ventrolateral (VL) thalamic nucleus. The VL projects to the primary motor corto(. The cerebral cortex influences the contralateral cerebellar hemisphere and dentate nucleus (pontocerebellum) by way of a relay in the pontine nuclei, The dentate nucleus projects to the contralateral VL thalamic nucleus. These connections determine that each side of the body is represented ipsilaterally in the cerebellum and that postural functions are localized in and near the midline, The cerebellum learns and executes instructions for movements, ensuring coordination of the force, extent, and duration of the contractions of muscles. A lesion in or near the midline cause disorders of posture and gait, whereas a lesion in a hemisphere causes defective control of movements of the ipsilateral limbs. 163 164 Reglonal Aflotorny of the Central Neruous system Although the cerebellum has an abundant input from sensory receptors, it is essentially a motor part of the brain, functioning in the maintenance of equilibrium and in the coordination of muscle contractions. The cerebellum makes a special contribution to synergy of muscle action (ie, to the synchronized contractions and relaxations of different muscles that make up a useful movement). The cerebellum ensures that there is contraction of the proper muscles at the appropriate time, each with the correct force. There is reason to believe that the cerebellum participates in the of patterns of neuronal activity needed for carrying out movements and also leaming in the execution of these encoded instructions. The cerebellum increased in size in the course of vertebrate evolution, The large size in the human brain coincides with the need for synergy of muscles, especially for maintenance of the erect posture and in leamed activities that require precisely orchestrated hand move_ ments. Despite their complexity, the activities of the cerebellum have long been thought to occur without any conscious awareness. This traditional viewpoint may not be entirely correct: imagined movements are accompanied by an increase in cerebellar blood flow that is larger than the increase detected in the motor areas of the cerebral cortex. Damage to the cerebellum causes disturbances of motor func- tion without voluntary paralysis. The cerebellum consists of a cortex, or surface layer, of gray matter contained in transverse folds or folia, a medullary center of white matter, and four pairs of central nuclei embedded in the medullary center. Three pairs of cerebellar peduncles, composed of nerve fibers, connect the cerebellum with the brain stem. oss Anatomy The superior cerebellar surface is elevated in the midline, conforming to the dural reflection or tentorium that forms a roof for the posterior cranial fossa. The inferior surface is deeply grooved in the midline; the remainder of this surface is convex on each side and rests on the floor ofthe posterior cranial fossa (Fig. I0-I1. Certain terms are useful to identify regions of the cerebellar surface. The region in and near the midline is known as the vermis and the remainder, as the hemispheres. The superior vermis is not demarcated from the hemispheres, but the inferior vermis lies in a deep depression (the vallecula) and is well delineated. The term paravermal zone is used for the medial parts of the hemispheres for I to 2 cm on either side of the vermis. Three major regions, the flocculonodular, anterior, and posterior lobes, are recognized in the horizontal plane (see Fig. I0-l). The flocculonodular lobe (or lobule) is a small component, the oldest phylogenetically, that lies at the rostral edge of the inferior surface. The nodule is the rostral portion of the inferior vermis, and the flocculi are irregularly shaped masses on each side. The cerebellum is deeply indented by several transverse fissures. The dorsolateral fissure (also called the posterolateral fissure) along the caudal border of the flocculonodular lobe is the firsr of these to appear during erhbryonic development. The main mass of the cerebellum (all but the flocculonodular lobe) consists of dnterior and posterior lobes. The anterior lobe is that part of the superior surface rosfial to the primary fissure,I The remainder of the cerebellum onbdth surfaces constitutes the poste- rior lobe. The roof of the rostral part of the fourth ventricle is formed by the superior cerebellar peduncles and by the superior medullary velum that bridges the interval between thern (Fig. I0-2; see also Fig. 7-10). The remainder of the roof consists of the thin inferior medullary velum, formed by pia mater and ependyma. This membrane, in which a deficiency constitutes the median apefture of the fourth ventricle or foramen of Magendie (see Fig. I This, despite its name, is the second fissure to appear during embryonic development. Chapter 10: Cerebellum f65 tthis r the )-t). fons and and 'su- Hemisphere the ina vell :is lres tar, red he Nodule all lat Flocculus I I Flo..ulo nodular lobe :e. le- Dorsolateral fissure 1y m Posterior lobe S. :d al Hemisphere st )- il :B is e Figure lo'1. The cerebellum. (A) superior surface. (B) Inferior ( x 0.66) surface. h I 6-4), commonly adheres to the inferiorvermis. The three pairs of peduncles are attached to the 1 cerebellum r in the interval between the floc- culonodular and anterior lobes. Other fissures outline further subdivisions or lobules, especially in the posterior lobe. The names given to these lobules by ear\ anatomists have no functional significance; neither is there uniform acceptance of a single system of nomenclature, Figure I0-3 is provided for reference, if smaller subdivisions of the cerebellum need to be identified. The position of the tonsils is clinically significant because these parts of the cerebellar hemispheres are close to the medulla and can compress this vital paft of the brain stem if the contents of the posterior fossa of the skull are displaced downward into the foramen magnum. The tonsil is also an angiographic landmark, imparting a characteristic curve to the course of the posterior inferior cerebellar artery. Three functional divisions of the cerebellum are recognized, based on the destinations of different categories of afferent fibers. Cortical histology is uniform throughout the cerebellum, unlike the cerebral cortex, in ...'n + Anterior lobe Lingula of vermis showing through the superior medullary velum w Superior peduncle TI th rh Middle peduncle Nodule Flocculus I J Flocculo- nodular th c( lobe Inferior peduncle c Dorsolateral fissure B la 8: Posterior lobe is Figure lo-2. The cerebeilum as viewed from in front and below, showing the cut surfaces of the cerebellar pedunct"r, n i O"O'OI VERMIS H EMISPHERE Central lobule Anterior part of quadrangular lobule Posterior part of quadrangular lobule (simple lobule) Culmen Superior semilunar lobule (crus I of ansiform lobule) Declive Folium Inferior semilunar lobule (crus ll of ansiform lobule) Biventral lobule Superior semilunar lobule (crus I of ansiform lobule) Inferior semilunar lobule (crus ll of ansiform lobule) 6.9.i5 Floccu lonodular Nl li.iiir:.i'.ljl lobe [-_l Anterior lobe Posterior lobe l-names of parts of the cerebellum. (The drawings, is a small, flattened portion oi the e central lobule and adherent to the superior 166 10_2). rt br c T R fc o; Chapter 10: Cerebellum which there are histologically different areas. The four central nuclei are likewise similar ar the cellular level. The cortex and nuclei are, therefore, described at this point, after which the functional divisions and their individual connections are discussed. Cerebellar Cort (Fig. l0-4). The nje cell layer consists of a single row ( sections) of bodies of purkinje cells, are the principal cells of the cerebellar cor The molecular layer contains some but is largely a synaptic zone, m up of profusely branching dendrites of cells and the axons of the term molecular is derived Because of the extensive folding of the cerebel- lar Surface in the form of thin transverse folia, 85olo of the coftical surface is concealed. There is, therefore, alatge cortical area, It is aboul three-quarters as extensive as that of the cere- bral cortex. neurons, of which the most abundant are the granule cells. The axons of these small inter_ neurons extend into the molecular laver. Of the afferent fibers to the cortex, mossy fibers terminate Cortical Layers Three ldyers are seen in histological sections. hom the surface to the white matter of the folium, these are the molecular layer, the layer of Purkirle cells, and the granule cell layer in synaptic contact with granule cells of the innermost layer, whereas ctimbing fibers enter the molecular laver and wind among the dendrites of purkinje ceils. the only libers that leave 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 Molecular layer Purkinje cell layer Cranule cell layer White matter of folium Figure l0'4. Transverse section of cerebellar folia showing the three layers of the cortex and the underlying white matter. (stained-with ciesyl violet, x 35) floc- 167 f68 Regional Anatomy ofthe Centtal Nervous culonodular lobe that proceed stem. System to the brain re contacted by mossy d axon enters the mofurcates and runs paral- Cytoarchitecture The five neuren types in the cerebellar cortex establish a complex but remarkably regular pattem of intracortical circuits. The precise three-dimensional orientation of dendrites and axons, as shown by the Golgi staining method and by electron microscopy, has en_ at ois indicated in Figure lO-5. Granule Celb and Mossg Fibers The granule cells are small and closely packed together in the deepest cortical layer. Each cell has a spherical nucleuswith a coaise chromatin pattern, and the scanty cytoplasm lacks clumps ot Nissl substance. The short dendrites have the cerebellum are mossy fibers that terminate in synaptic relation wi cells. While still in the fiber dMdes into sever enter the corto( the granule celll sheath and there Alon end, as rosettes, with whic pakg syn.ap_nc contact. t* #0fi:"i.;5;?l: i tion that includes the rosette of mossy"nUeq {e1d1ite9 of granute cells, and tne axcin oia qolgi cell (see Stellate and Golgi Cells) is known Ba-basket cetl Go-Golgi cetl Purkinje cell layer Interneurons Gr-granule cell P-Purkinje Granule cell layer Climbing fiber To a central nucleus of the cerebellum + Excitatory synapse - Inhibitory synapse Mossy fiber he cerebellar corto(, showing ercitatory and inhibitory from Kieman JA: Introduction to Human Neuroscience, 19871 Chapter 10: Cerebellum 169 Cranule cet I dendrite Astrocyte cytoplas,m Axonal rminal of Colgi cell Mossy fiber rosette Figure 10-6. Ultrastructure of a synaptic glomerulus in the granule cell layer. (axonal terminal) Mossy fiber released by the presynaptic boutons, therebv preventing diffusion to nearby glomeruli. Purki4je Cells and Climbing Fbers There a hey are cell easi bodies. c is profuse dendritic branching in the molecular layer, in a plane transverse to the folium. The Basket Cells The basket cells are scattered in the molecular layer near the bodies of furknje cells, The den_ drite of a basket cell branchesin the transverse granule cells and the transverse orientation of pect to a for a Pure number ule cell to contact many furkinje cells. The molecular layer is, therefore, a rich synaptic field to which stellate, basket, and Qolgi cells also contribute. Axons of Purkinje cells traverse the granule cell layer, acquire myelin sheaths, and terminate mainly in central cerebellar nuclei. Collateral branches given off by the axons synapse with adjacent furkinje cells and with Golgi cells in the outer part of the granule cell layer (see Fig, 10-5): As the mossy fibers have a special relation- Stellate and Golgi CeUs Granule, furkinje, and basket cells have special features, whereas stellate and Golgi celis are similar to small neurons elsewhereln the nervous system, lIl .i rl J rl .l '] 170 Regional Anatomy of the Centd Neruous Srstem potential potential mented macolog citatory transmitter is probably glutamate. All the other cerebellar interneurons make inhibitory synapses, with gamma-aminobutyric acid (CABA) as the probable transmitter. The excita- tory input to the intracortical circ and therefore su Figure l0-7. Cell body of a furkinje cellsituated between the molecular layer (aboue) and the granule cell layer of the cerebellar cortex. Most of the fibers surrounding the furkinje cell body are preterminal branches of basket cell €xons. (Cajal's silver nitrate method, x 450) There are scattered stellate cells in the superficial part of the molecular laver whose dendrites are contacted by axons of granule cells. Axons of stellate cells synapse mainly with furkinje cell dendrites; a few enter the innermost cortical layer and establish a feedback circuit bv synapsingwith granule cells. The Colgicells arb situated in the outer portion of the granule cell layer, and their dendrites extend into the molecular layer, where they are contacted by parallel fibers. Other afferents to Golgi cells are collateral branches of furkinie cell axons, Axons of Colgicells enter glomeruli, where they synapse with the dendrites of granule cells (see Fig. 10-6). I ntracortical Circuits Recordings from microelectrodes inserted into the cerebellar cortex have yielded information about whether synapses between specific types of neuron produce an excitatory postsyniptic and the degree of er<citation resulting from an incoming volley. Aminergic Ftbers Large numbers of noradrenergic axons enter the cerebellum through its superior peduncle. These unmyelinated fibers, which come from the locus coeruleus, end bybranching profusely in the molecular layer (see Fig. 1O-5). The noradrenaline released from these afferent fibers may have a modulatory action at the synapses between parallel fibers and furkinie cells. The cerebellar cortqr also contains ro-e ,erotonergic axons from the raphe nuclei of the n (see Ch. 9). lt has been sug- drenaline and serotonin mav ctions, the former amine enhancing and the latter reducing the excitatory action of glutamate on the dendrites of furkinie cells. Central Nuc]ei Four pairs of rruclei are embedded deep in the medullary center; in a medial to lateral direction, they are the fastigial, globose, emboliform, and dentare nuclei (Fig. l0-S). Chapter 10: Cerebellum Fastigial nucleus aptic )ple,har- and eby the lory. )ses exAJI ribi- rcid :ita- by llls, iorers .\\= :t'/ rlls, :lls ,lgi )ry )(to tje he _rd (-/- -.' o';o--:: \ .- V Figure l0-8. Central nucleiof the cerebellum. .,- tn The er e. n ly fastigial nucleus is nearly spherical, close.to the midline, and almost in contact with the roof of the fourth ventricle. The globose nucleus consists of two or three small cellular masses, and the larger emboliform 's nucleus is oval or plug-shaped. In mammals as high in the phylogenetic scale as the mon- e key, a single nucleus (the nucleus interpositus A the fastigial and dentate nuclei. In apes and B or interposed nucleus) is situated between humans, the interposed nucleus is represented by the globose and emboliform nuclei. The dentate nucleus is the most prominent of the central nuclei; this mammalian nucleus is Iargest in primates, especially in humans, The dentate nucleus has the irregular shape of a crumpled purse, similar to that of the inferior olivary nucleus, with the hilus facing medially. Its efferent fibers occupy the interior of the nucleus and leave through the hilus. The input to the cerebellar nuclei is from (a) sources outside the cerebellum and (b) the Purkinje cells of the cortex, The extrinsic input consists of pontocerebellar, spinocerebellar, and olivocerebellar fibers, together with fibers from the precerebellar reticular nuclei. Most of these ffirents are collateral branches of fibers proto the cerebellar cortu. A few rubrocerebellar fibers end in the globose and emboliform nuclei, and the fastigial nucleus receives afferents from the vestibular nerve and nuclei. The fastigial nucleus discharges to the brain stem through the inferior cerebellar peduncle, whereas efferents from the other nuclei leave the cerebellum through the superior peduncle ceeding and end in the brain stem and in the thalamus. Results of physiological studies have indicated that the input to the central nuclei from outside the cerebellum is excitatory, whereas the input from Purkinje cells, which use GABA as their transmitter substance, is inhibitorv. Crudely processed information in the .ent.il nuclei is re{ined by impulses received from the cortex. The combination of the two inputs maintains a tonic discharge from the central nuclei to the brain stem and thalamus. This l7l Llz Regional Anatomy ofthe Central Nerlous System Figure l0-9. Midline structures of the brain stem and cerebellum, showing the arb.or u.itae cerebelli in the vermis. The cut surface of the spe-imen has been 9$i1ed by a method that differentiates gray matter (dark'iand white matter (ughA. (x u. 1.5) al rl t( discharge changes constantly according to the afferent input to the cerebellum at any given time. at this point only as components of the cere- bellar peduncles. The inferior cerebellar peduncle con- sists mainly of fibers entering the cerebellum, ;1r i:: J The white matter is scanty in tl^e region of the vermis, where it produces a branching treelike pattern (the arbor vitae cerebelli) in a sagittal section (Fig. l0-9), Each hemisphere con- tains a large medullary center in which the dentate nucleus is embedded (Fig, I0-I0). This white matter consists of afferent fibers to the cortex, axons ofPurkinje cells proceeding to the central nuclei, and efferent fibers of the nuclei. The afferent and efferent systems are discussed in connection with the functional divisions of the cerebellum, They are identified with the largest contingent being of those that originate in the contralateral inferior olivarv complex of nuclei. The other components are the dorsal spinocerebellar tract, cuneocerebellar fibers, fibers from the vestibular nerve and nuclei, the arcuate nucleus,2 the nucleus of the spinal trigeminal tract, the pontine trigeminal nucleus, and the raphe and precerebellar retic- g. p si tI p fr n c d o fl e 2 The arcuate nucleus in the ventral part of the medulla was briefly discussed in Chapter 7. It has afferent and efferent connections similar to those of the pontine nuclei and is probably a caudally displaced part ofthis neuronal population. I 1 T Chapter l0: Cerebellum Li) Figure l0-I0. cerebellar surface in a sagittal plane through a hemi,p"h"r", stained to differentiate gray matter.(dark) and white matter in the white matter of it'igirt).'fn" dentate nucleus is shown, embedded the hemisphere. (x 1'5) I ular nuclei. The inferior cerebellar peduncle also contains efferent fibers that proceed from the flocculonodular lobe and fastigial nucleus to the vestibular nuclei and to the central group of reticular nuclei of the medulla and DONS. The middle cerebellar peduncle con- in cerebellar superior The the pontine nuclei, peduncle consists mainly of efferent fibers irom the globose, emboliform, and dentate sists of pontocerebellar fibers that originate nuclei. The other fibers in the superior peduncles are afferent to the cerebellum' They include the ventral spinocerebellar tract on the dorsolateral surface ofthe peduncle, fibers that originate in the locus coeruleus, a few fibers from the red nucleus, and fibers from the mesencephalic nucleus of the trigeminal nerve' natfionlall A'ntatclnraY Three divisions of the cerebellum are recognized on the basis of phylogeny (Fig' l0- I I ) ' The archicerebellum, which is the oniy component of the cerebellum in fishes and in Iower amphibians, consists of the flocculonodular lobe, together with a region of the inferior vermis known as the uvula (see Fig' l0-3). The paleocerebellum makes its flrst appearance in higher amphibians and is Iarger in reptiles and birds. In humans, it is represented by the superior vermis in the anterior lobe and by part of the inferior vermis in the posterior Iobe. The cerebellar hemispheres, together with the superior vermis in the poste- rior lobe, constitute the neocerebellum, which is found only in mammals and is largest in humans. These phylogenetic divisions of the cerebellum correspond in Iarge part with divisions based on the major sources of afferent {ibers (Fig, l0- l2). Thus the archicerebellumis identical to the vestibulocerebellum, which receives input from the vestibular nerve and nu- clei. Those parts of the vermis that constitute the paleocerebellum, together with the ad- 174 Regional Anatomy of the Central Neruous System ! Nl Archicerebellum l-l l0-ll. Paleocerebellum ! v"rtinrtocerebellum l--l Neocerebellum N Spino..rebellum Pontocerebellum Phylogenetic regions of the cer- Figure 10-f2. Functional regions of the cere- ebellum. (A) Superior surface. (B) Inferior sur- bellum. (A) Superior surface. (B) lnferior sur- face. face. jacent (neocerebellar) medial or paravermal zones of the hemispheres, make up the spinocerebellum. This is the site of termination of afferent fibers from these sources terminade in the fastigial nucleus, which also receives collateral branchei of the axons destined for the the spinocerebellar tracts and cuneocerebellar fibers, which convey proprioceptive and other sensory information, The remainder of the cortex of the vestibulocerebellum. The cortex and nucleus also receive afferents from the accessory olivary nuclei, Some Purkinje cell axons from the vestibulocerebellar cortex proceed to.the brain stem (an exception to the general rule.that such {ibers end in central nuclei), but most, terminate in the fastigial nucleus. Fibers from the cortex and the fastigial nucleus traverse the medial portion of the inferior cerebellar peduncle to their termination in the vestibular nuclear complex and in the central group of reticular nuclei. (One bundle of fastigiobulbar Figure neocerebellum (ie, the large lateral parts of the hemispheres and the superior vermis in the posterior lobe) constitutes the pontocere- bellum. The contralateralpontine nuclei send afferent fibers to this area. There is some overIapping of the three divisions; for example, some pontocerebellar fibers terminate in the cortex of the spinocerebellum. VESTIBULOCEREBEL The vestibulocerebellum receives afferent fi- fibers, known as the bers from the vestibular ganglion and from the vestibularnuclei of the same side (Fig. l0-13). Russelll, has an aberrant course. The fasciculus crosses the midline, passes through the other fastigial nucleus, and then curves over the root of the superior cerebellar peduncle to These enter the cerebellum in the medial part of the inferior cerebellar peduncle. Some of the uncinate fasciculus [of Chapter (To nuclei t0: Cerebellum of cranial nerves Iil, tv, vt) (To sptno- cerebellum) r-l-' I I Vestibular ganglion lnferior olivary Fastigial nucleus Vestibulospinal Cortex of flocculonodular Iobe tract (To spinal cord) Figure l0-13. connections of the vestibulocerebellum and vestibular nuclei. join other efferent fibers of the vestibulocere_ bellum in the contralateral inferior peduncle.) In summary, the vestibulocerebellum in_ fluences motorneurons through the vestibulospinal tract, the medial longitudinal fasciculus, and reticulospinal fibers. It is concemed with adjustrnent of muscle tone in response to ves_ tibular stimuli. It coordinates the actions of muscles that maintain equilibrium and partici_ in other motor responses to vestibular stimulation (see Ch. 22). pates SPINOCBREBBLL The following afferent systems project to the spinocerebellar cortex. (a) The dorsal and ven_ tral spinocerebellar tracts convey data from proprioceptive endings and from touch and pressure receptors (Fig. l0_f4). The dorsal tract, consisting of the axons of the neurons constitutingthe nucleus thoracicus in spinal segments Tl to L3 or L4, conveys information from the trunk and leg, The ventral tract, which arises in various parts of the lum_ bosacral gray matter (see Ch. 5), is involved mainly in conduction from the leg. (b) Cune- ocerebellar fibers from the aciessory cuneate nucleus (see Ch. 7) are equivalent, for the arm and neck, to those of ttre dorsal spi_ nocerebellar tract, Most of the fibers afferenito the cells of origin of the spinocerebellar and cuneocerebellar tracts have ascended in the dorsal funiculi of the spinal cord. l7S 176 Regional Anatorny ofthe Central Nervous System Posterior division of ventral lateral nucleus of se. ve thalamus m, Pontine reticu lotegmental SU Reticular formation (central group nucleus is of nuclei) ac m et Lateral and paramedian AI reticular nuclei Clobose di and emboliformnuclei Contralateral ve inferior c€ olivaru complex fa th rt $r tc bt til el Fastigial br ol D' nucteu5 br Vestibular nuclei r( Dorsal and ventral spinocerebel lar, cuneocerebella4 and Reticulospinal Vestibulospinal tract tract trigeminocerebeilar tracts Figure 10-14. Connections of the spinocerebellum. fi gi b e: n li p p c Data from cutaneous receptors are carried by spinoreticular fibers to the lateral and par:rmedian reticular nuclei (see Figs. 9_t and9-2) , from which fibers project ro thi cere_ bellum. These two nuclei also receive afferenr fibers from primary motor and sensory areas of the cerebral cortex. Another precerebellar reticular nucleus that projects to the vermis and medial pafis of the hemispheres is the re- ticulotegmental nucleus in the pons (see Fig. 9-1). This nucleus receives afferents from the cerebral cortex and from the vestibular nuclei. Finally, the spinocerebellum receives fibers from all three trigeminal sensory nerve and from the accessory olivary nuclei (in which spino -olivary tracts terminate ). CoIlateral branches of the axons from the various afferent sources terminate in the globose and emboliform nuclei, which also receive a small contingent of fibers from the red nucleus. Each half of the body is represented in the ipsilateral cerebellar cortex; if afferent fibers t tl T d r ir c T L t t I t Chapter 10: Cerebellum sented in two areas: one in and alongside the vermis inthe anteriorlobe, andthe otherinthe medial part of the hemisphere on the inferior sur{ace of the posterior lobe. The ,,head area,, is in the superior vermis and the immediatelv adjacent cortex of the posterior lobe. So_ matotopic representation in the spinocer_ ebellum is less clearly defined than ln some areas ofthe cerebral cortex; there is overlap of different inputs, so that ffains of impulses from various sources may reach the same purkinje cell. area of the cerebral cortex. The end result is conuol of muscle tone and synergy of collab_ orating muscles, as appropriate at any mo_ ment for the adjustment of posture and in many types of movement, including those of locomotion, PO CBREBELLUM Pontocerebellar fibers constitute the whole of rS, lei the vermis of the anterior and port.rio, tolll and throughout the cortex of the cerebellar hemispheres. The large lateral regions of the hemispheres constitute the pontocerebellum. tonus are effected bulbar connections tibiilocerebellum, F emboliform nuclei traverse the superior cere_ bellar peduncle and terminate in the central group of reticular nuclei. Thus the spinocerebellum may influence motor neurons through reticulospinal fibers and a similar projection to motor nuclei of cranial nerves. Alpha and garnma motor neurons are involved in cere_ bellar control of muscle action, and the influence of the spinocerebellum on the skeletal musculature is ipsilateral. Some fibers from the globose and embo_ liform nuclei traverse the superior cerebellar peduncle a4d end in the red nucleus. Others pass through or around the red,nucleus and continue to the ventral lateral nucleus of the thalamus, from which fibers project to the primary motor area of the cerebral cortex, The main projection of the red nucleus is through the central tegmental tractto the inferior olivary complex of nuclei. In summary, the spinocerebellum receives information from proprioceptive and exteroceptive endings and from the cerebral cortex. These data are processed in the circuitrv of the cerebellar cortex, which modifies and refines the discharge of nerve impulses from the central nuclei. Motor neurons are inlluenced mainly through relays in the vestibular nuclei, the reticular formation, and the primary motor Through corticopontine tracts that origi_ nate in widespread areas of the contralateial cerebral cortex (especially that of the frontal and p4rietal lobes) and the pontocerebellar projection, the cortex of a cerebellar hemi_ receive afferents from the superior colliculus and relay data used by the cerebellum in the control of visually guided movements. Purkinje cell axons from the pontocerebellar cortex terminate in the dentate nucleus, the efferent fibers of which compose most of the superior cerebellar peduncle. After travers_ ing the decussation of the peduncles, some den- tatothalamic fibers give off collateral branches that go to the red nucleus and the inferior olivary complex, but the great majority pass through or around the red nucleus andind in the ventral lateral nucleus of the thalamus. This thalamic nucleus projects in tum to the primary motor area of cortex in the frontal lobe. Through these connections. the ify activity in corand reticulospinal The output of the dentate nucleus, like that of the other cerebellar nuclei, fluctuates ac_ cording to the excitatory input from extra_ cerebellar sources and the refinement of dis_ charge by the inhibitory action of purkiqje cells. Main_ly through its inlluence on the cere_ 177 178 Reglonal Ailqtotny of the cenffal Neruous system pont Cerebral cortex (f rontal, parietal, and temporal Iobes) wher the cort€ fastil vary affer from ipsili Ventral lateral nucleus of T thalamus vary (posterior division) relat perf( cern Red nucleus ass syna TETS Central tegmental tract Cortex of cerebellar hemisphere I fiber sitio fiber exec mer ferer are Pontine nuclei mor neu' ceiv nuc tho: toc€ lnferior olivary the keyr Dentate nucleus comptex reac thrc tine Figure 10-15. Connections of the Pontocerebellum. dorr acot con bral motor cortex, the pontocerebellum en- licu sures a smooth and orderly sequence of muscle contractions and the intended precision in the force, direction, and extent of volitional movements. These functions are particularly important for the upper limb. A cerebellar hemisphere influences the musculature of the same side of the body because of the compensating decussations of the superior cerebellar peduncles and of the corticospinal tracts and other descending pathways. There is a large contingent of olivocerebellar fibers from the inferior olivary nucleus and from the dorsal and medial accessory olivary nuclei in the medulla. These cross the midline and enter the contralateral inferior cerebellar peduncle to be distributed to all parts of the cerebellar cortex. Olivocerebellar fibers also end in cerebellar nuclei. The principal inferior olivary nucleus projects to the cortex of the are *rr( visr corl tha fur cal pro Chapter pontocerebe whereas the the spinoce cortex and to the emboliform, globose, and fastigial nuclei. In addition to tlie rubro-oli_ vary fibers indicated in Figure I 0 - I 5 , the main afferents to the inferior olivary nucleus are from the sensorimotor strip of cortex of the ipsilateral cerebral hemisphere (see Ch. 7), The climbing fibers from the inferior oli- )0: Cerebellum lary, and urinary bladder responses, They are sympathetic in nature when the anterioilobe is stimulated and parasympathetic when the tonsils (see Fig. f 0-3) of the posterior lobe are stimulated. The postulated pathway includes the interposed nuclei, reticular formation and hypothalamus. Cerebellar Disorders Pathological conditions are classified broadlv into those that affect the vermis and flocculo_ nodular lobe (the vestibulocerebellum and spinocerebellum) and those that affect the synapseS, It has been suggested that transmit_ hemispheres (pontocerebellum), MIDLI execution and coordination of learned move_ ments are mediated by the mossy fiber af_ ferents, of which those from the poniine nuclei are the most numerous in primates. When a monkey makes an intended movement, the neurons in the dentate nucleus (which receives its excitatory afferents from the pontine nuclei) are active several millisecondi before those in the primary motor area. to th dorsal region LESIONS vermis. The patient has an unsteady, stagger_ ing ataxic gait, walks on a wide base, ana sways from side to side. Cerebellar nystag- mus is "pendular,,, with .y" -ou.-.nts of equal speed in both directions, usually in the horizontal plane, It is attributed to interrup_ tion of connections of the vermis with the ocu_ lar motor nuclei by way of the vestibular nuclei of the pontine nuclei convev acoustic data as well as visual data because of i connection from the inferior to the superior col_ liculus. Stimuli perceived by the eyei and ears are also able to influence the cerebellum through cofiicopontine Iibers that originate in visual and auditory areas of the cerebral cortex. Results of animal experiments have shown that the cerebellum also has a role in visceral functions. Under certain conditions, electri_ cal stimulation of the spinocerebellar cortex produces respiratory, cardiovascular, pupil- With re signs of intemrp i.sjllli,'j; destruction of the cortex and medullary center, or involve the central nuclei or the efferent pathways in the superior cerebellar peduncle. the motor disorder is seve lesion involves rior cerebellar unilateral, the signs of motor dysfunction are on the same side of the body. l7g 180 Regional Anatorny of the Central Nervous Systeffi The following signs, in varying degrees of severity, are those of a neocerebellar syndrome. Movements are ataxic (intermittent or jerky). There is dysmetria; for example, when the patient reaches out with the Iinger to an object, the finger overshoots the mark or deviates from it (past-pointing). Rapidly altemating movements, such as flexion and extension of the fingers or pronation and supination of the forearm, are performed in a clumsy manner (adiadochokinesis), Asynergy is separation of smoothly flowing voluntary movements into successions of mechanical or puppet-like movements (decomposition of movement). There may be hypotonia of muscles, which also tire easily. Cerebellar tremor, which occurs most frequently with ' demyelinating lesions in the cerebellar peduncles, usually occurs at the end of a particular movement (intention tremor). Dysarthria is evident if asynergy involves muscles used in speech, which is then thick and monotonous (sluning; scanning speech). There may be nystagmus/ if the lesion encroaches on the vermis. The deficits noted are superimposed on volitional movements that are themselves basicallv intact. SUGGESTED READING Brooks VB: The Neural Basis of Motor Control. New York, Oxford University Press, 1986 Decety J, Sjoholm H, Ryding E, Stenberg G, Ingvar DH: The cerebellum participates in mental ac- tivity: Tomographic measurements of regional cerebral blood flow, Brain Res 5i5t3l3-3l7, 1990 FitzGerald MJT: Neuroanatomy Basic and Clinica-. 2nd ed, London. Ballidre Tindall, 1992 Lalonder R, Botez MI: The cerebellum an! leaming processes in animals. Brain Res nev tf )32, L990 :lZl- Llin6s RR, Walton KD: Cerebellum. In Shepherd GM (ed): The Synaptic Organization of the Brain, 3rd ed., pp. 214-245. New york, Oxford University Press, 1990 Tledici G, Barajon I, Pizziti G, Sanguineti I: The organization of corticopontine fibres in man. Acta Anat 137:320-32), t99O Walton J: Introduction to Clinical Neuroscience, 2nd ed. London, Ballidre Ttndall. 1987