Cerebellum and Ataxia

Cerebellum and the Fourth Ventricle

The fourth ventricle lies posterior to the pons and upper half of the medulla oblongata and anterior to the cerebellum (see Plate 8-1 ). Its upper and lower ends become continuous, respectively, with the cerebral (sylvian, or mesencephalic) aqueduct and the central canal of the spinal cord in the lower half of the medulla. On each side, a narrow prolongation, the lateral recess , projects outward from its widest part and curves around the brainstem above the corresponding inferior (caudal) cerebellar peduncle ; its lateral aperture ( foramen of Luschka ) lies below the cerebellar flocculus and behind the emerging rootlets of the glossopharyngeal (IX) and vagus (X) nerves . The fourth ventricle has lateral boundaries, a roof, and a floor.

The lateral boundaries are formed on each side from above down by the superior cerebellar peduncle, the inferior cerebellar peduncle, and the cuneate and gracile tubercles.

Roof of Fourth Ventricle. The upper and lower parts of the V -shaped roof are formed by the superior and inferior medullary vela, which are thin laminae of white matter between the superior and inferior cerebellar peduncles. The lower part of the inferior velum has a median aperture (foramen of Magendie); cerebrospinal fluid escapes through this opening and the lateral aperture (foramina of Luschka) into the subarachnoid space. Because these are the only communications between the ventricular and subarachnoid spaces, their blockage can produce hydrocephalus.

The lower part of the roof and the posterior walls of the lateral recesses are invaginated by vascular tufts of pia mater, which form the T -shaped choroid plexus of the fourth ventricle.

The floor of the fourth ventricle is rhomboid shaped and is divided into symmetric halves by a vertical median sulcus. Its upper (pontine) and lower (medullary) parts are demarcated by delicate transverse strands of fibers, the striae medullares of the fourth ventricle.

On each side of the median sulcus is a longitudinal elevation, the medial eminence, lateral to which runs the sulcus limitans. Its superior part is the locus ceruleus, colored bluish-gray from a patch of deeply pigmented nerve cells. Also lateral to the upper part of the medial eminence is a slight depression, the superior fovea, and just below and medial to this fovea is a rounded swelling, the facial colliculus, which overlies the nucleus of the abducens (VI) nerve and the facial (VII) nerve fibers encircling it; the motor nucleus of the facial nerve lies more deeply in the pons. Inferolateral to the superior fovea is the upper part of the vestibular area, which overlies parts of the nuclei of the vestibulocochlear (VIII) nerve.

The lower (medullary) part of the medial eminence overlies the twelfth cranial nerve nucleus and is termed the hypoglossal trigone. Lateral to it is a slight depression, the inferior fovea, which, together with the neighboring vagal trigone, overlies parts of the dorsal nuclei of the glossopharyngeal and vagus nerves. Lateral to the inferior fovea is the lower part of the vestibular area, overlying parts of the vestibular nuclei of the vestibulocochlear nerve. On a deeper plane, parts of the trigeminal, solitary tract, and ambiguus nuclei also underlie the floor of the fourth ventricle. Some of the nuclei mentioned, such as the dorsal vagal and ambiguus nuclei, as well as others located in the nearby reticular formation, are concerned with cardiovascular, respiratory, metabolic, and other important functions, and are regarded as vital centers. Any lesion in this relatively small area of the brain may produce disastrous results.

Cerebellum Gross Anatomy

The cerebellum, from the Latin meaning little brain , is the largest part of the hindbrain occupying most of the posterior fossa. In the adult human brain, the cerebellum's volume is about 144 cm ³ , weighing 150 grams (10% total brain weight). However, its surface area is 40% of the cerebral cortex, containing half the total number of intracerebral neurons. The cerebellum, consisting of two hemispheres situated contiguously with the midline vermis, is separated from the overlying cerebrum by the tentorium cerebelli. The vermis (i.e., from the Latin, meaning worm ) is visible posteriorly and inferiorly in the vallecula, the deep groove separating the two cerebellar hemispheres. Superiorly, in contrast, the vermis appears as a low ridge straddling the midline, extending up 10 mm bilaterally.

A wide hollow within the anterior cerebellum is occupied by the pons and upper medulla oblongata, which are separated from the cerebellum by the fourth ventricle. Posteriorly, there is a narrow median notch, lodging the falx cerebelli. The cerebellum is connected to the brainstem by three white matter tracts: the superior, middle, and inferior cerebellar peduncles (described more fully in Plate 8-3 ). The cerebellum's superior and inferior surfaces meet within the caudal aspect of lobule crus I . The cerebellum forms a sphere, and therefore the vermal lobule I/II is separated anteriorly from lobule X by the fourth ventricle.

The cerebellum surfaces include numerous narrow folia separated by parallel, curved, deeply penetrating fissures. Each folium further consists of multiple, small subfolia. The folia are grouped into ten lobules divided by named fissures. These ten lobules form three lobes: the anterior, posterior, and flocculonodular lobes . Lobules I to V are the anterior lobe, lobules VI to IX are the posterior lobe, and lobule X is the flocculonodular lobe, including the flocculus, which is a small, semidetached portion lying close to the middle cerebellar peduncle. Earlier cerebellum nomenclatures were not uniform (one version is in the diagrams for comparison). These are replaced by a simplified, coherent numeric system existing across different species' brains. All lobules are identifiable at the vermis; lobules III to X are continuous across the hemispheres.

The primary fissure separating the anterior from the posterior lobe is deepest and most evident in the midsagittal plane but not as readily identifiable externally. The superior posterior fissure separating lobule VI from lobule VII is well seen on the posterior superior surface. The horizontal fissure , prominent on the posterior, inferior, and lateral hemisphere aspects divides lobule VIIA into two major components: lobule VIIAf at the vermis/crus I in the hemisphere, and lobule VIIAt at the vermis/crus II in the hemisphere. The paravermian sulcus on each side of the superior cerebellum surface is an indentation formed by the superior cerebellar artery medial branch. The retrotonsillar groove at the inferior and medial aspect of the cerebellum is caused by the rim of the foramen magnum and delineates the tonsil, a gross morphologic feature comprising lobule IX and part of lobule VIIIB that becomes clinically relevant with herniation syndromes .

The interior of the cerebellum contains a central mass of white matter, the medullary core, surrounded by the deeply folded cerebellar folia. The relationship of the folia to the white matter has a tree branch appearance, hence arbor vitae . The white matter core extends into the folia as narrow laminae, surrounded by the three-layered cerebellar cortex. The white matter consists largely of mossy and climbing fibers entering the cerebellum, and axons of Purkinje cells leaving the cerebellar cortex to the nuclei. There are no association fibers in the cerebellum linking cerebellar cortical areas with each other. The cerebellar nuclei within the medullary core include, medial to lateral, the fastigial, globose, emboliform, and dentate . These nuclei, together with other minor nuclei in the medullary core and vestibular nuclei in the posterior pons and medulla, are linked with the cerebellar cortex serving as the cerebellum's functional unit, namely, the corticonuclear microcomplex . Except for the vestibulocerebellum, these nuclei are the primary source of cerebellar efferents. These have highly organized connections with extracerebellar structures. The large, folded dentate nucleus is U -shaped. Its open end, or hilus, points medially, conveying fibers that, together with those from the fastigial, globose, and emboliform nuclei, form the superior cerebellar peduncle.

Cerebellar Peduncles

The cerebellum is linked with the spinal cord, brainstem, and cerebral hemispheres by three major fiber tracts—the inferior, middle, and superior cerebellar peduncles. These convey axons into the cerebellum (afferent) or away from it (efferent).

The inferior cerebellar peduncle (ICP) has two components. The larger is the restiform body, a purely afferent system, whereas the smaller juxtarestiform body carries both afferent and efferent fibers.

The restiform body (or ICP proper) is located in the dorsolateral medulla, lateral to the vestibular nuclei. Entering the cerebellum, it is situated medial to the middle cerebellar peduncle, conveying uncrossed mossy fiber afferents to cerebellum from the ipsilateral spinal cord and brainstem, and crossed climbing fiber inputs from the contralateral inferior olivary nucleus. Spinal cord inputs in the ICP are from the dorsal (posterior) spinocerebellar tract (DSCT), conveying information from the trunk and lower limbs. The rostral spinocerebellar tract carries information from the upper limbs and the central cervical tract arising from upper cervical segments. From the brainstem, the ICP conveys the cuneocerebellar tract arising in the external cuneate nucleus (also known as the lateral or accessory cuneate nucleus), which conveys information from the upper limb and the reticular formation (reticulocerebellar fibers), the trigeminal principal sensory nucleus (trigeminocerebellar fibers), and the midline raphe. Climbing fibers arise in the inferior olive, cross in the medulla, and course within the ICP to reach the contralateral cerebellar hemisphere.

The juxtarestiform body is a small aggregation of fibers situated medial to the restiform body that enters the cerebellum passing through the vestibular nuclei. It conveys afferent fibers to vermal lobule IX (uvula) and lobule X (the flocculonodular lobe). Primary vestibular afferents arise from the vestibular sense organs (the saccule and utricle) and terminate ipsilaterally; secondary vestibular fibers from the vestibular nuclei terminate bilaterally. Efferent fibers in the juxtarestiform body arise from the cerebellar cortex and fastigial nucleus. Cerebellar cortical axons in the juxtarestiform body emanating from Purkinje cells in the vestibulocerebellum (part of lobule IX, and lobule X) terminate in Deiters lateral vestibular nucleus, and, together with efferents from the anterior vermis, are the only instance of projections from cerebellar cortex bypassing the deep cerebellar nuclei to terminate on a target outside the cerebellum. Juxtarestiform body fibers arising from the fastigial nuclei lead to the vestibular and the reticular nuclei. Axons from the rostral half of the fastigial nucleus course to the ipsilateral brainstem in the fastigiobulbar tract . Axons from the caudal half of the fastigial nucleus cross to the contralateral cerebellum in the uncinate bundle , that is, the hook bundle of Russell, before traveling to the brainstem in the contralateral juxtarestiform body (see Plate 8-10 ).

The middle cerebellar peduncle (MCP) is a massive tract situated at the lateral aspect of the basis pontis. Axons leave the pontine nuclei, cross to the opposite side of the pons, and course in the contralateral MCP to the cerebellum. The pontine nuclei are an obligatory intermediate link between the ipsilateral corticopontine input via the cerebral peduncle and the contralateral pontocerebellar projections by way of the MCP. A minor projection from cerebellar nuclei back to the pons is also present.

The superior cerebellar peduncle (SCP) transmits efferents from and afferents to the cerebellum. It lies within the posterolateral wall of the fourth ventricle, ascends as the brachium conjunctivum to the midbrain, where it decussates and continues rostrally, carrying ascending projections from the cerebellum to the reticular nuclei in the pons and midbrain, red nucleus, hypothalamic area, and thalamus. The hilus of the dentate nucleus is continuous with the SCP, but there are also axons in the SCP arising from the fastigial, globose, and emboliform nuclei. A descending branch of the SCP leaves the larger ascending component in the rostral pons, descends in the pontomedullary tegmentum, and crosses obliquely to the opposite side of the ventral medulla to terminate in the inferior olive (the cerebello-olivary projection). Afferents to the cerebellum coursing in the SCP arise in the spinal cord and brainstem. These include crossed ventral (anterior) spinocerebellar tract fibers conveying information concerning the contralateral trunk and lower limbs, and both crossed and uncrossed fibers in the central cervical tract. Ipsilateral afferents include tectocerebellar projections from the superior and inferior colliculi in the midbrain, trigeminocerebellar fibers from the trigeminal mesencephalic nucleus, and coeruleocerebellar projections from the locus coeruleus in the pons.

The three peduncles are differentially affected by ischemic, compressive, demyelinating, neurodegenerative, and other disorders. Clinically, peduncle lesions manifestations are heterogeneous, reflecting the wide range of functions subserved by the information they convey between the cerebellum and the remainder of the neuraxis.

Cerebellar Cortex and Nuclei

The histology of the cerebellar cortex differs fundamentally from that of the cerebral cortex in that it has essentially the same paracrystalline structure throughout. The trilaminate cortex, the Purkinje cell layer lying between the innermost granular layer and the outermost molecular layer , is apposed on each side of a white matter lamella conveying fibers to and from the cortex (see Plates 8-4 and 8-5 ).

The Purkinje cell (PC) layer is a monolayer composed entirely of PCs, a 100 µm-thick sheet of 15 million neurons situated between the molecular and granular layers. The PC is the defining neuron of the cerebellum. It is among the largest cells in the nervous system, with a pear-shaped soma (35 × 70 µm) and a fanlike appearance of its dendritic tree. The proximal dendrite divides into two major dendrites that branch multiple times to form a flattened plate (400 × 20 µm) in the parasagittal plane oriented perpendicular to the long axis of the folium. Each PC has over 150,000 spines, with a density 25 times higher on distal dendrites, where parallel fibers (PFs) synapse, than on proximal dendrites, where climbing fibers (CFs) synapse. The PC is the only neuron with axons leaving the cerebellar cortex . The axon descends through a constricted region surrounded by the pinceau of basket cell axon terminals, acquires a myelin sheath, and descends to the deep cerebellar nuclei or vestibular nuclei. Recurrent collaterals course back toward the molecular layer, inhibiting interneurons as well as the soma and proximal dendrites of neighboring PCs.

The molecular layer is 300 µm thick. It contains granule cell axons and PFs, dendritic arborizations of PCs and Golgi interneurons , and cell bodies of basket, stellate, and supporting glial cells .

Parallel fibers are formed when the granule cell axon ascends through the PC layer into the molecular layer and branches in the shape of a T to form the PF, one of the thinnest vertebrate axons. It travels parallel to the long axis of the folium for 1 to 3 mm in the rat and cat, and possibly 6 to 8 mm in primates.

The basket cell lies in the lower third of the molecular layer just above the PCs. Its dendrites extend up into the molecular layer in a fan-shaped field 30 µm wide in the parasagittal plane (the same plane as the PC dendritic tree), giving off relatively few branches, interdigitating with the dendritic fields of the PCs, and contacted by the PFs. Its axon courses in the parasagittal plane among the lower dendrites of 9 or 10 PCs. It emits a succession of descending branches that envelope the PC somata in an axonal sheath with numerous synaptic contacts, giving the basket cell its name. Terminal axonal branches surround the initial segment of the PC axon in a dense fiber plexus with the appearance of an old paintbrush (French, pinceau ). This axo-axonic complex is unique in the mammalian nervous system. Sparse ascending collaterals from the basket cell axon synapse on secondary and tertiary PC dendrites.

Stellate cells are small, 5 to 10 µm in diameter, with short, profusely branching dendrites contacted by parallel fibers and axons that terminate on PC dendrites. Superficial stellate cells in the upper molecular layer have short axons oriented in the parasagittal plane. Deep stellate cells in the middle part of the molecular layer have long axons up to 450 µm in the parasagittal plane, providing ascending and descending collaterals early in their course, but they rarely enter the pericellular PC plexus and do not participate in the pinceau.

The granular layer is 200 µm to 300 µm deep and contains granule, Golgi, Lugaro, and unipolar brush cells. G ranule cells number about 50 billion, 3,000 per single PC. They have minimal cytoplasm, are among the smallest neurons in the brain (6-8 µm diameter), and are the most numerous. Their density renders the granule cell layer a deep blue on stains such as Nissl, which label nuclear material. The granule cell has three to five clawlike branched dendrites that participate in the granule cell glomerulus, pale islands between the granule cells containing a complex articulation between terminal rosettes of mossy fiber afferents, arborizations of granule cell dendrites, and Golgi cell axons. The granule cell axon ascends into the molecular layer where its PFs provide excitatory input to the PCs.

Golgi cells are irregularly rounded or polygonal inhibitory interneurons numbering approximately 1 per 1.5 PCs. In contrast to the PC, basket, and stellate cells, the Golgi cell dendritic tree has a three-dimensional configuration. Large Golgi cells, 10 to 24 µm in diameter, lie in the upper half of the granular layer, their dendrites arising as one or two main trunks with subsidiary branches that ascend to the outer zone of the molecular layer. Smaller Golgi cells, 9 to 18 µm, are in the depths of the granular layer, with dendrites that radiate out from the soma. One to three axons emerge from the Golgi cell body or from proximal dendrites and divide repeatedly, resulting in a multitude of fine branches that form an elaborate, dense plexus extending throughout the granular layer and participating in the granule cell glomerulus.

The Lugaro cell is a fusiform inhibitory interneuron measuring 10 × 30 µm, lying horizontally or obliquely in the outer third of the granular layer. Its dendrites originate from the tapering extremities at the two poles and extend horizontally for up to 600 µm at the level of the PC bodies in the infraganglionic plexus formed by the PC recurrent axon collaterals. It receives excitatory input from the granule cell axon and serotoninergic modulation acting through volume transmission. Its axon arises from the cell body or large proximal dendrite, forming two types of axonal plexuses. One parasagittal axon contacts the soma and dendrites of stellate and basket cells in the molecular layer; the other is transverse and contacts Golgi cells in the granular layer.

The unipolar brush cell (UBC) is the only excitatory interneuron in the cerebellum. The soma is 9 to 12 µm in diameter, with a single dendrite ending in a tight brushlike tip of dendrioles that have extensive synaptic contact with the mossy fiber rosette. Its axon synapses on granule and Golgi cells. The UBC is found in the vestibulocerebellum, vermis, and dorsal cochlear nucleus, and it is thought to amplify vestibular signals and provide feed-forward excitation to granule cells.

Glial cells in the cerebellum include protoplasmic astrocytes that envelope the PC perikaryon in a neuroglial sheath; Bergmann glial cells in the PC layer that are involved in neural migration and development of the cerebellar cortex, and that play a role in regulating glutamatergic neural transmission in the mature cerebellum; and oligodendroglia in cerebellar white matter and in the granular layer.

Cerebellar Cortical and Corticonuclear Circuitry

The neurons of the cerebellar cortex and nuclei are linked together in multiple repeating anatomic microcircuits—corticonuclear microcomplexes—that serve as the essential functional unit of the cerebellum. The key to their elucidation is the dual nature of the cerebellar inputs—the mossy fiber and climbing fiber systems. Monoaminergic fibers from the brainstem are an additional minor source of cerebellar afferents.

Climbing fibers (CFs) arise exclusively from the inferior olive . Axons of olivary neurons branch to form 7 to 10 CFs. Each CF provides extensive excitatory synaptic contact with the dendritic tree of a single PC (between 1,000 and 1,500 synaptic contacts between a CF and its PC). Climbing fibers enter the cerebellum through the inferior cerebellar peduncle (ICP), branch in the white matter, where they emit collaterals to the deep cerebellar nuclei (DCN), and ascend to the molecular layer. In the lower two thirds of the molecular layer the CF is tightly wound around the trunk and major proximal branches of the PC dendritic tree. Each varicosity of a CF synapses with several dendritic spines arising from the same dendritic branch. Fine tendrils that branch off from the CF in the molecular layer synapse with ascending branches of the basket cell axon and with the dendritic trees of stellate and Golgi cells. The olivocerebellar projection is organized according to a strict mediolateral parasagittal zonal pattern (see Plate 8-12 ).

Mossy fibers (MFs) are named for their thickened terminals that have thick, short, divergent, varicose branches resembling moss. Their synaptic arborizations are termed rosettes, have a variety of shapes, and are located along the course of the MF in the granular layer at the branch points and at their sites of terminations. MFs are heavily myelinated and convey excitatory afferents to the cerebellar cortex from the spinal cord, brainstem (except the olive), and the cerebral hemispheres. They enter through all three cerebellar peduncles, giving off 20 to 30 collateral branches in the white matter of the folium as they course toward the granular layer. They also provide collaterals to the deep cerebellar nuclei. Many MFs terminate bilaterally in the cerebellum after crossing in the cerebellar white matter. Unlike the CF, the MF provides excitatory input to the PC indirectly. The MF rosette is the central component of the granule cell glomerulus, the complex articulation between MF rosettes and the terminal arborizations of granule cell dendrites. Each MF rosette makes excitatory synaptic contact with 50 to 100 dendritic terminals from up to 20 granule cells and receives inhibitory feedback from the descending axons of Golgi cells. The granule cell axon ascends toward the molecular layer, making synaptic contact with dendrites of Golgi cells in the granular layer and spines of the proximal dendrites of PCs in the molecular layer. Upon reaching the molecular layer, the granule cell axon divides to form parallel fibers (PFs), the two branches traveling in opposite directions along the folium. The PFs make synaptic contact with one or two spiny branchlets on intermediate and distal regions of the dendritic trees of up to 300 PCs along the folium.

Each PC receives synaptic inputs from approximately 200,000 parallel fibers. In addition to excitatory inputs to the PCs from MFs and CFs, the PCs participate in a bidirectional corticonuclear projection, receiving excitatory feedback back from those regions of the DCN and vestibular nuclei to which the PC inhibitory projection is directed. The PCs receive inhibitory inputs from interneurons in the molecular layer—stellate cells, basket cells, and Lugaro cells, all of which receive excitatory afferents from the ascending granule cell axon and the PFs. Recurrent axon collaterals of the PCs are also inhibitory. The dendrites of the unipolar brush cell (UBC) in the vestibulocerebellum, the only excitatory cerebellar interneuron, receive MF inputs within the granule cell glomerulus. The net effect of these finely balanced interactions is that inputs to cerebellum are excitatory, output from the cortex via the PC is inhibitory, and output from the cerebellum via the DCN is excitatory to thalamus and brainstem but inhibitory to the inferior olive.

The output from the cerebellar cortex is derived exclusively from PCs, precisely organized, and directed toward the DCN and precerebellar nuclei. PCs in each lobule project to those parts of the deep cerebellar nuclei closest to them. Thus the vermis projects to the fastigial nucleus, the intermediate cortex to the globose and emboliform nuclei, and much of the lateral hemispheres project to the dentate nucleus. More detail on cerebellar corticonuclear circuits, modules, and microzones is presented in Plate 8-12 .

The two major neurotransmitters in the cerebellum are glutamate and γ-aminobutyric acid (GABA). Glutamate is excitatory and is found in the MFs, PFs, CFs, UBCs, and deep cerebellar nuclear neurons that project to thalamus, brainstem, and cerebellar cortex. GABA is inhibitory and is utilized by the PCs, all remaining cerebellar interneurons (stellate, basket, Golgi, Lugaro), and the DCN neurons that project to the inferior olive. Glycine is present in inhibitory interneurons in the DCN. A number of other peptide neurotransmitters are present also in the afferent fibers and neurons of the cerebellar cortex.

The PC generates two different classes of action potentials in response to its principal afferents. The input of hundreds of MFs produces a brief burst of repetitive simple spikes, 50 to 150 per second. The inhibitory basket and stellate cell interneurons produce inhibition of PCs locally and for some distance lateral to the longitudinal strip of active parallel fibers. Therefore MF-induced PC activity consists of a brief burst of action potentials along the course of active parallel fibers, surrounded by a band of inhibited cells. PC excitation is further restricted by Golgi cells, which receive excitatory PF synapses on their apical dendrites and provide a mostly tonic inhibitory input to the glomerulus, decreasing the excitability of granule cells to MF afferents. PF input is thought to provide information about incoming signals, such as direction and speed of limb movement. In the cognitive domain, the PF may provide the PC with the context in which behaviors occur.

Climbing fiber input to the PC induces a complex spike with the very low frequency of 0.5 to 2 spikes per second, the same rate of firing as the olivary neurons from which the CF originates. The CF input to the PC is thought to signal the occurrence of errors. The MF-CF inputs to the PC are relevant to synaptic plasticity involved in learning and memory. Long-term depression (LTD) is characterized by the persistent depression of synaptic transmission from PFs to the PC that occurs when parallel and climbing fiber activation are concurrent. Long-term potentiation (LTP) has also been described. The balance of LTD and LTP enables the cerebellar cortex to adapt to errors by regulating cortical output either down or up.

These corticonuclear circuits and physiology are the basis of theories that the cerebellum functions as an adaptive filter, utilizing internal models to maintain behaviors around a homeostatic baseline, and optimizes cerebellar influence upon motor, cognitive, or limbic behaviors appropriate to the prevailing context. The paracrystalline structure of cerebellar cortical architecture and organization has led to the idea that it has a general signal-transforming ability, a universal cerebellar transform , which is applied to multiple domains of neurologic function . The role of the cerebellum in the nervous system is a result, then, of the combination of the uniform cerebellar structure and function and the complex and varied connections of the cerebellar microcircuits, with extracerebellar areas conveyed by the mossy and climbing fiber inputs and the corticonuclear outputs.

Cerebellum Subdivisions and Afferent Pathways

The cerebellum is divided into three lobes: anterior, posterior, and flocculonodular (see also Plate 8-2 ). It is also divided into three mediolateral subregions on the basis of phylogeny and function. The archicerebellum , or vestibulocerebellum, includes vermal and hemispheric parts of lobule X (flocculonodular lobe), parts of vermal lobule IX (uvula), and lobule I/ II (lingula); it is linked with the vestibular nuclei and is concerned with eye movements and equilibrium. The paleocerebellum , or spinocerebellum, in vermal and paravermal lobules III through VI and lobule VIII, receives cutaneous and kinesthetic afferents from spinal cord, brainstem, and cerebral hemispheres. The anterior vermis is linked with the rostral fastigial nucleus , influences the medial motor system through brainstem vestibulospinal and reticulospinal projections, and controls trunk and girdle muscles enabling balance and gait. Paravermal areas are linked with the interpositus nuclei, the posterior part of the dentate nucleus, red nucleus, and primary motor cortex, influencing descending lateral motor systems and controlling distal limb movements. The neocerebellum (pontocerebellum) includes lobules VI and VII at the vermis and hemispheres. It receives afferents from cerebral cortex through the pons. Lateral cerebellar hemispheres project via the ventral dentate nucleus to thalamus and cerebral association areas; the posterior vermis is linked through the caudal fastigial nucleus with limbic areas. It appears that the neocerebellum is involved in cognition and emotion.

Knowledge of cerebellar connections with extracerebellar structures is critical to understanding the diverse roles of the cerebellum and the consequences of cerebellar injury. Afferents to cerebellum are conveyed predominantly by mossy fibers and climbing fibers that are organized in a fundamentally different manner (see Plates 8-6 and 8-7 ).