Cerebellar Afferents

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The cerebellar cortex receives afferent input from most parts of the peripheral (proprioceptive, cutaneous, vestibular, and visual, except olfactory) and central nervous system. The afferent fibers consist of mossy fibers, the climbing fibers, and monoaminergic fibers.

The two most important types of excitatory cerebellar afferents, mossy and climbing fibers, have been defined based on the morphology of their terminal fibers. Mossy * The term "interposed nucleus" refers to the cerebellum in the cat, rabbit, and monkey. The corresponding nuclei in the human cerebellum are called emboliform and globose nucleus.

Figure 17-1 The cerebellar cortex, unfolded into one plane, showing the fields of termination of cerebellar afferent projections (= mossy fiber system): pontocerebellar(open contours), spinocerebellar fibers(dottedareas) and vestibulocerebellar fibers(hatched areas). LOB POST = Posterior lobe; LOB ANT = anterior lobe; lobl simplex = lobulus simplex; lobl semilun sup = superior semilunar lobule; lobl semilun inf = inferior semilunar lobule; lobl grac = gracile lobule; lobl bivent = biventer lobule; tons cbl = cerebellar tonsil; fiss I = primary fissure; fiss postlat = posterolateral fissure; Li = lingula; Ce = central lobe; Cu = culmen; De = declive; Fo = folium vermis; Tu = tuber vermis; Py = pyramis; No = nodulus; Uv = (With permission from Nieuwenhuys R, Voogd J, van Huijzen C: The Human Central Nervous System. A Synopsis and Atlas. Berlin, Springer, 1988, p 162.)

fiber terminals are characterized by their mosslike appearance on histological examination, whereas climbing fibers climb in a characteristic way along the cell body and dendrites of the Purkinje cell. Mossy fibers terminate at numerous granular cells in the cerebellar cortex. Climbing fibers directly contact Purkinje cells in an one-to-one relationship. Climbing fibers originate solely from the inferior olive in the brain stem. y Mossy fibers form the major cerebellar input. They originate from different brain stem nuclei (pontine, vestibular, trigeminal and reticular nuclei), deep cerebellar nuclei, and neurons in the spinal cord. y

Some vestibular and spinal cord afferents send fibers more or less directly to the cerebellum. From the trunk and legs, the dorsal and ventral spinocerebellar tract enter the cerebellum through the ipsilateral inferior and contralateral superior cerebellar peduncle, respectively. From the arms and neck, the cuneocerebellar and rostral spinocerebellar tracts enter the cerebellum through the inferior

Figure 17-2 (Figure Not Available) The cerebellum has three functional components based on different inputs; the vestibulocerebellum, the spinocerebellum and the cerebrocerebellum or pontocerebellum, and based on different outputs, a medial zone ( vermis) projecting to the fastigial nucleus, an intermediate (

paravermal parts of hemispheres) zone projecting to the interposed nucleus and a lateral zone (

lateral part of hemisphere) projecting to the dentate nucleus. The main efferents from the flocculonodular lobe project directly to the vestibular nfFrom Ghez C: The cerebellum. In Kandel ER, Schwartz JH, Jessell TM (eds.): Principles of neural science, 3rd ed. Norwalk: Appleton & Lange, 1991, Fig. 41-7, p 633.)

and superior cerebellar peduncles. Many afferent pathways have additional relay stations (pontine nuclei, inferior olives) before they enter the cerebellum. The pontine nuclei represent the most important relay for corticocerebellar pathways. From the pontine nuclei, corticopontine projections enter the cerebellum mainly through the contralateral middle cerebellar peduncle. The inferior olives receive afferents mainly from the spinal cord and also from several other cortical and brain stem areas. From the inferior olive, climbing fibers enter the cerebellum through the contralateral inferior cerebellar peduncle.

The primary destination of afferent fibers is the Purkinje cell located in the Purkinje cell layer of each lobe of the cerebellar cortex.

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