Cerebellar Afferents

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1. Mossy Fiber Afferent System

As stated previously, the vast majority of cerebellar afferents reach the cerebellar cortex as mossy fibers. Spinocerebellar and trigeminal cerebellar mossy fibers convey interoceptive, proprioceptive, and exterocep-tive information from limbs, trunk, and face. The input is directed to the cerebellum by second-order neurons and not directly by primary sensory neurons. The organization of the vestibulocerebellar system is different. The labyrinth sends afferents directly to the cerebellar cortex; in addition to these primary vestibular afferents, secondary vestibular fibers originating from the four vestibular nuclei also reach the cerebellum. These latter nuclei are also unique in the cerebellar organization since they are the only ones, besides the deep cerebellar nuclei, that receive Purkinje cell axons. This specific organization is confined to the oldest cerebellar region, termed the vestibulo- or archicerebellum.

Many different cell groups located within the reticular formation give origin to mossy fibers. The lateral reticular nucleus is the recipient of mainly spinal afferents but it also receives relevant amounts of fibers from the cerebral cortex and the red nucleus. Mossy fibers from the lateral reticular nucleus terminate bilaterally in the cerebellum with a clear ipsilateral

Figure 4 Schematic reconstruction of the circuitry of the cerebellar cortex. A single cerebellar folium has been sectioned vertically, both in longitudinal and transverse planes. (Lower left) Enlargement of a single glomerulus (modified with permission from L. Heimer, The Human Brain and Spinal Cord, Fig. 120, p. 213. Copyright © 1983 by Springer-Verlag).

Figure 4 Schematic reconstruction of the circuitry of the cerebellar cortex. A single cerebellar folium has been sectioned vertically, both in longitudinal and transverse planes. (Lower left) Enlargement of a single glomerulus (modified with permission from L. Heimer, The Human Brain and Spinal Cord, Fig. 120, p. 213. Copyright © 1983 by Springer-Verlag).

prevalence. The paramedian reticular nucleus conveys information from the somatosensory and frontal cortices mainly to the vestibulocerebellum. The pontine reticular cells provide visual-related information from cortical and subcortical structures mainly to vermal lobules VII and VIII.

Many of the cerebral cortical areas are connected with the contralateral cerebellum through the pontine mossy fiber system, the largest group of mossy fibers. Corticopontine fibers originate from layer V pyramidal cells and their terminal fields are organized topographically with the anterior cortical region located medially and the posterior one located later ally. The topographic organization is maintained in the pontocerebellar projection to the contralateral cerebellum (Fig. 8). However, this projection is not strictly contralateral since both ipsilateral and bilateral fibers have been described.

2. Climbing Fiber Afferent System

There is only one known source of climbing fibers—the inferior olive. This structure receives information from a wide range of brain structures funneling data from the periphery (spinal cord, dorsal column nuclei, trigeminal and vestibular nuclei, and

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