Cerebellum Subdivisions and Afferent Pathways (Continued)

Trigeminocerebellar projections from the principal trigeminal sensory nucleus travel in the ICP to the face representation in caudal lobule V and lobule VI; in contrast, cerebellar projections from the mesencephalic trigeminal nucleus travel in the SCP. The tectocerebellar tract from the superior and inferior colliculi bilaterally projects to lobules VIII and IX (the posterior, or dorsal, paraflocculus and uvula), and the vermal visual area in lobule VII.

Reticulocerebellar projections act over wide areas of the cerebellum and DCN. They originate from the lateral reticular nucleus (LRN) and paramedian reticular nucleus in the medulla, the nucleus reticularis tegmenti pontis (NRTP) in the pons, and the medial (magnocellular) reticular formation. Feedback projections arise from the DCN, particularly the fastigial nucleus.

The NRTP receives cerebral cortical afferents from sensorimotor, frontal lobe, and superior parietal regions, and subcortical afferents from vestibular and visual- or eye-movement–related nuclei, including the superior colliculus. Inputs from limbic-related structures include the cingulate gyrus and mammillary bodies. NRTP fibers enter cerebellum through the middle cerebellar peduncle and project widely, with a focus in vermal lobules VI and VII, and lobule X. The NRTP is involved in ocular vergence and accommodation and the visual guidance of eye movements. The limbic relay provides cerebellum with emotionally salient information (see Plate 8-15).

The lateral reticular nucleus (LRN) receives inputs from the spinal cord, lateral vestibular nucleus, red nucleus, superior colliculus, and cerebral cortex. Its fibers enter the cerebellum through the ipsilateral ICP; many cross to the contralateral side, providing collaterals to the DCN and terminating in multiple parasagittal zones. LRN connections are somatotopically arranged: the ventrolateral parvicellular region conveys afferents from the lumbar cord to primary and secondary hindlimb representations bilaterally; the dorsomedial, magnocellular part conveys inputs from the cervical cord to cerebellar forelimb regions. LRN neurons resemble VSCT or RSCT neurons but lack group I muscle input, are excited by descending vestibulospinal fibers, and respond to stimulation of larger body surface areas.

The paramedian reticular nucleus in the medulla receives afferents from the vestibular nuclei and somatosensory regions of the cerebral cortex and projects through the ICP to the vermis.

Perihypoglossal Nuclei. These medullary nuclei related to the control of extraocular muscles receive vertical and horizontal gaze information from midbrain and pontine nuclei and face regions of the sensorimotor cortex. They are reciprocally interconnected with vermal and hemispheric components of cerebellar lobule X (nodulus and flocculus) and the fastigial and interposed nuclei.

Arcuate Cerebellar Tract. Fibers from the arcuate nucleus in the ventral medulla form the striae medullares visible on the posterior surface of the medulla. They enter cerebellum via the ICP and terminate in ipsilateral hemispheric lobule X. The arcuate nucleus is involved in central reflex chemosensitivity and cardiorespiratory activity.

Vestibulocerebellar Pathways. Vestibular input to cerebellum arises from primary vestibular afferent fibers and projections of neurons in the vestibular nuclei. These fibers carry information from receptors of the vestibular labyrinth, which signal the position and motion of the head in space (see Plate 8-11).

Pontocerebellar Pathways. Basis pontis neurons receive input from multiple areas of the cerebral cortex and project as pontocerebellar fibers via the contralateral MCP to the cerebellar cortex. The organization, somatotopy, and functional relevance of the corticopontocerebellar system are considered in Plate 8-13.

CLIMBING FIBER PATHWAYS

Climbing fibers from the inferior olives project via the contralateral ICP to the cerebellar cortex. Climbing fiber anatomy, connections, and physiology are shown in Plates 8-6 and 8-7.

MONOAMINERGIC FIBERS

Minor dopaminergic inputs to cerebellum arise in the substantia nigra—noradrenergic inputs from the locus coeruleus project diffusely to the vermis and lateral hemispheres, and serotonergic fibers from raphe nuclei project diffusely to most regions of cerebellar cortex.

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