Cerebellovestibular Pathways

VESTIBULOCEREBELLAR PROJECTIONS

There are five peripheral vestibular end organs; the cristae in the three orthogonally oriented semicircular canals detect movement in the sense of angular rotation in the horizontal, pitch, and roll planes, and the maculae in the two otoliths that sense the effect of the linear acceleration of gravity during roll-tilt (utricle) and pitch (saccule).

Vestibular afferents from these end organs combine in the vestibular nerve. One branch terminates in the ipsilateral cerebellar cortex, providing profuse primary vestibular afferents from otoliths to vermal lobule IX and from semicircular canals to vermal lobule X. The other branch terminates with varying degrees of intensity in different subregions of all four vestibular nuclei: medial, lateral, superior, and inferior. These nuclei, with the exception of the posterior part of the lateral vestibular nucleus (Deiters nucleus), provide cholinergic secondary vestibular afferents bilaterally to the vermal and hemispheric regions of lobules IX and X and to the anterior vermis. Together with the perihypoglossal nucleus, they also project to the fastigial nucleus as collaterals of vestibulocortical fibers. Axons of both pathways terminate as diffusely projecting mossy fibers in granule cell glomeruli within the cerebellar cortex. Tertiary vestibular afferents reach vermal lobules IX and X as climbing fibers derived from subregions of the medial accessory olive (beta subnucleus and dorsomedial cell column). These olivary nuclei receive inhibitory input from the parasolitary nucleus that, in turn, receives projections from the labyrinth. The olivocerebellar terminations are arranged in discrete parasagittal zones, relaying information from the vertical and anterior semicircular canals.

CEREBELLOVESTIBULAR PROJECTIONS

Projections from vermal and hemispheric parts of lobules IX and X are directed to all the vestibular nuclei. The anterior lobe vermis projects to the fastigial nucleus, and in addition, Purkinje cells in zone B of the anterior vermis project directly to the part of the lateral vestibular nucleus that is devoid of vestibular afferents and gives rise to the lateral vestibulospinal tract. Strong topographically arranged projections to the vestibular nuclei are also derived from the fastigial nucleus. The rostral fastigial nucleus, linked with the spinal recipient anterior vermis, projects to the medial, superior, and perihypoglossal nuclei. The caudoventral region of the fastigial nucleus, devoted to oculomotor control, projects to the inferior vestibular nucleus and to the part of the lateral vestibular nucleus that receives zone B cortical inputs. The projections of the rostral portion of the fastigial nucleus are ipsilateral, whereas fibers from the caudal fastigial nucleus cross in the hook bundle of Russell to excite contralateral vestibular neurons. Thus Purkinje cells of the cerebellar vermis can depress the activity of neurons in the vestibular nuclei either by direct inhibition or by inhibiting the discharge of neurons in the fastigial nucleus, thereby decreasing the excitatory activity reaching vestibular neurons via fastigio-vestibular pathways, an example of disfacilitation.

FUNCTIONAL CONSIDERATIONS

The vestibular system is critical for the control of eye movements for orientation in intrapersonal and extrapersonal space and for control of the axial musculature, essential for balance. Vestibulocerebellar connections provide the cerebellum with a topographic map of space, serving as an anatomic substrate for modulation of postural reflexes evoked by vestibular and optokinetic stimulation. The vestibulocerebellum predicts spatial environments and, by modulating the amplitude of movements produced by reflexes such as the vestibulo-ocular reflex, compensates for head movements to optimally guide behavior. The clinical relevance of these vestibulocerebellar circuits is exemplified by the loss of plastic changes in the horizontal vestibulo-ocular reflex in individuals with damage to lobule X (flocculus) and by the inability to remember postural adjustments to a previously maintained head position in space after damage to the vermal lobules IX and X. Acute injury to the vestibular system produces violent nausea, vomiting, and vertigo. The paleocerebellum receives a modest amount of vestibular afferent input but extensive input from spinocerebellar tracts. The principal action of the paleocerebellum on the vestibular system is to regulate vestibular activity in relation to proprioceptive and exteroceptive information about the head, trunk, and extremities. This is critical for posture, balance, and equilibrium.

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