The Vestibulocerebellar Module

The vestibulocerebellum (archicerebel-lum) is phylogenetically the oldest cere-bellar module. The flocculonodular and adjacent vermian cerebellar cortex receive information about the orientation of the head and body in space from the vestibular ganglion, and receive other inputs from the contralateral accessory olivary nuclei (see p. 12). The vestib-ulocerebellum also receives inputs concerning eye movements from the basal pons.

The vestibulocerebellum sends cere-bellar corticovestibular efferents from the flocculonodular lobe to the vestibular nuclei. It also sends cerebellar corti-conuclear fibers to the fastigial nucleus, which in turn projects excitatory fibers to the vestibular nuclei. The cerebellar afferents are GABAergic inhibitory fibers. The fastigial body also projects bilateral excitatory fibers to the vestibular nuclei and to nuclei in the midbrain reticular formation. From the vestibular nuclei fibers descend as the medial and lateral vesti-bulospinal tracts. The vestibular nuclei project bilaterally to the cranial nerves, namely III, IV, and VI, in tracts that ascend in the medial longitudinal fasciculus.

In addition to the flocculonodular lobe, the vermis also sends efferents to the vestibular nuclei, and receives afferents from them. The lateral vermian zones receive ipsilateral secondary vestibular afferents, and send reciprocal efferents to these nuclei. The vermis projects also to the contralateral ventral lateral thalamus, and from there to the trunk areas of the motor cortex. Both the flocculonodular node and the lateral vermian zones project efferents to the ipsilateral fastigial nuclei. The fastigial nucleus and the vestibulocerebel-lar module therefore work together to exert their functional effects. The vesti-bulocerebellum receives information about head movements from the semicircular canals, and about its spatial orientation from the vestibular system.

Through the descending medial and lateral vestibulospinal tracts, the vesti-bulocerebellar module controls postural equilibrium, balance, and the judgement of limb movements. This is achieved through synaptic contact with ventral horn motoneurons, which innervate axial and proximal limb muscles. These functions are revealed after injury to midline structures, for example the fastigial nucleus, or to the flocculonodular node. Symptoms range from tremor to complete inability to maintain posture.

Damage to the flocculonodular node produces equilibrium loss. The patient cannot stand steadily (astasia), and cannot maintain equilibrium while walking (abasia). The gait when walking is unsteady and reminiscent of alcohol toxicity (truncal or axial ataxia; flocculonodular syndrome). The ataxia results from the inability of the muscles to act in coordination. The patient tends to fall over to the side, or backwards, or forwards when ambulating. When standing, patients may stand with feet wider apart than normal; this is to compensate for a loss of balance. In addition to these symptoms, the patient may exhibit axial and head tremors (ti-tubation). Eye movement abnormalities may be seen, including nystagmus, a rapid, uncoordinated movement of the eyes.

The interruption of pathways to and from the flocculonodular node results in characteristic abolition of certain reflexes. For example, patients whose floc-culonodular nodes and part of the uvula have been damaged no longer suffer from motion sickness.

ventral lateral nucleus of thalamus flocculonodular. node vestibular _ nuclei accessory olivary nucleus.

ventral lateral nucleus of thalamus flocculonodular. node accessory olivary nucleus.

Reicular Formation Nucleus

fastigio-thalamic fibers fastigial -nucleus fastigio-vestibular -fibers fastigio-oliva ry -fibers vestibular _ nuclei olivocerebellar" fibers olivocerebellar" fibers fastigiospinal fibers lateral medial vestibulospinal tracts

RF= reticular formation fastigio-thalamic fibers fastigial -nucleus fastigio-vestibular -fibers fastigio-oliva ry -fibers

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