Hair cells have synaptic endings and release the neurotransmitter glutamate in response to appropriate stimulation. Even so, historically they have not been classified as neurons. By convention, first-order neurons of the vestibular pathway are designated as the afferent fibers originating from cell bodies in the vestibular ganglion located near the base of the utricle and saccule (see Fig. 1). The peripheral processes receive postsynaptic stimulation from hair cells, and resulting action potentials are propagated along the peripheral dendrite and on to central axonal processes, both of which are myelinated. The central vestibular fibers join central auditory fibers to form cranial nerve VIII as it projects to the brain stem at the junction between the pons and the medulla.
Cell bodies of the vestibular nuclei occupy a substantial area on the lateral aspect of the medulla (Fig. 5). Four divisions of the nucleus are recognized. The lateral vestibular nucleus receives input from all vestibular organs and sends out descending fibers in the lateral vestibulospinal tract. These second-order vestibular processes terminate on motor neurons in the spinal cord and provide the excitatory drive to maintain body posture. The lateral vestibular nucleus also receives inhibitory innervation from the cerebellum. This serves to coordinate and control the normal excitatory output of the lateral vestibular nucleus. Patients who have head injuries that damage the incoming cerebellar fibers to the vestibular nuclei suffer a pronounced motor imbalance in the extremities called decerebrate rigidity. Decerebration (loss of input from higher centers) removes the inhibitory input from the cerebellum and other cortical areas, leaving the motor neurons exposed
cranial nerves: -trochlear occulomotor abducens reticular formation
medial vestibulospinal tract to motor neurons innervating neck muscles
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