Definition

The vestibular nuclei function in the maintenance of equilibrium and posture, the perception of head position and acceleration, as well as general muscle tone.

Current Knowledge

The vestibular nuclei function in conjunction with the cerebellum to maintain equilibrium and posture, convey perception of head position and acceleration, and modify muscle tone. Bipolar neurons of the vestibular ganglion (also called Scarpa’s ganglion) receive input through peripheral branches coursing from specialized receptor cells in the semicircular canals and the utricle and saccule. Axons from the vestibular ganglion come together with axons projecting from the auditory neurons to form the vestibulocochlear nerve (cranial nerve VIII). The vestibular neurons project to the four vestibular nuclei, that is, superior, inferior, medial, and lateral, which lie under the floor of the fourth ventricle in the pons and rostral medulla.

The lateral vestibular nucleus receives input from the semicircular canals and utricle. Its neurons form the lateral vestibulospinal tract, which extends the length of the spinal cord, and end in the medial parts of the ventral horn of the spinal gray. Tonic excitation of those neurons affects motor neurons that innervate gravity-opposing muscles in the limbs. The medial and superior vestibular nuclei receive input from the semicircular canals. The medial vestibulospinal tract arises from medial vestibular nucleus, and these axons make connections in the cervical region of the spinal cord with motor neurons that innervate muscles of the neck. This pathway is important in mediating the reflex movements of the neck that help stabilize the position of the head in space. Ascending fibers from the superior and medial vestibular nuclei go to the motor nuclei of the oculomotor, trochlear, and abducens cranial nerves which supply the muscles of the eyes. This pathway also mediates the vestibule-ocular reflex in which eye movements are adjusted automatically for changes in head position. Ascending pathways also relay information via nuclei of the thalamus to the cerebral cortex. The inferior vestibular nucleus receives input from the semicircular canals, saccule, utricle, as well as the vermis of the cerebellum. This nucleus appears to be a site where vestibular inputs are integrated with inputs from other sensory systems as well as inputs from the cerebellum. Axons from the inferior vestibular nucleus project into the vestibulospinal and vestibuloreticular pathways. Finally, there also exist commissural projections from the contralateral vestibular nuclei to principally the superior and medial vestibular nuclei which have inhibitory influences on contralateral vestibular neurons. They also are important in vestibular compensation, a process by which reflexes and postural control that are impaired because of unilateral loss of vestibular receptor function through trauma or disease are gradually restored.

Skull fractures that pass through the internal auditory meatus can sever the eighth cranial nerve and result in rapid unilateral removal of the function of one labyrinth. When this occurs, one experiences acute symptoms including extreme dizziness, nausea and vomiting, deviation toward the side of the lesion when walking, and a brisk spontaneous nystagmus. Eventually, there is adaption to having only one vestibular labyrinth through vestibular compensation that begins almost immediately. This appears to be a learned modification in the reflexes such that the unbalanced inputs from the vestibular system are ignored and visual and proprioceptive inputs are relied upon completely.

Cross References

Cranial Nerves

Eye Fields

Vestibular System

Vestibulocochlear Nerve