The vestibulocochlear nerve is a cranial nerve that supplies both hearing and balance information to the brain. All cranial nerves are paired, meaning one nerve supplies the right side and the other nerve supplies the left. The vestibulocochlear nerve is the eighth of 12 paired cranial nerves and is called cranial nerve VIII. The vestibulocochlear nerves are purely sensory and have two divisions: vestibular and cochlear. The vestibular portion deals with the sensation of balance, while the cochlear division is used for auditory information.
Anatomy and Physiology
Within the skull and deep to the external ear, the vestibulocochlear nerve’s sensory receptors are located in the membranous labyrinth. This is a very delicate small structure that is tubular, filled with endolymph, and connected to a series of tunnels within the petrous portion of the temporal bone. The tunnel walls are called the bony labyrinth, which is connected to the inner ear by two openings, the oval window with the stapes bone and the round window with a flexible membrane called the round window membrane. The stapes vibrates when a sound wave hits it, resulting in a pressure wave into both the bony and membranous labyrinths. This pressure wave travels through the channels and makes the round window membrane vibrate. In turn, the endolymph moves, resulting in the sound wave being propagated.
The sensory receptors in the cochlea and vestibular structures are small and can be easily damaged. The outputs of these sensory receptors travel a short distance to their receiving neurons, which are located in the cochlea and the semicircular canals. The cells in the cochlea make up the spinal ganglion, while the neurons in the base of the semicircular canals make up the vestibular ganglion. These neurons’ axons bundle and travel together, making up cranial nerve VIII. The nerve travels through the internal auditory meatus with the facial nerve. Both the vestibulocochlear and facial nerves then enter the brainstem at the junction of the pons and medulla. This is where the two divisions of the vestibulocochlear nerve begin to diverge from each other.
Vestibular Component of the Nerve
The fibers from the vestibular division terminate in the vestibular nuclear complex within the floor of the fourth ventricle. This makes up the vestibulocerebellar tract. The axons from the vestibular nuclear complex terminate in several nuclei within the brainstem and spinal cord to affect the muscles used for maintaining balance. The lateral vestibulospinal tract is made of ipsilateral fibers from the lateral vestibular nucleus to terminate down the spinal cord onto neurons that control extensor muscles. The medial and inferior vestibular nuclei have shared connections to the cerebellum to control and coordinate balance while the body is moving. Finally, all vestibular complex nuclei project to the three cranial nerve nuclei used to control the muscles of the eyes. This makes up the medial longitudinal fasciculus, which is critical in maintaining the body’s orientation in space as well as maintaining fixation of an object during head movement. These fibers terminate onto both the left and right nuclei of cranial nerves III, IV, and VII. These interconnections show how the vestibular division is highly integrated with vision.
Cochlear Component of the Nerve
The fibers from the cochlear division terminate in the dorsal and ventral cochlear nuclei. The dorsal cochlear nucleus receives high-frequency information, while the ventral cochlear nucleus receives information about low frequencies. The pathway to the cerebral cortex from here is not well understood. However, a few synapses have been studied in patients with cortical deafness, as described in the following. The outputs of the dorsal and ventral cochlear nuclei cross to the other side of the brainstem and ascend toward the brain. This forms the lateral lemniscus, a tract of ascending axons to the cerebral cortex. Some axons cross over and synapse in the contralateral trapezoid body or superior olivary nucleus before joining the lateral lemniscus. There are few fibers that do not cross over and terminate in the ispilateral superior olivary nucleus and ascend in the ipsilateral lateral lemniscus. From here, axons of both the left and right lateral lemnisci terminate in the inferior colliculus. The inferior colliculus axons terminate into the thalamus, which sends its axons to the transverse temporal gyrus. It is here where sound is interpreted in the brain.
Clinical Symptoms and Treatment
If the vestibular component of the nerve is damaged, the result is reduction or complete loss of balance. In particular, patients will feel dizzy, may fall more often, and have abnormal eye movements. Patients may also have nausea, causing them to vomit. The most common lesion to the vestibulocochlear nerve is a tumor of the Schwann cells surrounding the nerve. Removal of the tumor may be necessary for balance function to return. Finally, patients may be trained to overcome their balance deficit by using their eyes more effectively, as vision can override vestibular issues. This is because the eyes can see the horizon and realize that it is stable and not truly moving.
If the cochlear component of the nerve is damaged, the result is reduction or complete loss of hearing. This usually occurs from skull fractures or ear infections. Tumors can occur in the internal auditory meatus, which can damage both divisions of the vestibulocochlear nerve as well as the facial nerve. Finally, lesions to the lateral lemniscus typically result in a characteristically partial deafness on the contralateral side. This is because the small amount of ipsilateral fibers is spared on the affected side and can carry the auditory information to the brain. Treatment may include antibiotics to heal the infection or hearing aids.
Jennifer L. Hellier
See also: Auditory System; Balance; Cochlea; Cranial Nerves; Dizziness; Nerves; Vestibular System
Liang, Barbara. (2012). The 12 cranial nerves. Retrieved from http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP11504
Yale University School of Medicine. (1998). Cranial nerves. Retrieved from http://www.yale.edu/cnerves/