Vestibulocochlear nerve (VIII)

At a glance

English

Vestibulocochlear nerve (VIII)

Latin (TA2)

Nervus vestibulocochlearis (VIII)

System

Nerve

Region

Head

Side

Paired, left and right

About the vestibulocochlear nerve (viii)

Cranial nerve VIII is a special sensory nerve made of two divisions that travel together: the cochlear nerve for hearing and the vestibular nerve for balance. Both consist largely of bipolar neurons and arise from the otic placode of the embryo. Older books call it the acoustic or auditory nerve, names now avoided because they ignore its role in balance. A small set of efferent fibres, the olivocochlear bundle, runs the other way, from the superior olivary complex to the cochlea, and adjusts the sensitivity of its hair cells.

The cochlear nerve starts in the spiral ganglion inside the modiolus of the cochlea, whose cells collect signals mainly from inner hair cells in the spiral organ (of Corti). The vestibular nerve starts in the vestibular ganglion and supplies five receptor patches: the cristae of the three semicircular canals, which respond to rotation, and the maculae of the utricle and saccule, which respond to linear acceleration and gravity. The two divisions join in the internal acoustic meatus, which they share with the facial nerve and the labyrinthine artery, usually a branch of the anterior inferior cerebellar artery.

Leaving the meatus, the nerve crosses the subarachnoid space of the cerebellopontine angle, the corner where pons, medulla and cerebellum meet, and enters the brainstem at the pontomedullary junction, lateral to the facial nerve. Cochlear fibres end in the dorsal and ventral cochlear nuclei; vestibular fibres end in the vestibular nuclei and partly in the cerebellum.

Damage can cause hearing loss, tinnitus, vertigo, nystagmus and unsteadiness, especially in the dark when vision cannot make up for the loss. A vestibular schwannoma in the angle classically begins with one-sided hearing loss and ringing and may later press on the facial and trigeminal nerves. Hearing is screened at the bedside with tuning forks. In the Rinne test, air conduction normally exceeds bone conduction; if bone is better, the loss is conductive. In the Weber test the sound is heard louder in a conductively deaf ear but in the better ear with sensorineural loss. Formal assessment adds otoscopy, tympanometry, otoacoustic emissions, pure-tone audiometry and speech testing.

Course and supply

Arises from

The spiral ganglion of the cochlea and the vestibular ganglion in the internal acoustic meatus; the nerve enters the brainstem at the pontomedullary junction, in the cerebellopontine angle.

Ends in

Centrally the cochlear nuclei and vestibular nuclei of the pons and medulla, with some vestibular fibres going to the cerebellum; peripherally the spiral organ, maculae and ampullary crests.

Supplies

Special sense of hearing through the cochlear division and of balance and head movement through the vestibular division; efferent olivocochlear fibres tune the cochlea.

Blood supply

The labyrinthine artery, normally a branch of the anterior inferior cerebellar artery, accompanies it through the internal acoustic meatus.