Cochlear nerve

At a glance

English

Cochlear nerve

Latin (TA2)

Nervus cochlearis

System

Nerve

Region

Head

Side

Single, on the midline

About the cochlear nerve

Also called the auditory or acoustic nerve, the cochlear nerve forms one of the two components of the eighth cranial nerve; its partner, the vestibular nerve, carries signals about head position and movement. The cochlear nerve is purely sensory and carries information about sound from the spiral organ to the brainstem.

Its nerve cells sit inside the cochlea, in the spiral ganglion, which winds around the bony core (the modiolus) following the turns of the cochlea. Each cell has two myelinated processes. The peripheral one reaches the hair cells of the spiral organ, where it receives glutamate released at specialised ribbon synapses. The central one runs down through the modiolus to the base of the cochlea, where the fibres gather into a trunk about 2.5 cm long. This trunk passes through the internal acoustic meatus with the vestibular nerve and the facial nerve, crosses the cerebellopontine angle and enters the brainstem at the junction of pons and medulla. Unusually, the electrical signal travels straight through the cell body rather than starting beyond it, so the peripheral process behaves more like an axon than a dendrite.

Two classes of fibre are recognised. Type I fibres form the great majority: large, myelinated and bipolar, each contacting a single inner hair cell, while each inner hair cell receives many of them. These carry most of what we hear. Type II fibres are few, thin, unmyelinated and unipolar, and each branches to many outer hair cells.

All fibres end in the cochlear nuclei on the same side, at the dorsolateral margin of the upper medulla: the dorsal, anteroventral and posteroventral nuclei. The cochlea's map of pitch is kept here, with high and low frequencies reaching different parts of each nucleus. Damage to the nerve, most often from a vestibular schwannoma (acoustic neuroma) growing in the meatus, causes hearing loss, ringing in the ear and sometimes imbalance on that side; the loss is sensorineural, so bone conduction is reduced as well as air conduction.

Course and supply

Arises from

The bipolar cells of the spiral ganglion in the modiolus of the cochlea, whose peripheral processes contact the hair cells of the spiral organ.

Ends in

Ends in the dorsal, anteroventral and posteroventral cochlear nuclei of the same side, at the pontomedullary junction.

Supplies

Special sensory: carries hearing (sound frequency and loudness) from the cochlea to the brainstem.

Blood supply

The labyrinthine artery, usually a branch of the anterior inferior cerebellar artery, which travels with the nerve in the internal acoustic meatus.