| Cochlear nerve | |
|---|---|
 Diagrammatic longitudinal section of the cochlea. (Cochlear nerve is in center, shown as striped.) | |
 Part of the cochlear division of the acoustic nerve, highly magnified. | |
| Details | |
| From | Vestibulocochlear nerve |
| Identifiers | |
| Latin | nervus cochlearis |
| MeSH | D003056 |
| TA98 | A14.2.01.133 |
| TA2 | 6318 |
| FMA | 53431 |
| Anatomical terms of neuroanatomy | |
Thecochlear nerve (alsoauditory nerve oracoustic nerve) is one of two parts of thevestibulocochlear nerve, acranial nerve present inamniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information from thecochlea of theinner ear directly to thebrain. The other portion of the vestibulocochlear nerve is thevestibular nerve, which carries spatial orientation information to the brain from thesemicircular canals, also known as semicircular ducts.[1]
In terms of anatomy, an auditorynerve fiber is eitherbipolar orunipolar, with its distal projection being called theperipheral process, and its proximal projection being called theaxon; these two projections are also known as the "peripheral axon" and the "central axon", respectively. The peripheral process is sometimes referred to as adendrite, although that term is somewhat inaccurate. Unlike the typical dendrite, the peripheral process generates and conductsaction potentials, which then "jump" across the cell body (orsoma) and continue to propagate along the central axon. In this respect, auditory nerve fibers are somewhat unusual in that action potentials pass through the soma. Both the peripheral process and the axon aremyelinated.
In humans, there are on average 30,000 nerve fibers within the cochlear nerve.[2] The number of fibers varies significantly across species; the domestic cat, for example, has an average of 50,000 fibers. The peripheral axons of auditory nerve fibers form synaptic connections with the hair cells of thecochlea viaribbon synapses using the neurotransmitterglutamate. The central axons form synaptic connections with cells in thecochlear nucleus of the brainstem.
The cell bodies of the cochlear nerve lie within the cochlea and collectively form thespiral ganglion, named for the spiral shape it shares with the cochlea. These central axons exit the cochlea at its base and form anerve trunk, which, in humans, is approximately one inch long. This travels in parallel with the vestibular nerves through theinternal auditory canal, through which it connects to the brainstem. There, its fibers synapse with the cell bodies of thecochlear nucleus.
In mammals, cochlear nerve fibers are classified as either type I or type II.
In mammals, the axons from each cochlear nerve terminate in the cochlear nuclear complex that is ipsilaterally located in the medulla of the brainstem. The cochlear nucleus is the first 'relay station' of the central auditory system and receives mainly ipsilateral afferent input.
The three major components of the cochlear nuclear complex are (see figure below):
Each of the three cochlear nuclei are organized tosort sound according to a specific spacial arrangement. As such, sound frequencies detected by the cochlea are transmitted electrically to specific positions in the cochlear nuclei. The axons from the low-frequency region of the cochlea project to the ventral portion of the dorsal cochlear nucleus and the ventrolateral portions of the anteroventral cochlear nucleus. The axons from the high-frequency region project to the dorsal portion of the anteroventral cochlear nucleus and the uppermost dorsal portions of the dorsal cochlear nucleus. The axons from the intermediate frequency region project to intermediate targets. Through this process, a spatial representation of sound is created by electrical nerve impulses through the cochlear complex.
