In humans, the number of nerve fibers within the cochlear nerve averages around 30,000. The number of fibers varies significantly across species—for example, the domestic cat has some 50,000 fibers. Auditory nerve fibres provide synaptic connections between the hair cells of the cochlea and the cochlear nucleus within the brainstem. The cell bodies of the cochlear nerve lie within the central aspect of the cochlea and are collectively known as the spiral ganglion. This name reflects the fact that the cell bodies, considered as a unit,has a spiral (or perhaps more accurately, a helical) shape, reflecting the shape of the cochlea. The terms "cochlear nerve fiber" and "spiral ganglion cell" are used, to some degree, interchangeably, although the former may be used to more specifically refer to the central axons of the cochlear nerve. These central axons exit the cochlea at its base, where it forms a nerve trunk. In humans, this aspect of the nerve is roughly one inch in length. It projects centrally to the brainstem, where its fibers synapse with the cell bodies of the cochlear nucleus. A good anatomical description of human auditory nerve fibers is provided by Spoendlin and Schrott (1985). Important earlier work was done by Schuknecht.
It was once believed that most of the cochlear nerve fibres were directed to the outer hair cells, but it is now understood that at least 90% of the cochlear ganglion cells terminate on inner hair cells, the rest terminating on the outer hair cells.
The transmission between the inner hair cells and the neurons is chemical, using glutamate as a neurotransmitter.
The three major components of the cochlear nuclear complex are: (see figure below)
Each of the three cochlear nuclei are tonotopically organised. The axons from the lower frequency area of the cochlea innervate the ventral portion of the dorsal cochlear nucleus and the ventrolateral portions of the anteroventral cochlear nucleus, while the higher frequency axons project into the dorsal portion of the anteroventral cochlear nucleus and the uppermost dorsal portions of the dorsal cochlear nucleus. The mid frequency projections end up in between the two extremes, in this way the frequency spectrum is preserved.
Nonsyndromic Isolated Unilateral Cochlear Nerve Aplasia Without Narrow Internal Auditory Meatus: A Previously Overlooked Cause of Unilateral Profound Deafness in Childhood
Nov 01, 2005; Objectives: Juvenile or adolescent unilateral profound sensorineural deafness (worldwide prevalence, 0.1% to 0.2%) has been...
Auditory Brainstem Implantation after Unsuccessful Cochlear Implantation of Children with Clinical Diagnosis of Cochlear Nerve Deficiency
Oct 01, 2013; Objectives: We compared the perceptual auditory abilities of 21 children with suspected cochlear nerve deficiency (CND) and a...
Imaging case study of the month: Does cochlear nerve aplasia always occur in the presence of a narrow internal auditory canal?
Apr 01, 2001; The current approach to management of deafness in childhood is to identify the cause of the auditory deficit and its exact...