Cochlear nerve

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20 Cochlear nerve

Auditory System

The auditory system comprises the cochlea, the cochlear nerve, and the central auditory pathway from the cochlear nuclei in the brainstem to the cortex of the temporal lobe. The central auditory pathway is more elaborate than the somatosensory or visual pathway. This is because the same sounds are detected by both ears. In order to signal the location of a sound, a very complex neuronal network is in place, with numerous connections (mainly inhibitory) between the two central pathways in order to magnify minute differences in intensity and timing of sounds that exist during normal, binaural hearing.

The cochlea

The main features of cochlear structure are seen in Figures 20.1 and 20.2. The cochlea is pictured as though it were upright, but in life it lies on its side, as shown earlier in Figure 19.1. The central bony pillar of the cochlea (the modiolus) is in the axis of the internal acoustic meatus. Projecting from the modiolus, like the flange of a screw, is the osseous spiral lamina. The basilar membrane is attached to the tip of this lamina; it reaches across the cavity of the bony cochlea to become attached to the spiral ligament on the outer wall. The osseous spiral lamina and spiral ligament become progressively smaller as one ascends the two and one half turns of the cochlea, and the fibers of the basilar membrane become progressively longer.

The basilar membrane and its attachments divide the cochlear chamber into upper and lower compartments. These are the scala vestibuli and the scala tympani, respectively, and they are filled with perilymph. They communicate at the apex of the cochlea, through the helicotrema. A third compartment, the scala media (cochlear duct), lies above the basilar membrane and is filled with endolymph. It is separated from the scala vestibuli by the delicate vestibular membrane.

Sitting on the basilar membrane is the spiral organ (organ of Corti). The principal sensory receptor epithelium consists of a single row of inner hair cells, each one having up to 20 large afferent nerve endings applied to it. The hair cells rest upon supporting cells, and there are ancillary cells too. The organ of Corti contains a central tunnel, filled with perilymph diffusing through the basilar membrane. On the outer side of the tunnel are several rows of outer hair cells, attended by supporting and ancillary cells.

All of the hair cells are surmounted by stereocilia. Unlike the vestibular hair cells, they have no kinocilium in the adult state. The stereocilia of the outer hair cells are embedded in the overlying tectorial membrane. Those of the inner hair cells lie immediately below the membrane.

The outer hair cells are contractile (in tissue culture), and they have substantial efferent nerve endings (Figure 20.2). In theory, at least, oscillatory movements of outer hair cells could influence the sensitivity of the inner hair cells through effects on the tectorial or basilar membrane.

Cochlear nerve

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