4 Neural processing of auditory information
In this section we will look at how the frequency selectivity found along the basilar membrane is preserved or modified by the auditory nerve and how information about the intensity of the signal is encoded in the response of the auditory nerve fibres.
The nerve that communicates with or innervates the hair cells along the basilar membrane is called the vestibulocochlear nerve or VIIIth cranial nerve. It enters the brainstem just under the cerebellum and conveys information from the hair cells in the inner ear as well as from the vestibular organs of the inner ear. The cochlear portion of the nerve (auditory nerve ) contains two basic types of auditory nerve fibres: afferent fibres that carry information from the peripheral sense organ (organ of Corti) to the brain; and efferent fibres that bring information from the cerebral cortex to the periphery. Afferent fibres arise from nerve cell bodies in the spiral (or cochlear) ganglion (Figure 21) and contact the hair cells. The hair cells themselves do not have axons and therefore do not generate action potentials. Action potentials are first produced by the axons of afferent fibres. Recall that about 10 per cent of the ion channels are open when the hair cell is unstimulated (Section 3.5). This means that in the auditory nerve, there is a continuous low level of discharge of action potentials even when hair cells are unstimulated. Depolarisation of hair cells in response to stereocilia shearing causes an increase in the discharge rate of action potentials above this spontaneous rate (excitation) while hyperpolarisation of hair cells leads to a decrease in the discharge rate of action potentials below the spontaneous discharge rate (inhibition) (Figure 22).
The inner hair cells are innervated by 95 per cent of the afferent fibres. In humans an average of 8 fibres innervate 1 inner hair cell. They therefore make a many-to-one connection with inner hair cells (Figure 21). The other 5 per cent of afferents innervate the outer hair cells.