Hearing
Hearing

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Hearing

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).

Figure 21
Figure 21 Innervation of the organ of Corti. Afferent fibres arise from nerve cell bodies within the spiral ganglion. Ninety-five per cent of afferents contact inner hair cells, each of which consists of the sole terminus for up to ten axons. Five per cent of afferents contact the outer hair cells
Figure 22
Figure 22 In unstimulated hair cells there is a low level of discharge of action potentials in the axons of the auditory nerve fibres. When the cell is stimulated, depolarisation results in an increase in the discharge rate of action potentials (excitation) while hyperpolarisation results in a decrease in discharge rate of action potentials (inhibition)

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.

SD329_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has over 40 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus