3.7 Hair cell tuning
We have determined that the location of the peak of the travelling wave on the basilar membrane is determined by the frequency of the originating sound. The hair cells run the length of the basilar membrane. When a certain frequency sound stimulates a point on the membrane, it responds by moving, and the hair cells at that site are stimulated by the shearing force that this movement creates (Figure 18). Groups of hair cells therefore only respond if certain frequencies are present in the originating sound. The frequency sensitivity of a hair cell can be displayed as a tuning curve. To construct a tuning curve, a single hair cell is stimulated repeatedly with pure tone stimuli of various frequencies. For each frequency, the intensity of the stimulus is adjusted until the response of the hair cell reaches some predefined level. The tuning curve is then the graph of sound intensity against stimulus frequency (Figure 19). Tuning curves for hair cells are characteristically V-shaped. The tip represents the frequency to which the cell is most sensitive.
A sound of this frequency will elicit a response from the cell even when it is of very low intensity. Sounds of greater or lesser frequency require higher intensity to excite the cell to the predetermined level.
Adjacent piano strings are tuned to frequencies some 6 per cent apart. On average, successive inner hair cells differ in characteristic frequency by about 0.2 per cent.
The great majority of neurons that carry information from the cochlea to higher levels of the auditory system connect to the inner hair cells. Thus most, if not all, information about sounds is conveyed to the brain via the inner hair cells. Given that the outer hair cells greatly outnumber the inner hair cells, it seems paradoxical that most cochlear output is derived from the inner cells. However, ongoing research suggests that outer hair cells do play an important role in the transduction process. Membranes of the outer cells contain a motor protein that changes the length of the outer hair cells in response to stimulation. This change in length effects a change in the mechanical coupling between the basilar and tectorial membranes. Outer hair cells are sometimes said to constitute a cochlear amplifier by amplifying the response of the basilar membrane. This causes the sterocilia on the inner cells to bend more, creating a bigger response in the auditory nerve (Figure 20).