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3.4 The organ of Corti and hair cells

We have established that the vibration patterns of the basilar membrane carry information about frequency, amplitude and time. The next step is to examine how this information is converted or coded into neural signals in the auditory nervous system. To do so, we must look at the organ of Corti in some detail since it is here that the auditory receptor cells that convert mechanical energy into a change in membrane polarisation are located.

As we saw in Section 2, the receptor cells responsible for the transduction of mechanical energy into neural energy are called hair cells. A typical hair cell is shown in Figure 13. The outer hair cells are closest to the outside of the cochlea and are arranged in 3 rows whereas the inner hair cells form a single row (see Figure 7). There are about 12 000 outer hair cells and about 3500 inner hair cells in the human ear. The tips of the tallest row of cilia of each hair cell are in contact with the tectorial membrane which is situated at the top of the organ of Corti and has a soft, ribbon-like structure.

Figure 13
Figure 13 The structure of a hair cell. Inside the cell body can be seen the nucleus, a synaptic bar, and synaptic vesicles which carry the neurotransmitter molecules