Functions of the basilar membrane

The basilar membrane sits inside of the cochlea, and contains over $10,000$‍ sensory hair cells that project axons into what eventually becomes the auditory nerve. The basilar membrane varies in stiffness, and is stiffest nearest the oval window, and floppiest near the apex. This allows it to function as a frequency spectrum analyzer. When exposed to a high frequency signal, the basilar membrane resonates where it is stiff, resulting in the excitation of nerve cells close to the oval window. Likewise, low frequency sounds excite nerve cells at the far end of the basilar membrane. This makes specific fibers in the cochlear nerve respond to specific frequencies. This organization is called the place principle, and is preserved throughout the auditory pathway into the brain.

Another information encoding scheme is also used in human hearing, called the volley principle. Nerve cells transmit information by generating brief electrical pulses called action potentials. A nerve cell on the basilar membrane can encode audio information by producing an action potential in response to each cycle of the vibration. For example, a $200$‍-hertz sound wave can be represented by a neuron producing $200$‍ action potentials per second. However, this only works at frequencies below about $500$‍ hertz, the maximum rate that neurons can produce action potentials. The human ear overcomes this problem by allowing several nerve cells to take turns performing this single task. For example, a $3000$‍-hertz tone might be represented by ten nerve cells alternately firing at $300$‍ times per second. This extends the range of the volley principle to about $4$‍ kHz, above which the place principle is exclusively used. (Adapted from: http://www.dspguide.com/ch22/1.htm)