ABSTRACT
The inner ear is a transducer of mechanical force to appropriate neural excitation. The key element is the receptor cell, or hair cell, which has cilia on the apical surface and afferent (and sometimes efferent) neural synapses on the lateral walls and base. Generally for hair cells, mechanical displacement of the cilia in the forward direction toward the tallest cilia causes the generation of electrical impulses in the nerves, while backward displacement causes inhibition of spontaneous neural activity. Displacement in the lateral direction has no effect. For moderate frequencies of sinusoidal ciliary displacement (20 to 200 Hz), the neural impulses are in synchrony with the mechanical displacement, one impulse for each cycle of excitation. Such impulses are transmitted to the higher centers of the brain and can be perceived as sound. For lower frequencies, however, neural impulses in synchrony with the excitation are apparently confused with the spontaneous, random firing of the nerves. Consequently, there are three mechanical devices in the inner ear of vertebrates that provide perception in the different frequency ranges. At zero frequency, that is, linear acceleration, the otolith membrane provides a constant force acting on the cilia of hair cells. For low frequencies associated with rotation of the head, the semicircular canals provide the proper force on cilia. For frequencies in the hearing range, the cochlea provides the correct forcing of hair cell cilia. In nonmammalian vertebrates, the equivalent of the cochlea is a bent tube, and the upper frequency of hearing is around 7 kHz. For mammals, the upper frequency is considerably higher, 20 kHz for man but extending to almost 200 kHz for toothed whales and some bats. Other creatures, such as certain insects, can perceive high frequencies, but do not have a cochlea nor the frequency discrimination of vertebrates.
