Mechanoreceptors are sense organs that respond to mechanical stimuli, impinging upon them as forces or movements. They include receptors responsible for the senses of touch, hearing, acceleration, gravity, proprioception (sensitivity to movements and forces within the body) and pressure. It is now generally assumed that the transduction process in mechanoreceptors involves a direct action of the mechanical stimulus on ion channels in the membrane of the sensory cell or nerve terminal, although in most cases we know very little as yet about the nature of the channels involved.

The acoustico-lateralis system of vertebrates

In vertebrates, the sensory receptors involved in hearing, equilibrium reception and the detection of water movements are all of one basic type and are clearly of common evolutionary origin. In each case the receptor cells, or hair cells (fig. 14.1) possess fine processes, stereocilia or ‘hairs’, whose movement leads to modification of the sensory output. The receptor cells are all connected to sensory neurons which the enter the brain via the cranial nerves; neurons concerned with hearing and balance enter via the eighth nerve and those concerned with the detection of water currents enter via the various lateralis branches.

It is a fascinating feature of the acoustico-lateralis system that we can see how a particular type of receptor cell can be used to respond to a whole variety of different mechanical stimuli, largely by evolutionary elaboration of the accessory structures in the various sense organs (see fig. 14.2).