Mechanoreceptors respond to physical deformations of their membrane produced by touch, stretch, pressure, motion, and sound. Mechanoreceptors are found in all multicellular animals, and even bacteria have pressure-sensitive protein receptors in their cell membrane. In both bacteria and multicellular organisms, evidence suggests that the protein receptor is also a sodium ion channel. Thus, deformation of the receptor membrane opens sodium channels directly, causing a depolarization of the endings of the cell’s dendrites (Fig. 36.2b). Early pressure-sensitive protein receptors in bacteria sensed internal cell pressure: When water moving into the cell by osmosis increases the risk of bursting (Chapter 5), the stretching of the membrane opens channels that let water leave the cell. Other mechanoreceptors in roundworms and anemones are linked to externally projecting cilia that sense forces at the animal’s body surface.

One well-studied example of a mechanoreceptor is a touch receptor in the roundworm Caenorhabditis elegans. Roundworms, also called nematodes, are one of the most diverse and widespread groups of animals on Earth. Deformation of the surface of the worm—its cuticle—exerts pressure on proteins connected to an ion channel. The mechanical force changes the shape of the ion channel, causing it to open and produce a change in membrane potential.

A mechanoreceptor in humans and other mammals is the sensory receptor found in the skin that senses touch and pressure. The cell bodies of these neurons are located in ganglia near the spinal cord, with extensions going in two directions: to the skin and to the spinal cord. In the skin, the neuron has branched tips containing ion channels sensitive to deformations of the membrane. These branched tips are the initial sensors of touch and pressure. If the stimulus is strong enough, local depolarization leads to the firing of an action potential that travels all the way to the spinal cord. Thus, in contrast to taste chemoreceptors, this type of mechanoreceptor transmits an action potential. The axon is much too long for simple depolarization to spread to its other end.

Mechanoreceptors are also found at the base of whiskers that rodents, cats, dogs, and other mammals use to sense touch with their snouts. Stretch receptors found in muscles are also mechanoreceptors that influence a muscle’s motor activation, helping to control its length and force. A very different group of specialized mechanoreceptors called hair cells are the sensory receptors for balance, gravity sensing, and hearing, discussed in section 36.3.