sensory receptors account for our ability to see, hear, taste, and smell, and to sense touch, pain, temperature, and body position. They also provide the unconscious ability of the body to detect changes in blood volume, blood pressure, and the levels of salts, gases, and nutrients in the blood.

These specialized cells are exquisitely adapted for the detection of particular physical or chemical events outside the cell. They are connected to nerve cells, or are themselves nerve cells. Many of them are enclosed in sense organs. Others are the endings of nerve fibres that ramify within the skin, the muscles, bones, and joints and the other organs of the body. Yet others are nerve cells within the brain that are sensitive temperature, to dissolved gases, salts, and other substances in the fluid around them.

In human beings there are just four basic types of sensory receptor — sensitive to mechanical stimulation, light, chemicals, and temperature — but they vary enormously in their form. The particular kind of stimulus to which they respond is largely determined by the structure of the sense organ around them or by their location in the body. Some animals have receptors sensitive to magnetic fields or to electrical fields.

All sensory receptors in the human body operate on the same general principles. Their membranes contain particular protein molecules that are activated and change their shape when the appropriate physical force or chemical substance comes into contact with them. For instance, light falling on the retina causes rotation of a small part of molecules called photopigments, which lie within the internal membranes of the rod and cone receptor cells. Olfactory neurons in the nose have fine hairs covered in a huge variety of protein molecules to which inhaled odorant molecules attach in a ‘lock-and-key’ fashion. Specialized proteins in the membranes of hair cells in the cochlea and the vestibular apparatus of the inner ear are sensitive to the mechanical forces caused by sound or movements of the head, respectively. Other types of mechanoreceptive nerve endings that detect touch and vibration of the skin, movements and stretch of muscles, tendons, and joints, and the pressure of blood in the blood vessels and heart, employ similar stretch-sensitive proteins in their membranes.

Activation of the specific protein receptors in the cell membrane is followed by a sequence of reactions, collectively called transduction, leading to the initiation of nerve impulses (action potentials), which are transmitted along a fibre towards (or within) the central nervous system. The essential step, resulting directly or indirectly from the activation of the receptor molecules, is the opening or closing of tiny pores (ion channels) in the cell membrane. This causes a change in the movement of charged ions (usually sodium ions), which alters the voltage inside the receptor cell. The amplitude of this receptor potential varies with the intensity of the stimulus. This then leads to the firing of nerve impulses, either in the sensory cell itself or in an adjacent nerve cell. The sensory stimulus is thus translated into a train of impulses whose frequency varies with the stimulus strength.

The sensitivity of a sensory receptor usually depends on how much it has recently been stimulated. Hence, if a receptor (say a nerve fibre in the skin) is exposed to a constant stimulus (such as pressure on the skin) the rate of nerve impulses quickly falls to a much lower level, or even ceases altogether. This phenomenon, called adaptation, leads receptors to be more sensitive to changing than to steady stimulation. Hence they usually measure the stimulus as a percentage of its deviation from the background signal, rather than signalling its absolute intensity. This means that our sensory receptors are sensitive to small changes in signal strength but tune out constant signals. Everyone is familiar with this effect: it is the reason you cease to notice a constant background noise, quickly desensitize to strong smells, and are gradually able to see in a darkened room after leaving one that is brightly lit. It is one example of the way in which our sensory systems ‘economize’ in their use of nerve impulses.

Frances M. Ashcroft, and Colin Blakemore

See also baroreceptors; chemoreceptors; eyes; hearing; sense organs; somatic sensation; taste and smell; vestibular system; vision.