Candi Smith-Wiggleswire
2-12-2009
Adaptation of Sensory Receptors

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.