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Receptors and Stimulation

The first step in sensing the world is to translate energy from the environment into information that can be used by the brain. The first link in the chain, for every sensory system, is a specialized type of cell called a sensory receptor.

What is a sensory receptor?

A sensory receptor is a living cell that responds to a particular type of energy. Receptors are often housed in a sense organ, a specialized structure that is constructed in such a way that it collects one type of energy particularly well, so the receptor cells can respond to it. The eye and the ear are examples of sense organs. The skin may also be considered a sense organ (for the sense of touch), and some of our other senses such as the sense of balance or equilibrium have sense organs that are hidden inside the body.

What are the four basic types of receptors?

Four types of receptors are known. Photoreceptors are those sensitive to light. Chemoreceptors respond to particular chemicals or molecular shapes of particles. Mechanoreceptors respond to physical touch or movement. Finally, magnetoreceptors respond to magnetic fields. Magnetoreceptors are found in species such as honeybees, mollusks, hornets, salmon, tuna, turtles, salamanders, and homing pigeons. Receptors for magnetoreception contain magnetite, a mineral which is magnetic itself and that therefore can respond to magnetic fields. Humans have receptors that contain magnetite, but there is no evidence yet that humans can detect magnetic fields.

What is a stimulus? A specific stimulus?

A receptor responds to a stimulus. The word stimulus comes from the Latin stylus for prod or poke and can refer to any measurable input to the nervous system. Each type of receptor responds with exquisite sensitivity to a particular type of stimulation. The form of energy to which a receptor is most sensitive is called the specific stimulus for that receptor.

Although each receptor has a specific stimulus, it can be made to respond to other types of stimulation that are very strong. For example, a poke in the eye (a mechanical stimulus) activates photoreceptors in the eye, producing a flash of light (the proverbial "stars" a person sometimes sees immediately after a blow to the head). However, photoreceptors are billions of times more sensitive to light than they are to touch. Therefore light is the specific stimulus for photoreceptors.

What does it mean to say that senses are "optimized"?

The senses of most animals are optimized. In other words, they are as sensitive as they could possibly be without causing problems for the organism.

Rob de Ruyter showed this by embedding a fly in orange wax and monitoring the neurons of its visual system for the 3 to 4 days the fly survived. Ruyter found that neurons in the fly's visual system were so sensitive that they were close to the theoretical limit imposed by their optical system and bandwidth of neural pathway in which they operated (Flam, 1993).

What would we hear, if our ears were more sensitive?

The same is true of human senses. Human hearing, for example, is so sensitive that if it was any more powerful, we would hear the movements of molecules in the air (Brownian movement) hitting our eardrums.

How sensitive are human photoreceptors?

Light can be conceived as waves (light waves) or as a stream of particles (photons). Photons are extremely tiny units of energy. A 50 watt incandescent bulb, almost too dim for reading, puts out 3,800,000,000,000,000,000 photons per second. Yet a single photoreceptor in the human eye can respond to three or four photons, when the eye is completely adapted to darkness.

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