Table 1 lists and classifies five basic types of sensory receptors: (1) mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor; (2) thermoreceptors, which detect changes in temperature, with some receptors detecting cold and others warmth; (3) nociceptors (pain receptors), which detect physical or chemical damage occurring in the tissues; (4) electromagnetic receptors, which detect light on the retina of the eye; and (5) chemoreceptors, which detect taste in the mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration, and other factors that make up the chemistry of the body.
Table1. Classification of Sensory Receptors
In this chapter, we discuss the function of a few specific types of receptors, primarily peripheral mechanoreceptors, to illustrate some of the principles by which receptors operate. Other receptors are discussed in other chapters in relation to the sensory systems that they sub serve. Figure 1 shows some of the types of mechanoreceptors found in the skin or in deep tissues of the body.
Fig1. Several types of somatic sensory nerve endings.
DIFFERENTIAL SENSITIVITY OF RECEPTORS
How do two types of sensory receptors detect different types of sensory stimuli? The answer is “by differential sensitivities.” That is, each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to other types of sensory stimuli. Thus, the rods and cones of the eyes are highly responsive to light but are almost completely nonresponsive to normal ranges of heat, cold, pressure on the eyeballs, or chemical changes in the blood. The osmoreceptors of the supraoptic nuclei in the hypothalamus detect minute changes in the osmolality of the body fluids but have never been known to respond to sound. Finally, pain receptors in the skin are almost never stimulated by usual touch or pressure stimuli but do become highly active the moment tactile stimuli become severe enough to damage the tissues.
Modality of Sensation—The “Labeled Line” Principle
Each of the principal types of sensation that we can experience—pain, touch, sight, sound, and so forth—is called a modality of sensation. Yet, despite the fact that we experience these different modalities of sensation, nerve fibers transmit only impulses. Therefore, how do different nerve fibers transmit different modalities of sensation?
The answer is that each nerve tract terminates at a specific point in the central nervous system, and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system to which the fiber leads. For instance, if a pain fiber is stimulated, the person perceives pain regardless of what type of stimulus excites the fiber. The stimulus can be electricity, overheating of the fiber, crushing of the fiber, or stimulation of the pain nerve ending by damage to the tissue cells. In all these instances, the person perceives pain. Likewise, if a touch fiber is stimulated by electrical excitation of a touch receptor or in any other way, the person perceives touch because touch fibers lead to specific touch areas in the brain. Similarly, fibers from the retina of the eye terminate in the vision areas of the brain, fibers from the ear terminate in the auditory areas of the brain, and temperature fibers terminate in the temperature areas.
This specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle.
