Neural activity is initiated at the border between the nervous system and the outside world by sensory receptors. Since some receptors respond to changes in the internal environment, the "outside world" with regard to the sensory receptors can also mean, for example, distension of a blood vessel in our body.
Information about the external world and about the body's internal environment exists in different energy forms—pressure, temperature, light, sound waves, and so on. Receptors at the peripheral ends of afferent neurons change these energy forms into graded potentials that can initiate action potentials, which travel into the central nervous system. The receptors are either specialized endings of afferent neurons themselves (Figure 9-1a) or separate cells that affect the ends of afferent neurons (Figure 9-1b).
To avoid confusion in the remainder of this chapter, the reader must recall from Chapter 7 that the term "receptor" has two completely different meanings. One meaning is that of "sensory receptor," as just defined. The second usage is for the individual proteins in the plasma membrane or inside the cell to which specific chemical messengers bind, triggering an in-tracellular signal transduction pathway that culminates in the cell's response. The potential confusion between these two meanings is magnified by the fact that the stimuli for some sensory receptors (for example, those involved in taste and smell) are chemicals that bind to protein receptors in the plasma membrane of the sensory receptor. If you are in doubt as to which meaning is intended, add the adjective "sensory" or "protein" to see which makes sense in the context.
To repeat, regardless of the original form of the energy, information from sensory receptors linking the nervous system with the outside world must be translated into the language of graded potentials or action potentials. The energy that impinges upon and activates a sensory receptor is known as a stimulus. The process
L Vesicle containing chemical messenger
Receptor cell —
Sensory receptors. The sensitive membrane that responds to a stimulus is either (a) an ending of an afferent neuron or (b) on a separate cell adjacent to an afferent neuron (highly schematized).
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
The Sensory Systems CHAPTER NINE
by which a stimulus—a photon of light, say, or the mechanical stretch of a tissue—is transformed into an electrical response is known as stimulus transduction.
There are many types of sensory receptors, each of which is specific; that is, each type responds much more readily to one form of energy than to others. The type of energy to which a receptor responds in normal functioning is known as its adequate stimulus.
Specificity exists at still another level. Within the general energy type that serves as a receptor's adequate stimulus, a particular receptor responds best (that is, at lowest threshold) to only a very narrow range of stimulus energies. For example, individual receptors in the eye respond best to photic energy of one range of light wavelengths.
Most sensory receptors are exquisitely sensitive to their specific energy form. For example, some olfactory receptors respond to as few as three or four odor molecules in the inspired air, and visual receptors can respond to a single photon, the smallest quantity of light.
Virtually all sensory receptors, however, can be activated by several forms of energy if the intensity is sufficiently high. For example, the receptors of the eye normally respond to light, but they can be activated by an intense mechanical stimulus, like a poke in the eye. Note, however, that one experiences the sensation of light in response to a poke in the eye. Regardless of how the receptor is stimulated, any given receptor gives rise to only one sensation.
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