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Sensory Receptors, Neuronal Circuits for Processing Information
Our perceptions of signals within our bodies and of the world around us are mediated by a complex system of sensory receptors that detect such stimuli as touch, sound, light, pain, cold, and warmth. In this chapter, we discuss the basic mechanisms whereby these receptors change sensory stimuli into nerve signals that are then conveyed to and processed in the central nervous system.
Types of Sensory Receptors and the Stimuli they Detect
Table 47-1 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 detecting 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.
Table 47-1
Classification of Sensory Receptors
| I.Mechanoreceptors |
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| II.Thermoreceptors |
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| III.Nociceptors |
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| IV.Electromagnetic Receptors |
|
| V.Chemoreceptors |
|
We will discuss the function of a few specific types of receptors, primarily peripheral mechanoreceptors, to illustrate some of the principles whereby receptors operate. Other receptors are discussed in other chapters in relation to the sensory systems that they subserve. Figure 47-1 shows some of the types of mechanoreceptors found in the skin or in deep tissues of the body.
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 example, 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 cases, 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 .
Transduction of Sensory Stimuli Into Nerve Impulses
Local Electrical Currents at Nerve Endings—Receptor Potentials
All sensory receptors have one feature in common. Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane electrical potential of the receptor. This change in potential is called a receptor potential .
Mechanisms of Receptor Potentials
Different receptors can be excited in one of several ways to cause receptor potentials: (1) by mechanical deformation of the receptor, which stretches the receptor membrane and opens ion channels; (2) by application of a chemical to the membrane, which also opens ion channels; (3) by change of the temperature of the membrane, which alters the permeability of the membrane; or (4) by the effects of electromagnetic radiation, such as light on a retinal visual receptor, which either directly or indirectly changes the receptor membrane characteristics and allows ions to flow through membrane channels.
These four means of exciting receptors correspond in general to the different types of known sensory receptors. In all cases, the basic cause of the change in membrane potential is a change in membrane permeability of the receptor, which allows ions to diffuse more or less readily through the membrane and thereby to change the transmembrane potential .
Maximum Receptor Potential Amplitude
The maximum amplitude of most sensory receptor potentials is about 100 mV, but this level occurs only at an extremely high intensity of sensory stimulus. This is about the same maximum voltage recorded in action potentials and is also the change in voltage when the membrane becomes maximally permeable to sodium ions.
Relation of the Receptor Potential to Action Potentials
When the receptor potential rises above the threshold for eliciting action potentials in the nerve fiber attached to the receptor, then action potentials occur, as illustrated in Figure 47-2 . Note also that the more the receptor potential rises above the threshold level, the greater becomes the action potential frequency .
Receptor Potential of the Pacinian Corpuscle—an Example of Receptor Function
Note in Figure 47-1 that the Pacinian corpuscle has a central nerve fiber extending through its core. Surrounding this central nerve fiber are multiple concentric capsule layers; thus, compression anywhere on the outside of the corpuscle will elongate, indent, or otherwise deform the central fiber.
Figure 47-3 shows only the central fiber of the Pacinian corpuscle after all capsule layers but one have been removed. The tip of the central fiber inside the capsule is unmyelinated, but the fiber does become myelinated (the blue sheath shown in the figure) shortly before leaving the corpuscle to enter a peripheral sensory nerve.
Figure 47-3 also shows the mechanism whereby a receptor potential is produced in the Pacinian corpuscle. Observe the small area of the terminal fiber that has been deformed by compression of the corpuscle, and note that ion channels have opened in the membrane, allowing positively charged sodium ions to diffuse to the interior of the fiber. This action creates increased positivity inside the fiber, called the “receptor potential.” The receptor potential in turn induces a local circuit of current flow, shown by the arrows, that spreads along the nerve fiber. At the first node of Ranvier, which lies inside the capsule of the Pacinian corpuscle, the local current flow depolarizes the fiber membrane at this node, which then sets off typical action potentials that are transmitted along the nerve fiber toward the central nervous system.
Relation Between Stimulus Intensity and the Receptor Potential
Figure 47-4 shows the changing amplitude of the receptor potential caused by progressively stronger mechanical compression (increasing “stimulus strength”) applied experimentally to the central core of a Pacinian corpuscle. Note that the amplitude increases rapidly at first but then progressively less rapidly at high stimulus strength.
In turn, the frequency of repetitive action potentials transmitted from sensory receptors increases approximately in proportion to the increase in receptor potential. Putting this principle together with the data in Figure 47-4 , one can see that very intense stimulation of the receptor causes progressively less and less additional increase in numbers of action potentials. This exceedingly important principle is applicable to almost all sensory receptors. It allows the receptor to be sensitive to very weak sensory experience and yet not reach a maximum firing rate until the sensory experience is extreme. This feature allows the receptor to have an extreme range of response—from very weak to very intense.
Adaptation of Receptors
Another characteristic of sensory receptors is that they adapt either partially or completely to any constant stimulus after a period of time. That is, when a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until, finally, the rate of action potentials decreases to very few or to none at all.
Figure 47-5 shows typical adaptation of certain types of receptors. Note that the Pacinian corpuscle adapts very rapidly, hair receptors adapt within a second or so, and some joint capsule and muscle spindle receptors adapt slowly.
Furthermore, some sensory receptors adapt to a far greater extent than others. For example, the Pacinian corpuscles adapt to “extinction” within a few hundredths of a second, and the receptors at the bases of the hairs adapt to extinction within a second or more. It is probable that most mechanoreceptors eventually adapt almost completely, but some require hours or days to do so, and they are called “nonadapting” receptors. The longest measured time for almost complete adaptation of a mechanoreceptor is about 2 days, which is the adaptation time for many carotid and aortic baroreceptors; however, some physiologists believe that these specialized baroreceptors never fully adapt. Some of the nonmechanoreceptors—the chemoreceptors and pain receptors, for example—probably never adapt completely.
Mechanisms by Which Receptors Adapt
The mechanism of receptor adaptation is different for each type of receptor in much the same way that development of a receptor potential is an individual property. For example, in the eye, the rods and cones adapt by changing the concentrations of their light-sensitive chemicals (discussed in Chapter 51 ).
In the case of the mechanoreceptors, the receptor that has been studied in greatest detail is the Pacinian corpuscle. Adaptation occurs in this receptor in two ways. First, the Pacinian corpuscle is a viscoelastic structure, so that when a distorting force is suddenly applied to one side of the corpuscle, this force is instantly transmitted by the viscous component of the corpuscle directly to the same side of the central nerve fiber, thus eliciting a receptor potential. However, within a few hundredths of a second, the fluid within the corpuscle redistributes, and the receptor potential is no longer elicited. Thus, the receptor potential appears at the onset of compression but disappears within a small fraction of a second, even though the compression continues.
The second, much slower mechanism of adaptation of the Pacinian corpuscle results from a process called accommodation , which occurs in the nerve fiber itself. That is, even if by chance the central core fiber should continue to be distorted, the tip of the nerve fiber gradually becomes accommodated to the stimulus. This probably results from progressive “inactivation” of the sodium channels in the nerve fiber membrane, which means that sodium current flow through the channels causes them to close gradually, an effect that seems to occur for all or most cell membrane sodium channels, as was explained in Chapter 5 .
Presumably, these same two general mechanisms of adaptation also apply to the other types of mechanoreceptors. That is, part of the adaptation results from readjustments in the structure of the receptor, and part results from an electrical type of accommodation in the terminal nerve fibril.
Slowly Adapting Receptors Detect Continuous Stimulus Strength—the “Tonic” Receptors
Slowly adapting receptors continue to transmit impulses to the brain as long as the stimulus is present (or at least for many minutes or hours). Therefore, they keep the brain constantly apprised of the status of the body and its relation to its surroundings. For example, impulses from the muscle spindles and Golgi tendon apparatuses allow the nervous system to know the status of muscle contraction and load on the muscle tendon at each instant.
Other slowly adapting receptors include the following: (1) receptors of the macula in the vestibular apparatus; (2) pain receptors; (3) baroreceptors of the arterial tree; and (4) chemoreceptors of the carotid and aortic bodies.
Because the slowly adapting receptors can continue to transmit information for many hours, or even days, they are called tonic receptors .
Rapidly Adapting Receptors Detect Change in Stimulus Strength—the “Rate Receptors,” “Movement Receptors,” or “Phasic Receptors.”
Receptors that adapt rapidly cannot be used to transmit a continuous signal because they are stimulated only when the stimulus strength changes. Yet, they react strongly while a change is actually taking place . Therefore, these receptors are called rate receptors, movement receptors, or phasic receptors . Thus, in the case of the Pacinian corpuscle, sudden pressure applied to the tissue excites this receptor for a few milliseconds, and then its excitation is over, even though the pressure continues. Later, however, it transmits a signal again when the pressure is released. In other words, the Pacinian corpuscle is exceedingly important in apprising the nervous system of rapid tissue deformations, but it is useless for transmitting information about constant conditions in the body.
Predictive Function of the Rate Receptors
If the rate at which some change in the body’s status is taking place is known, the state of the body a few seconds or even a few minutes later can be predicted. For example, the receptors of the semicircular canals in the vestibular apparatus of the ear detect the rate at which the head begins to turn when a person runs around a curve. Using this information, a person can predict how much he or she will turn within the next 2 seconds and can adjust the motion of the legs ahead of time to keep from losing balance. Likewise, receptors located in or near the joints help detect the rates of movement of the different parts of the body. For example, when a person is running, information from the joint rate receptors allows the nervous system to predict where the feet will be during any precise fraction of the next second. Therefore, appropriate motor signals can be transmitted to the muscles of the legs to make any necessary anticipatory corrections in position so that the person will not fall. Loss of this predictive function makes it impossible for the person to run.
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