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Figure 46-2

Typical relation between receptor potential and action potentials when the receptor potential rises above threshold level.

Figure 46-3

Excitation of a sensory nerve fiber by a receptor potential produced in a pacinian corpuscle. (Modified from Loewenstein WR: Excitation and inactivation in a receptor membrane. Ann N Y Acad Sci 94:510, 1961.)

Lamellar Corpuscle Action Potential
Stimulus strength (per cent)

Figure 46-4

Relation of amplitude of receptor potential to strength of a mechanical stimulus applied to a pacinian corpuscle. (Data from Loëwenstein WR: Excitation and inactivation in a receptor membrane. Ann N Y Acad Sci 94:510, 1961.)

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 46-4, one can see that very intense stimulation of the receptor causes progressively less and less additional increase in numbers of action potentials. This is an exceedingly important principle that 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 allows the receptor to have an extreme range of response, from very weak to very intense.

Adaptation of Receptors

Another characteristic of all 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 often to none at all.

Figure 46-5 shows typical adaptation of certain types of receptors. Note that the pacinian corpuscle adapts extremely rapidly and hair receptors adapt

Lamellar Corpuscle Action Potential

Figure 46-5

Adaptation of different types of receptors, showing rapid adaptation of some receptors and slow adaptation of others.

Figure 46-5

Adaptation of different types of receptors, showing rapid adaptation of some receptors and slow adaptation of others.

within a second or so, whereas 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 hun-dredths of a second, and the receptors at the bases of the hairs adapt to extinction within a second or more. It is probable that all other mechanoreceptors eventually adapt almost completely, but some require hours or days to do so, for which reason they are called "non-adapting" receptors. The longest measured time for complete adaptation of a mechanoreceptor is about 2 days, which is the adaptation time for many carotid and aortic baroreceptors. Conversely, some of the nonmechanoreceptors—the chemoreceptors and pain receptors, for instance—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 instance, in the eye, the rods and cones adapt by changing the concentrations of their light-sensitive chemicals (which is discussed in Chapter 50).

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, so that 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 mechanism of adaptation of the pacinian corpuscle, but a much slower one, 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 itself 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 gradually to close, 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 apply also to the other types of mechano-receptors. That is, part of the adaptation results from readjustments in the structure of the receptor itself, and part 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 instance, 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 (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, 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 these receptors 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. But later, 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.

Importance of the Rate Receptors—Their Predictive Function.

If one knows the rate at which some change in bodily status is taking place, one can predict in one's mind the state of the body a few seconds or even a few minutes later. For instance, the receptors of the semicircular canals in the vestibular apparatus of the ear detect the rate at which the head begins to turn when one 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 instance, when one 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.

Nerve Fibers That Transmit Different Types of Signals, and Their Physiologic Classification

Some signals need to be transmitted to or from the central nervous system extremely rapidly; otherwise, the information would be useless. An example of this is the sensory signals that apprise the brain of the momentary positions of the legs at each fraction of a second during running. At the other extreme, some types of sensory information, such as that depicting prolonged, aching pain, do not need to be transmitted rapidly, so that slowly conducting fibers will suffice. As shown in Figure 46-6, nerve fibers come in all sizes between 0.5 and 20 micrometers in diameter—the larger the diameter, the greater the conducting velocity. The range of conducting velocities is between 0.5 and 120 m/sec.

General Classification of Nerve Fibers. Shown in Figure 46-6 is a "general classification" and a "sensory nerve classification" of the different types of nerve fibers. In the general classification, the fibers are divided into types A and C, and the type A fibers are further subdivided into a, b, g, and § fibers.

Type A fibers are the typical large and medium-sized myelinated fibers of spinal nerves. Type C fibers are the small unmyelinated nerve fibers that conduct impulses at low velocities. The C fibers constitute more than one half of the sensory fibers in most peripheral nerves as well as all the postganglionic autonomic fibers.

The sizes, velocities of conduction, and functions of the different nerve fiber types are also given in Figure 46-6. Note that a few large myelinated fibers can transmit impulses at velocities as great as 120 m/sec, a distance in 1 second that is longer than a football field. Conversely, the smallest fibers transmit impulses as slowly as 0.5 m/sec, requiring about 2 seconds to go from the big toe to the spinal cord.

Alternative Classification Used by Sensory Physiologists.

Certain recording techniques have made it possible to separate the type Aa fibers into two subgroups; yet

Myelinated

Unmyelinated

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