G. moderate movement

mV 50

mV 50

FIGURE 3 Characteristics of Pacinian corpuscles and Ruffini's endings: (A) to (D) experimental analysis of rapidly adapting Pacinian corpuscles, and (E) to (G) slowly adapting Ruffini's endings. Each example shows the receptor on the left and three recordings on the right: the movement (top), generator potentials (middle), and action potentials (bottom). In (A) and (E), a tiny movement produces small generator potentials, but these are too small to start action potentials. In (B) and (F), a larger movement produces larger generator potentials. This causes a burst of action potentials in the Pacinian corpuscle and a continuous stream of action potentials in the Ruffini's endings. With a still larger movement, the generator potentials in (C) and (G) are even greater, and the action potentials occur at higher frequency. In (D), with most of the corpuscle removed, the Pacinian receptor responds in a manner similar to that for a Ruffini's ending.

begin firing (see below). Heat receptors have small, unmyelinated fibers with relatively slow conduction rates that are designated as type IV or C fibers (Fig. 1). Cold receptors respond preferentially to temperatures of 18 to 30°C and cease firing entirely below 10°C, which is one of the reasons why cold temperature acts as an effective anesthetic. Cold receptors have fibers of medium-sized diameter and limited layers of myelin and are thus designated as type III fibers (Fig.1).

Besides the conscious perception of temperature changes provided by thermoreceptors in the skin, other thermoreceptors clustered in the hypothalamus and spinal cord monitor internal temperatures. Changes in core body temperature lead to subconscious reflexes that regulate temperature but provide no conscious sensations.

These receptors show exquisite sensitivity, effectively signaling changes as small as 0.01 °C. A stable core temperature close to 37°C is particularly important for maintaining normal brain function. Temperatures above 40.5° C cause serious dysfunction; brain temperatures below 30°C depress all neuronal activity, including that necessary for maintaining minimal respiratory rate.

Pain Receptors

Receptors that convey information about noxious or potentially noxious stimuli are called nociceptors (from the Latin nocere, ''to hurt''). Nociceptors are divided into four major subtypes based on the types of painful stimuli to which they respond. Three of the subtypes—mechanoreceptors, thermal receptors, and chemoreceptors—respond to a single modality, whereas the fourth subtype is polymodal and is capable of responding to mechanical, thermal, and chemical features of the noxious stimuli.

Although other receptor types respond to similar modalities, nociceptors are most sensitive to a higher range of stimulus intensities, specifically those that could be expected to cause tissue damage. For example, general thermal receptors in the skin normally respond to temperatures below 45° C, whereas nociceptive thermal receptors respond preferentially to temperatures above that level. Similarly, general mechanoreceptors respond to most types of pressure, but nociceptive mechanoreceptors are preferentially sensitive to pressure from sharp objects. It is interesting to note that as nociceptive pathways become activated, perceptions change accordingly. The sensation of "hot" changes to that of "scalding" at approximately 45° C. Likewise, moderate pressure from a smooth object can feel pleasant, but it feels uncomfortable when the same pressure is applied from a sharp object.

A number of different general chemical receptors are found throughout the body, including sensors for specific compounds, such as carbon dioxide in the blood. Nociceptive chemoreceptors are particularly sensitive to potential irritants—namely, high levels of potassium, extremes in tissue pH, bradykinins, and histamines.

All nociceptors have either free nerve endings or simple, nonencapsulated end organs. They are located throughout the body except for brain and bone tissue. In general, all nociceptors subtypes have fibers that fall into two categories; the first includes type III fibers that generate the sensation of pricking pain or tickle, and the second includes type IV (C) fibers that generate the sensation of slow burning pain (see Fig. 1).

Nociceptors are unique among sensory receptors in that their sensitivity to stimuli increases after tissue in the area near the nerve ending has been damaged. This

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