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Pain Receptors are also called free nerve endings. These simple receptors are found in the dermis around the base of hair follicles and close to the surface of the skin (epidermis) where the hair emerges from the skin. These free endings respond to changes in temperature (warmth and cold) and other events associated with tissue damage. Separate receptors for warmth and cold exist; with the cold receptors located close to the surface of the skin in the epidermis and the warmth receptors located deep within the dermis. The rate of adaptation of free nerve endings is uncertain.
Psychophysics is the study of sensations that are elicited by physical stimuli. These studies are interested in the subjective qualities of sensory stimulation, and in the relationship between the perception and the quantity of stimulation applied. Psychophysical studies of pain receptors (Darian-Smith, 1984) indicate that the normal temperature range of skin is 30-36 degrees centigrade. There is generally no experience of temperature within this range. In the range of 15-30 or 36-45 degrees centigrade, sensations of warmth or cold are explicit. Any temperature outside of these ranges is perceived as painful. In addition to temperature, free nerve endings that trigger signals associated with the perception of pain respond to a number of different substances that are released by damaged tissue. These include the pain peptides or endorphins, prostaglandins, histamine, and substance P. Free nerve endings have receptors for each of these substances. In general, thermoreceptors have small receptive fields.
The sensation of pain is associated with activation of afferent C-fibers found in the skin. These fibers are stimulated by both mechanical and heat stimulation. When these C-fibers were activated by laser stimulation, the level of activation was highly correlated with the reported subjective level of pain (Olausson, 1998). This correlated response occurred optimally when the stimulation site ranged in diameter from 1millimeter to 3 millimeters in diameter. No pain sensation was reported when the stimulation site was less than 1 millimeter in diameter. This finding suggests the importance of the spatial summation of activity across multiple afferent C-fibers in the perception of pain. In this psychophysical study, however, the magnitude level of pain sensation reported in response to the laser stimulation varied among the individual subjects. Thus, as has been suggested before, subjective sensation of pain is highly variable across people.
Darian-Smith, I (Ed.). (1984). The handbook of physiology: sensory processes (Volume III, Parts 1 and 2). The American Physiological Society.
Olausson, B. (1998). Recordings of polymodal single c-fiber nociceptive afferents following mechanical and argon-laser heat stimulation of human skin. Experimental Brain Research, 122(1), 44-54.