Nociceptors are bare or free nerve endings; therefore, they do not adapt, or stop responding, to sustained or repeated stimulation. This is beneficial in that it keeps the individual aware of the damaging stimulus for as long as it persists. Nociceptors are widely distributed in the skin, dental pulp, periosteum, joints, meninges, and some internal organs. The three major classes of nociceptors are:

• Thermal nociceptors

• Mechanical nociceptors

• Polymodal nociceptors

Thermal nociceptors are activated by extreme temperatures, especially heat. One group of these receptors is stimulated by noxious heat (>45°C) and a second group is stimulated by noxious cold (<5°C). These are the temperatures at which the tissues begin to be damaged. Mechanical nociceptors are activated by mechanical damage, such as cutting, pinching, or tissue distortion, as well as by intensive pressure applied to the skin. Their firing rates increase with the destructiveness of the mechanical stimulus. Polymodal nociceptors are activated by all types of damaging stimuli (thermal, mechanical, chemical), including irritating exogenous substances that may penetrate the skin. Endogenous substances that may stimulate these receptors to elicit pain include potassium released from damaged cells; bradykinin; histamine; substance P; acids; and proteolytic enzymes (see Table 8.1). Stimulation of polymodal nociceptors elicits sensations of slow, burning pain.

Two types of afferent neurons are associated with nociceptors:

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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