Pain receptors or nociceptors are free sensory nerve endings that are present in most tissues of the body (Fig.Two types of afferent fibers conduct pain impulses: The myelinated A delta fibers that transmit impulses very rapidly and the unmyelinated C fibers that transmit impulses slowly, Acute pain-the sudden, sharp, localized pain related to thermal and physical stimuli primarily from skin and mucous membranes-is transmitted by the A delta fibers, whereas chronic pain-often experienced as a diffuse, dull, burning or aching sensation
--is transmitted by C fibers.Thermal means refer to extremes of temperature, mechanical means could refer to pressure, and chemical sources could include acids or compounds produced in the body, such as bradykinin, histamine, or prostaglandin. 4-1).