Nociceptors

The bare ending of small diameter afferents is nociceptors that are the receptors for noxious tissue-damaging, pain-producing stimuli. The Nociceptor afferents are explained as high threshold as they need intense stimulation to excite them. Unlike thermoreceptors, usually they have no background firing. They are categorized by what excites them:

- The Mechanical nociceptors are stimulated by intense mechanical forces and those in skin give increase to sharp, pricking pain. Each nociceptor is the ending of one of 5–20 branches of an Aδ afferent with low conduction velocities. The Mechanoreceptors in the visceral peritoneum which invests the gut react to excessive distension.

- The Thermal nociceptors fall into two groups, one excited by temperatures greater than 450 C, and the other by temperatures less than 50 C. They also react to intense mechanical stimuli. Their afferents are Aδ or C fibers.

- The Polymodal nociceptors in skin react to puncture, temperatures in excess of 480 C, and to a wide range of molecules liberated as an outcome of tissue damage involving: K⁺, H⁺, prostaglandins, bradykinin, serotonin, and histamine. The Stimulation of these nociceptors causes burning or aching pain. Their afferents are C fibers that conduct at less than 1.0 ms^-1. Since C fiber conduction is too slow, the pain they generate arrives last after a blow. They are also responsible for the visceral and muscle pain, and toothache.

- The Itch receptors belong to an individual class of C fiber which responds to histamine released from the mast cells.