NEURAL MECHANISMS INVOLVED IN ITCH, “ITCHY SKIN,” AND TICKLE SENSATIONS

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Abstract
The sensation of itch has 2 subjectively distinguishable components, "pricking" and "burning." These correspond to the 2 qualities of cutaneous pain, and are mediated respectively by the 2 types of nerve fibers involved in the transmission of pain from the skin. Cutaneous tickle and "itchy skin" do not differ qualitatively from itch, and are mediated by the same neural structures. When itching is present, the pain threshold at the site is lower than during itch-free intervals. Tickle, "itchy skin," and itch are abolished by pin pricks in adjacent skin. In the case of itch, this abolition occurs if the skin is pricked anywhere in the dermatome which contains the site of itching. Tickle, "itchy skin," and itch do not occur in areas of secondary hyperalgesia. Itch occurs when pain receptors are weakly stimulated. It is suggested that the sensation results from the presence in the central nervous system, probably in the spinal cord, of impulses traveling in circuits of internuncial neurons.