Itch is the sensation that provokes an urge to scratch, which distinguishes it from pain (which induces a withdrawal response). The physiology of itch is imperfectly understood. Itch receptors are apparently unspecialized nerve endings in the skin and cornea that lead to unmyelinated C fibers that travel in the spinothalamic tracts. Both histamine and interleukin-1 are mediators of itch; prostaglandin E1 lowers the itch sensory threshold; and substance P can cause itching by inducing the release of histamine. Endogenous opioids and exogenous opiates can cause itching.
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