Itching for an explanation

Itch is a distinct sensation arising from the superficial layers of skin and mucous membranes. It is elicited by histamine and probably other endogenous chemicals that excite subpopulations of unmyelinated primary afferents and spinal neurones projecting through the anterolateral quadrant to the brain. The two popular views, which propose either that itch is signalled by a labelled line system of peripheral and central itch-specific neurones or that itch is the subliminal form of pain, both fail to explain convincingly many known features. Alternative theories emphasize central processes that extract the relevant information from afferents with broad sensitivity spectra for pruritogenic and noxious stimuli. Thus, itch presents an irritating challenge for the specificity theory of somatosensation.