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 itchspecific neurones or that itch is the subliminal form of pain, both fail to explain convincingly many known features. Alternative theories emphasize centra!;~ mcesses 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.

Itch is a well-appreciated but poorly understood sensation. Yet itch represents a major clinical problem affecting skin, mucous membranes and the upper respiratory tract, and its understanding is important for theories of somatosensation. This review will describe what is known about itch, and discuss the possible underlying neural mechanisms. To date there is no universally accepted single hypothesis that both explains all the observed features of itch and is clearly supported by all experimental evidence.