From the NY Times, April 7, 2009 (Health Section)
Scratching Relieves Itch by Quieting Nerve Cells
By Benedict Carey
A new study "reports that specialized cells in the spinal cord appear to be critically involved in producing the sensation of itch and the feeling of relief after the application of fingernails, at least in healthy individuals. The study appears in the current issue of the journal Nature Neuroscience.
It suggests that itch, like pain, may be a “gated” system in which signals from other nerve cells can interfere with or moderate the sensation. Scratching the skin near, but not directly on, the spot that itches often provides relief, just as rubbing an aching limb can reduce pain. Perceptions in the brain, too, probably moderate the urge to scratch: some chronic, compulsive cases of itching suggest that the brain is not properly reading the effect of outside signals at all but is instead acting on a mistaken internal representation of what is happening to the skin.
Full NY Times Article
Nature/Neduroscience Article:
Davidson S, Zhang X, Khasabov SG, Simone DA, Giesler GJ Jr. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons.
Itch is relieved by scratching, but the neural mechanisms that are responsible for this are unknown. Spinothalamic tract (STT) neurons respond to itch-producing agents and transmit pruritic information to the brain. We observed that scratching the cutaneous receptive field of primate STT neurons produced inhibition during histamine-evoked activity but not during spontaneous activity or activity evoked by a painful stimulus, suggesting that scratching inhibits the transmission of itch in the spinal cord in a state-dependent manner.