How scratching relieves itch

Scratching decreases activity in some spinal cord neurons only during itchiness, according to research published in Nature Neuroscience this week. The work suggests that itchiness itself may induce a specific state in the spinal cord that allows scratching to decrease the activity of the neurons transmitting the itch sensation to the brain.

Everyone knows that scratching relieves itch, but the physiological mechanisms for how this works are poorly understood. Work with both humans and primates has suggested that a specific part of the spinal chord the spinothalamic tract is important for the sensation, and that neurons here are more active when itchy substances are applied to the skin.

Glenn J. Giesler, Jr. and colleagues now show that scratching the skin of primates blocks the activity of spinothalamic neurons during itchiness. Interestingly, the activity of these neurons was not stopped by scratching under conditions when the primate was not itchy.