There are as many explanations for why scratching relieves itchiness as there are causes of itching, with some of the favorites being that scratching releases painkilling endorphins or distributes itch-causing histamines so the high local concentration is diffused. (A New Yorker article last year explored the world of itching and scratching so thoroughly you’ll need calamine lotion after reading it). But a new paper in Nature Neuroscience makes a good case for a dark horse explanation: scratching decreases activity in some spinal cord neurons that transmit the itch sensation to the brain.
Although the physiological mechanisms for how scratching relieves itch are poorly understood, scientists have at least figured out that neurons in a specific part of the spinal cord—the spinothalamic tract, or STT—are more active when we itch, transmitting that information to the brain. To see if scratching acts on those neurons, neuroscientist Glenn J. Giesler of the University of Minnesota at Minneapolis and colleagues injected histamine into monkeys (they used crab-eating macaques, or Macaca fascicularis). They found that scratching the skin blocks the activity of spinothalamic neurons but, interestingly, only when the skin is itchy as a result of the histamine. That suggests, the write, that “scratching inhibits the transmission of itch in the spinal cord in a state-dependent manner." The “state-dependent” qualifier means that the activity of STT neurons was not reduced by scratching if the monkey did not itch.
Instead, the reduced activity in the neurons as a result of scratching occurred only when the skin was itchy and the neurons were carrying that information to the brain. Scratching blocks the transmission of that "I itch!" signal, bringing relief.