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. |