Scratching is the most instinctive reflex to an itch. Over the course of a day, we may find ourselves consciously or subconsciously scratching any region of the body from a dozen to hundreds of times. But do you ever wonder how scratching helps that nagging itchy feeling? The reasons are not completely known, but a team of researchers at the University of Minnesota recently performed a few experiments to help elucidate the mechanisms, which they recently published in Nature Neuroscience.
The research group tested the effect of scratching on signal transmission along the spinothalamic tract (STT). The STT is a sensory pathway within the spinal cord where neural signals for pain, temperature, and touch travel to the thalamus in the brain (hence, the name “spino”-“thalamic” tract).
The experiments involved injecting an area of skin with histamine, a bodily substance released during an allergic reaction. This action would stimulate the itching sensation and related reactions locally. The researchers would then scratch the affected area to evaluate the frequency of nerve conductions. It appeared that scratching of a histamine-injected area resulted in fewer nerve signals conducted along the STT. However, when scratching occurred in the absence of histamine, there was no inhibition of nerve signals.
In another experiment, capsaicin was injected. Capsaicin is an irritant which causes prolonged neural activation with eventual depletion of neurotransmitters. This can lead to a reduction in pain sensation to the affected area. The researchers wanted to evaluate whether a signal “fatigue” was involved in the effects of scratching. Interestingly, in nerves responsive to capsaicin, scratching had the opposite effect of increasing nerve discharges.
So what does this all mean? The researchers summarize their findings: “Our data suggest that relief of itch by scratching is the result of a reduction in the discharge rate of STT neurons responding to an itch-producing stimulus.” This occurs only in the presence of histamine, “suggesting the itch produces a state during which scratching engages a central inhibitory mechanism“.
- Source
- Davidson S, Zhang X, Khasabov SG, Simone DA, Giesler GJ Jr. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons. Nat Neurosci 2009. [Epub ahead of print]
