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When I was a kid, my mom would say, “Don’t scratch that itch, it will only make it worse.” And multiple elementary school teachers told me, “Holding an injury doesn’t help. Leave it alone.”
But it seems that my body knew some neuroscience that my parents and teachers didn’t: that scratching and grabbing actually do help, which is why evolution wired those adult-annoying behaviors into us.
Let’s start with itches
Itching arises with local inflammation from an insect bite, allergy, or other irritant. Itch sensations travel along small-diameter, slow neurons to the spinal cord and thence to the brain via the spinothalamic pathway [13]. Any patch of skin that itches also has mechanoreceptors that detect touch and vibration, whose signals make their way to the spinal cord via fast, large-diameter neurons that spinal neurons relay to the brain via a pathway called the dorsal columns [13].
Within the spinal cord, slow/itch and fast/touch sensory neurons from the skin synapse on interneurons that cross-connect the two pathways: mechanoreceptors for touch and vibration activate neurons that inhibit signals from the itch pathway, and “itch” receptors inhibit those same inhibitory neurons [1-4].
Thus, an unscratched (no mechanoreceptor input) itch travels unimpeded from your skin to your brain, knocking down anti-itch activity in the spinal cord. When the itch hits your brain, you scratch, activating the mechanoreceptors that ratchet up the itch inhibition, lessening the unpleasant feelings. The inhibition of itch sensations from simultaneous activation of touch sensations is called the gate theory [5-9], where touch “gates off itch.”
But, there’s more to scratching than gating in the spinal cord. Deep brain structures stimulated by touch send signals back down the spinal cord to further inhibit upward transmission of itchy sensations [5-9]. Moreover, fMRI studies show that scratching activates positive reward centers in the brain, making it “pleasurable” to scratch an itch.
Bottom line: Modern neuroscience now knows what you already knew. Parents and teachers notwithstanding, scratching works, and does not necessarily make things worse.
Holding an injury is similar, but not quite the same
For years, neuroscientists assumed that itching was just a type of low-grade pain that traveled along the same pathways as more severe pain sensations, but recent research reveals that itch and pain travel through similar, but distinct, neural pathways. Whereas itching activates itch-specific primary sensor neurons (e.g., MrgprA3 receptors), pain is sensed by different nociceptive primary nerve fibers (e.g., A-delta, C-nociceptive), which connect to different interneurons in the spinal cord [5-9].
However, as with itches, inputs from mechanoreceptors (touch and vibration) “gate off” pain signals in spinal interneurons, albeit a unique set of interneurons. Similarly, descending signals from deep brain regions (such as the periaqueductal gray) further inhibit pain sensations in the spinal cord [7-9].
Scratching and touching also help with itches and pain that are “all in your head”
Several neurological conditions, such as neuropathy and thalamic stroke, can cause itching and pain sensations that do not originate in irritated or damaged skin but rather in damage to neural circuits downstream of skin receptors. These unpleasant sensations are called central itching and pain because they arise in the central nervous system [10-12]. Intriguingly, scratching the skin where there is central itch and touching the skin where there is central pain also alleviates the unpleasant sensations. These findings strongly suggest that the activity of inhibitory spinal interneurons that “turn off” itching and pain plays a role in itch and pain reduction, even when the source of the discomfort is in the brain itself [10-12].
Conclusion
Central itching and pain are of great interest to me right now because, as I write this, my skin is itching like mad, all over, simply because I am writing about itching and am acutely aware of sensations I’d otherwise ignore. And whether the itches are imaginary or real, scratching definitely helps. So, one piece of personal advice I can offer is this: If you want to avoid itching, don’t write about it!
