Itch or Pain? Your Brain Knows the Difference—Here’s How

If you have ever brushed your hand against a thorn or been bitten by a mosquito, you know that feeling pain vs. an itch are two different experiences. A new study published in Nature Communications has uncovered how the brain tells the difference. Using mice, the team found that specialized cells in an area just behind the frontal lobe called the anterior cingulate cortex selectively respond to itch or pain, while others seem to respond to both. Understanding how these distinct pathways trigger the urge to scratch an itch or move away from pain may help develop more targeted therapies for chronic pain and itchiness.

The Difference Is in How It Feels

It is easy to think that an itch is just a mild form of pain. Although that is true in some sense, research shows that when these sensory signals reach the brain, an itch activates different pathways than pain. Bug bites, dryness, allergies, or simply coming into contact with something that feels itchy creates irritation and activates nerve fibers embedded in the skin. These receptors send signals to the brain that trigger the impulse to scratch, temporarily relieving that itchy feeling.

Pain, on the other hand, can be a sign of potential tissue damage in the body. Sharp throbbing, aching, or burning generates an instantaneous reaction to move your body away from the source of pain, like when you touch a hot pan. Pain receptors in the skin send more intense and urgent signals to the brain. While pain may be acute (lasting for a short time) or chronic (persistent over time), it often causes more immediate distress than itching.

How Your Brain Separates Itch From Pain

Both pain and itch signals are transmitted through the spinal cord as they travel to the brain. How the brain processes these sensations not only influences perception but also our response. Signals from the body are first sent to the thalamus, the brain’s hub for sensory processing. Neurons here relay information to higher brain regions that help identify the source and intensity of the pain or itch. Both sensations activate many of the same areas throughout the brain.

It is not until these signals reach the anterior cingulate cortex that pain and itch seem to be differentiated. Located in the front part of the brain, the anterior cingulate cortex analyzes and adds emotional context to what we are feeling. This new study suggests that it contains two groups of specialized neurons—one that detects pain and the other that detects itch.

The team began their study by observing the differences in how mice respond to an itch vs. pain. The mice were exposed to two chemicals, one that causes itchiness and the other mild pain at the site of injection. When the animals either scratched the itch or tensed up in response to the pain stimulus, the team observed a sharp increase in neural activity in the anterior cingulate cortex. A closer look revealed that the same neurons fired when the animal felt itchy but not pain and vice versa. This finding led the team to speculate whether there may be two groups of neurons that distinguish between itch and pain.

Next, the team injected a chemical that selectively deactivates itch-specific neurons into this brain region. This reduced the animal’s impulse to scratch even after being exposed to the itch-inducing stimulus repeatedly. However, there was no change in how the animal responded to pain. The reverse was seen when pain-specific receptors were suppressed. It is clear that the brain processes itch and pain differently.

What the team was surprised to find was that these are not the only neurons that selectively respond to itch or pain. The team found evidence of entirely separate brain circuits for processing either sensation. Pain- and itch-selective neurons in the anterior cingulate gyrus seem to be connected to corresponding neurons in the thalamus that receive the initial signal. The circuits that these neurons create are likely what triggers a scratching vs. withdrawal response.

Conclusion

The next time you feel the urge to scratch, remember that there is an extensive network of neurons that tells your brain whether you are feeling an itch or pain. This study gives us a glimpse into how our brain tells the difference between the two. These findings may lead to more targeted treatments that can relieve chronic itching or pain symptoms associated with a range of conditions