About Itch

Pathogenesis of Itch

Human skin is composed of two layers: an outer layer called the epidermis and a deeper layer called the dermis. The region in between these two layers contains all the small nerves that send itch signals to your brain that contributes to your feeling of itch.

Inflammation refers to your body’s immune system being activated in response to some trigger. While inflammation can occur from infections, injuries, or damage to cells in your body, certain individuals are also more predisposed to it.

Increased inflammation is a common cause of itch, since inflammation can activate the small nerves in the skin and result in the itchy sensation.

As mentioned previously, the sensation of itch is inherently connected to nerves in the skin; however, here’s a more in depth explanation of exactly how this process happens.

The first image shown represents how itch travels from skin cells to nerves. When stimulated, many cells in the skin release several itch-inducing protein complexes. These complexes then bind to a multitude of receptors linked to sensory nerve fibers in the skin.

The second image shown depicts how the spinal cord is involved in the sensation of itch. A variety of itch pathways, including mechanical itch pathways (the itch you might feel after brushing against a leaf or wearing a scratchy sweater), chemical itch pathways (the itch that occurs when your body produces histamine, such as when you’re bitten by a mosquito), scratching pathways, pain pathways, cooling pathways, and even descending inhibitory pathways (which mitigate pain perception), all interact through interneurons (neurons that transmit impulses) at the spinal cord. These connections regulate itch signaling, meaning that they control any itchy sensation you might be feeling.

The final image reveals how itch signals ultimately interact with the brain. The aforementioned itch signals travel through several neural pathways, including the spinothalamic tract, thalamus, spinoparabrachial pathway, and parabrachial nucleus. They’re then transmitted to many different parts of the brain, which are all involved in itch processing and ultimately drive the demand to scratch!