Would you ever have thought that the itchingthat annoying and unpleasant tingling sensation you feel protects from insect-borne diseases and infections, such as mosquitoes? Well yes, and what’s more we share this annoyance with other species, which by scratching themselves or moving their tails drive insects away. It can be triggered by chemical stimuli (such as foreign substances and allergens) or mechanics (like the wool sweater your aunt gave you) and even if the activation mechanism is different, the urge to scratch and the pleasure that comes from it follow the same path. So let’s see what itching is and why scratching gives us relief and pleasure.
What is itching?
Itching is a physiological protection mechanismwhich allows us to remove annoying and potentially harmful elements from our body. It can be caused by chemical and mechanical stimuli, such as insect bites, irritants, drugssome systemic diseases up to inflammation and skin diseases, such as seborrheic dermatitis and psoriasis.
Also there dry skin triggers itching, because microlesions are created that trigger the inflammatory process. In some cases, itching and the compulsion to scratch have no identifiable causes, but are triggered by psychological causes and stress with mechanisms still the subject of studies and debates.
How itching gets to the brain is complicated and still unclear. In particular for mechanical itching, which for years was thought to travel on the same nerve fibers that transmit pain. In reality, although they are closely connected, they follow different paths.
The “chemical” itch: the mosquito bite
Many molecules trigger itching: endorphinsopiates, serotonin and the best known of all, thehistamine. Each of these substances has its own receptor to bind to and its own signaling pathway to the brain. For example, allergens and foreign substances, such as those contained in mosquito salivaactivate the immune system and the release of histamine by mast cells.
Among other actions of the immune and inflammatory response, histamine activates specific C-neuron fibers that carry the itch signal to the neurons of the spinal cordand from here the signal is carried by neurons that release gastrin (GRP) to the thalamus. The thalamus then sends the signal to the hand of move to ward off danger and scratch.
Mechanical Itch and the Nobel Prize
If the path of chemical itching seemed complex to you, prepare yourselves, because the mechanical one is even more tangled and the result of very recent research, which has been worth the Nobel Prize for Medicine of 2021 to Drs. David Julius and Ardem Patapoutian.
Mechanical itching arises from touch on the skinfor example of a wool fiber or an insect walking on our arm. This touch is perceived by specific sensors called mechanoreceptorswhose mechanical deformation activates a specific signaling. In addition to the itch receptors, this family also includes the receptors of touchfor example.
The itch receptors are part of a family of receptors called PIEZO and are located on the membrane of sensory neurons in the skin. When an insect walks on our skin, the PIEZOs are activated and carry the signal to the brain according to two distinct paths.0
- The PIEZO1 are connected to the communication pathway of the chemical itch: once activated, they release a molecule, natriuretic polypeptide type B (Nppb) which sends the itch signal to the brain through the GRPR neurons of chemical signaling.
- The activation of PIEZO2, instead, transmits the signal to two other groups of neurons in the spinal cord. It may seem strange, but the stimulus is sent to both inhibitory and excitatory neurons. The former, however, produce a molecule (neuropeptide Y) that maintains silenced excitatory neurons. In this way, you avoid feeling constant itchingfor example when we wear clothes, but is only activated when the stimulus is high.
Why We Like to Scratch and What Problems It Can Cause
Analyzing the magnetic resonance imaging of some volunteers before and after the “scratching”, researchers from Wake Forest University have noticed that the action of Scratching activates areas of the brain linked to pleasure and the reward circuitry and inhibits those related to pain.
Further studies explain why the more we scratch, the more we feel the need to do so. We anticipated that itching and pain are linked: when we scratch we activate pain sensorsinterfering with the transmission of the itch signal. We are obviously talking about mild pain, but our brain is programmed to respond to pain, of any degree, by producing serotonin.
Temporarily the pain is reduced and we feel like we have some relief, but since Serotonin can also bind to itch receptorsreactivates the urge to scratch, creating a vicious circle of itching-scratching.
This becomes a problem in the presence of mosquito bites (or insect bites in general) or dermatological pathologies, because the continuous urge to scratch can cause injuries of the skin, secondary infectionserythema and redness which do nothing but increase the inflammation, in fact worsening the pre-existing condition.
Understanding the delicate mechanisms that regulate itching is essential for the search for therapies for the chronic itchinga condition often linked to skin or body-wide diseases, with repercussions on the mood and quality of life of those who suffer from it.
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