Itching is a sensation that everyone is familiar with. Typically relieved with a mild physical rubbing or scraping of the skin, itching is generally negligible and forgettable. Some more bothersome and persistent itches are treatable with topical medications, and, in more obstinate cases, with orally administered systemically-acting anti-itch treatments.
While itching is normally an obvious result of superficial skin lesions, there is a central nervous system component to itching that has been targeted with neurologically active medications such as anticonvulsants and other centrally acting pharmacological agents. Recent studies point out that some itches may actually have more of a nervous system component than previously thought.
The Peripheral Nervous System and the Spine in Itch Sensation
The itch receptors that have been identified include histaminergic and non-histaminergic receptors that are stimulated by a variety of known and identified pruritrigens, which produce a depolarization that results in synaptic spread of the input to the spine. It turns out that recently discovered itch receptors that also respond to several pruritogens synapse at the dorsal root ganglion of the spinal cord. Neuromodulators for itch include gastric releasing peptide, a molecule that binds to a receptor and sends the message through the peripheral and central nervous systems.
The Brain and Integration of Itch Perception
One of the relatively well-understood central nervous system activities in regards to itching is that the reward centers of the brain are activated with the act of scratching. These reward centers are generally known to be activated with pleasure as well as with relief, and scratching provides a type of relief.
New techniques for studying itch through mechanisms such as fMRI have pointed to involvement of the anterior cingulate cortex, the insula, the dorsolateral prefrontal cortex, and even the cerebellum. Further studies have pointed not only to the anatomy of activation, but also to increased connectivity between the anterior insular cortex and the basal ganglia.
Practical Application of the Link Between Itching and the Brain
There is a well-known and nevertheless curious link between the nervous system and the skin that is believed to be related to embryological stages of development. Yet the relevant details of this link are still sparse. While identifying the structures and metabolism of sensory modalities in the peripheral nervous system, the spinal cord and the brain are indeed interesting, the practical application in the context of itch is important for the well-being of patients.
This information can be of extreme value in the study of better treatments for persistent itch among patients whose pruritis interferes with quality of life. In fact, in atopic dermatitis, a common dermatological disorder, control of the itch through central mechanisms has been found to provide a degree of relief for patients. The current mainstays of anticonvulsant treatment for persistent itch is primarily focused on the side effect profile in a manner that applies equally to all patients. Identifying the regions and the neurochemistry of the nervous system in the context of itching can potentially help to better direct treatment of pruritis based on targeted efficacy.
Reference: Mueller SM, et al. Functional magnetic resonance imaging in dermatology: the skin, the brain and the invisible. Exp Dermatol. 2017 Jan 21.