Brain processing of itch and scratch

Abstract

Itch is defined as an unpleasant sensation that evokes the desire to scratch. Intractable itch and scratching can affect sleep, mood, and personal relationships, signifi cantly reducing quality of life of the chronic pruritic diseases such as atopic derma titis. Pruritogens activate certain receptors on small itch–selective unmyelinated C fibers. Peripheral itch stimuli are transmitted by sensory neurons to the spinal cord dorsal horn. After undergoing processing in the spinal cord, itch signals are conveyed through the spinothalamic tract to the thalamus and through the spinoparabrachial pathway to the parabrachial nucleus. Itch processing activates many brain areas such as the prefrontal cortex (PFC), supplementary motor area (SMA ) , premotor cortex (PM), primary motor cortex (MI), primary somatosensory cortex (SI), parietal cortex, cingulate cortex, precuneus, opercular cortex (OPC) including the secondary somato sensory cortex (SII) and insular cortex (IC), claustrum, basal ganglia including the striatum, thalamus, and cerebellum. Itch was suppressed during and after scratching. It proposed two possible mechanisms by inhibitory circuits of the spinal dorsal horn and descending inhibitory pathway originated from brain such as periaqueductal gray matter (PAG), the raphe nuclei and locus ceruleus. Scratching temporarily relieves itch and can also be rewarding and even addictive. The degree of pleasure obtained by scratching is correlated with itch intensity. In addition, activation of areas of the brain reward system (eg, midbrain and striatum) is observed when an itch is scratched. In the brain, chronic itch modulates activation of particular brain areas, including the anterior cingulate cortex (ACC), posterior cingulate cortex (PCC), and PFC; alter-nates functional brain connectivity;