What is sensitisation?

Sensitisation refers to increased responsiveness of neurons to their normal input, or recruitment of a response to normally subthreshold inputs. Clinically, this is seen as hyperalgesia or allodynia.

Hyperalgesia is an increased pain response to normally painful stimuli (eg touching sunburnt skin with a sharp needle may be perceived as more painful than the same sharp touch to adjacent normal skin).

Allodynia is a painful experience in response to normally nonpainful stimuli (eg light breeze or cottonwool touching the skin).

Sensitisation is subdivided into peripheral and central sensitisation.

Peripheral sensitisation refers to reduced threshold and increased responsiveness of nociceptive neurons to stimuli in the periphery. For example, sunburn causes tissue damage and may itself generate nociception but, in addition, the inflammatory mediators released sensitise peripheral nociceptors so that even warm water or light touch in the burn area is transmitted as nociception and may be perceived as painful. This is localised to the injured tissue and resolves after healing.

Central sensitisation refers to increased responsiveness of nociceptive neurons in the central nervous system. It can occur irrespective of whether the signals were generated by nociceptive or neuropathic processes. Central sensitisation is induced by complex processes within the central nervous system that amplify the nociceptive signals reaching the brain, causing hypersensitivity to a trigger (eg acute trauma). Sometimes central sensitisation processes continue after healing the original trigger, and chronic pain may occur; see The transition from acute to chronic pain.