Somatic pain–pathogenesis and prevention

Abstract

because it is a key component of the body’s normal defence mech- anisms, protecting the body from a potentially hostile external environment by initiating behavioural and reflex avoidance strategies. This protective mech- anism operates as a result of the presence of a specific set of primary sensory neurones which encode the intensity, duration and quality of any noxious stimulus and, by virtue of their topographically organized projections to the spinal cord, its location [108]. Absence of these nociceptors, as in patients with congenital analgesia or peripheral neuropathies, is associated with tissue damage and poor healing as a consequence of the absence of the normal protective reflexes and behavioural responses elicited by the nociceptors. This “ouch” pain is, therefore, an important and adaptive element of the normal nervous system which, clinically, only needs to be temporarily suppressed or disabled during surgical procedures where damage is deliberately produced. The nociceptors terminate in a highly ordered way in the dorsal horn of the spinal cord with the thinly myelinated A ending in laminae I and V and the unmyelinated C-fibres in lamina II. These high threshold sensory fibres activate a large number of second order interneurones and projection neurones in the spinal cord, some of which are activated exclusively by noxious stimuli (