Mechanisms of acute visceral pain.

Acute visceral pain is dull, aching, ill-defined, badly localized and often referred to remote areas of the body. These properties indicate that the representation of internal organs within the CNS is very imprecise. There is evidence for the existence of specific visceral nociceptors in some viscera and for the existence of non-specific receptors in other internal organs. Some visceral receptors are 'silent' in normal viscera but become active following acute injury or inflammation of the internal organ that they innervate. The number of nociceptive afferent fibres in viscera is very small but these few nociceptive afferents can excite many second order neurones in the spinal cord which in turn generate extensive divergence within the CNS, sometimes involving supraspinal loops. Such a divergent input activates several systems--sensory, motor and autonomic--and thus triggers the general reactions that are characteristic of visceral nociception: a diffuse and referred pain, and prolonged autonomic and motor activity.