The involvement of the sympathetic nervous system in pain. Possible neuronal mechanisms

The sympathetic nervous system is defined anatomically as the thoraco-lumbar autonomic nervous system with only efferent neurones projecting into the lower extremities. The afferent fibres travelling in the sympathetic nerves and supplying visceral organs are named visceral afferents. They are probably involved in visceral nociception. Some afferents in the sympathetic trunk which have been regarded by some as sympathetic afferent neurones from deep structures in the extremities in fact seem to innervate structures in the retroperitoneal space such as the vertebral column e.g. Arguments for and against the direct modulatory influence of sympathetic activity on sensory receptors in skin and skeletal muscle are discussed. It is concluded that an efferent sympathetic control of receptors in the periphery does not play a significant role in mammals. The possible involvement of the sympathetic nervous system in the pathogenesis of reflex sympathetic dystrophy is discussed in the light of recent experimental findings. The different symptoms like pain, dysregulation of blood flow and sweating, the trophic changes in skin and subcutaneous tissues are considered with respect to the complex morphological and functional changes found in primary afferent neurones, dorsal horn neurones and postganglionic vasoconstrictor neurones in animals with nerve lesions. The clinical picture of postsympathectomy pain is presented. It is proposed that the basic peripheral neuronal mechanism leading to this pain may be very much the same as it may be in different types and stages of reflex sympathetic dystrophy.