Systemic and specific autonomic reactions in pain: efferent, afferent and endocrine components.

The sympathetic nervous system is hierarchically organized. At the bottom of this organization are the sympathetic pre-post-ganglionic channels which supply the autonomic target organs, at the top the hypothalamus and cortical structures. Each level of this hierarchy contains neuronal programmes which govern the sympathetic activity in a patterned fashion. According to this hierarchy the reactions to noxious, tissue-damaging events, which are organized by the hypothalamus and suprahypothalamic brain structures, are of a more general character than those organized by the spinal cord, these being of a more specific character. The spinal cord is probably much more important than hitherto believed for integration and in determining the characteristic discharge patterns of the sympathetic pre- and post-ganglionic neurones. The defence reaction is described as a general reaction to noxious stimuli. Its sympathetic components consist of the patterns of discharge in sympathetic neurones supplying skin, skeletal muscle, viscera and adrenal medulla. It is organized in the hypothalamus and upper brain stem. The spinal cord integrates more specific somato-sympathetic, viscero-sympathetic and viscero-visceral reactions to noxious stimuli which are of a protective character. Visceral thoraco-lumbar afferents in 'sympathetic' nerves innervate visceral organs in the pelvic abdominal and thoracic cavities. Some innervate deep somatic structures of the ventral compartment of the vertebral column. No conclusive experimental evidence exists to show that they innervate skin and deep tissues of the extremities. Noxious events in the visceral domain are encoded by these visceral afferents. However, no convincing experimental evidence exists to show that this occurs by 'specific' visceral nociceptive afferents. It may well be that visceral noxious events are encoded by the intensity of the discharges in these neurones. No conclusive experimental evidence exists to show that peripheral nociceptors are controlled by activity in sympathetic post-ganglionic neurones. In certain pathophysiological situations, however, it may happen that activity in sympathetic post-ganglionic neurones, which supply an extremity, leads to excitation of afferent axons, thus establishing a vicious circle between primary afferent neurones, spinal cord and sympathetic outflow. This situation may occur after partial lesions of peripheral nerves in a syndrome which is called 'reflex sympathetic dystrophy'.