Organization of the Sympathetic Nervous System: Peripheral and Central Aspects

Abstract

Abstract The sympathetic nervous system is involved in many autonomic regulations leading to homeostasis and adaptation of the homeostatic regulations to the internal and external demands on the body. The basic peripheral building blocks of these regulations are the final sympathetic pathways, consisting of populations of pre- and postganglionic neurons that are functionally defined according to the effector cells they supply (vascular smooth muscle cells, non-vascular smooth muscle cells, secretory epithelia, etc.). Each group of target cells is innervated (and regulated) by one (rarely two) sympathetic final pathways. Neurons of a specific, functionally defined sympathetic pathway exhibit a typical discharge pattern, which is the result of integrative processes in distinct neural circuits of the spinal cord, brain stem, hypothalamus, and telencephalon. Impulse activity in preganglionic neurons is transmitted in the sympathetic ganglia to postganglionic neurons of the same functional type. There is no communication between functionally different sympathetic pathways in the autonomic ganglia. Activity in sympathetic postganglionic axons is transmitted to the effector cells largely via anatomically and functionally defined neuroeffector junctions. Spinal cord, brain stem, and hypothalamus contain functionally distinct neural circuits that are connected to the final sympathetic pathways. Only a few of these central circuits have been explored and can be described in detail. The lowest level of central integration occurs in the spinal cord. Spinal circuits are probably integrated in every sympathetically mediated regulation. The lower brain stem (pons and medulla oblongata) contains the neural circuits involved in (1) homeostatic cardiovascular regulation and its integration with the regulation of respiration, (2) regulation of body core temperature, (3) regulation of pelvic organs, and (4) regulation of the gastrointestinal tract (including regulation of food intake and metabolism). Neural circuits in spinal cord and lower brain stem, which represent the different types of homeostatic regulation, are integral components of complex regulations represented in the hypothalamus and mesencephalon, which include neuroendocrine systems and somato-motor systems and constitute elementary behaviors. The telencephalon (neocortex and limbic system) adapts these functions to the external state of the organism. The sympathetic nervous system is involved in the regulation of protection of body tissues against external and internal threatening events. This function is closely related to the neural control of the immune system. During fast defense (confrontational defense, flight, quiescence), the body is prepared for protection and during slow defense the organism switches to recuperation. Both involve the sympatho-neural systems and the sympatho-adrenal (SA) system and are related to the neural control of inflammation and sensitivity of nociceptors (hyperalgesia). The sympathetically mediated protective functions are controlled by neural circuits in the spinal cord, brain, and hypothalamus. These in turn are potentially under telencephalic control.