Key Concepts
The part of the nervous system that controls visceral (internal organ) functions of the body. The autonomic nervous system is a component of the peripheral nervous system, or PNS, which, along with the central nervous system, or CNS, represents the two main divisions of the nervous system. The PNS is further divided into the somatic nervous system and the autonomic nervous system. As a distinct system, the autonomic nervous system functions primarily at a subconscious level to innervate smooth muscle, cardiac muscle, and glands. The system regulates visceral processes involuntarily including those associated with cardiovascular activity, digestion, metabolism, and thermoregulation (bodily temperature). See also: Cardiovascular system; Central nervous system; Digestion; Gland; Metabolism; Muscle; Muscular system; Nervous system (vertebrate); Thermoregulation
The autonomic nervous system is traditionally partitioned into the sympathetic system and the parasympathetic system, based on the region of the brain or spinal cord in which the autonomic nerves have their origin (see illustration). The sympathetic system is defined by the autonomic fibers that exit thoracic and lumbar segments of the spinal cord. The parasympathetic system is defined by the autonomic fibers that either exit the brainstem via the cranial nerves or exit the sacral segments of the spinal cord. Groups of nerve cell bodies, called ganglia, that are located in various parts of the body outside of the CNS (brain and spinal cord) serve as the localized control centers for the autonomic nervous system. Within the brainstem, the hypothalamus is an important autonomic nervous system control center. The enteric nervous system is sometimes considered a branch of the autonomic nervous system but can also be understood as a distinct nervous system dedicated to digestion. See also: Brain; Cranial nerve; Ganglion; Parasympathetic nervous system; Spinal cord; Sympathetic nervous system
In general, the sympathetic and parasympathetic branches of the autonomic nervous system mediate opposite effects on internal organs, maintaining a dynamic balance of function. The sympathetic system activates the "fight-or-flight" response by enabling the body to rise to emergency demands encountered in flight, combat, pursuit, and pain. In contrast, the parasympathetic nervous system is more active during "rest-and-digest" periods, promoting homeostatic processes including immune system regulation, organ repair and maintenance, and the digestion of food. In this way, the sympathetic system can be characterized as being important for short-term survival, while the parasympathetic system can be characterized as important for long-term survival. See also: Homeostasis
Through interactions with the rest of the autonomic nervous system and the CNS, the enteric nervous system handles the local control of food digestion and influences hunger and satiety. Receptors detect the chemical signature of contents in the gastrointestinal tract or gut, in turn regulating the release of digestive secretions, the contraction of muscles to produce gut motility (the movement of consumed contents through the esophagus, stomach, and intestines), and the opening of muscular sphincters. See also: Digestive system; Food
Phylogeny provides insights into the functional and anatomical organization of the autonomic nervous system. The autonomic nervous system may be phylogenetically organized by proceeding from primitive structures that conserve metabolic resources and regulate visceral homeostasis, to structures that mobilize for fight-or-flight behaviors, and finally to structures found only in mammals that promote social and emotional behavior. Paralleling these functional shifts are increases in the interaction between visceral and somatic neurons and increases in influences from higher brain structures. See also: Neurobiology; Neuron; Phylogeny
