The autonomic nervous system (ANS or visceral nervous system or involuntary nervous system) is the part of the peripheral nervous system that acts as a control system. It functions largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal. Most autonomic functions are involuntary but a number of ANS actions can work alongside some degree of conscious control. Everyday examples include breathing, swallowing, and sexual arousal, and in some cases functions such as heart rate.
Within the brain, the ANS is located in the medulla oblongata in the lower brainstem. The medulla's major ANS functions include respiration (the respiratory control centre, or "rcc"), cardiac regulation (the cardiac control centre, or "ccc"), vasomotor activity (the vasomotor centre or "vmc"), and certain reflex actions (such as coughing, sneezing, vomiting and swallowing). These then subdivide into other areas and are also linked to ANS subsystems and nervous systems external to the brain. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving ANS regulatory input from the limbic system to do so.
The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS) which operate independently in some functions and interact co-operatively in others. In many cases, the two have "opposite" actions. When one activates a physiological response, the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization used is that the sympathetic nervous system is a "quick response mobilising system" and the parasympathetic is a "more slowly activated dampening system"; but even this also has exceptions, such as in sexual arousal and orgasm where both play a role. The enteric nervous system is also sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.
The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first neuron will begin at the “outflow” and will synapse at the postganglionic, or second neuron’s cell body. The postganglionic neuron will then synapse at the target organ.
General visceral afferent sensations are mostly unconscious visceral motor reflex sensations from hollow organs and glands that are transmitted to the CNS (see Figure 1 for a depiction of a typical nerve fiber, including general visceral afferent fibers). While the unconscious reflex arcs are normally undetectable, in certain instances, they may send pain sensations to the CNS masked as referred pain. If the peritoneal cavity becomes inflamed or if the bowel is suddenly distended, the body will interpret the afferent pain stimulus as somatic in origin. This pain is usually non-localized. The pain is also usually referred to dermatomes that are at the same spinal nerve level as the visceral afferent synapse.