Autonomic nervous system

The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal. The fight-or-flight response, also known as the acute stress response, is set into action by the autonomic nervous system.

The autonomic nervous system is regulated by integrated reflexes through the brainstem to the spinal cord and organs. These functions include control of respiration, cardiac regulation, vasomotor activity, and certain reflex actions such as coughing, sneezing, swallowing and vomiting. Those are then subdivided into other areas and are also linked to autonomic subsystems and the peripheral nervous system. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving autonomic regulatory input from the limbic system.

Although conflicting reports about its subdivisions exist in the literature, the autonomic nervous system has historically been considered a purely motor system, and has been divided into three branches: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. The enteric nervous system however is a less recognized part of the autonomic nervous system. The sympathetic nervous system is responsible for setting off the fight-or-flight response.