The autonomic nervous system (ANS) is the part of the peripheral nervous system that controls involuntary functions that are critical for survival. The ANS participates in the regulation of heart rate, digestion, respiratory rate, pupil dilation, and sexual arousal, among other bodily processes.
Within the brain, the ANS is located in the medulla oblongata in the lower brainstem. The medulla's main functions are to control the cardiac, respiratory, and vasomotor centers, to mediate autonomic, involuntary functions, such as breathing, heart rate, and blood pressure, and to regulate reflex actions such as coughing, sneezing, vomiting, and swallowing.
The hypothalamus acts to integrate autonomic functions and receives autonomic regulatory feedback from the limbic system to do so. The ANS is classically divided into two subdivisions, the sympathetic division and the parasympathetic division.
The brain stem with pituitary and pineal glands
The medulla is a subregion of the brainstem and is a major control center for the autonomic nervous system.
The sympathetic division of the ANS is often referred to as the sympathetic nervous system (SNS). The SNS provides noradrenergic drive to the ANS. It is often referred to as a quick response mobilizing system that initiates the body's fight-or-flight response.
PSNS input to the ANS is responsible for the stimulation of feed-and-breed and rest-and-digest responses, as opposed to the fight-or-flight response initiated by the SNS. The parasympathetic division of the ANS (PSNS) acts to complement and modulate the drive provided by SNS neurotransmission within the ANS.
As a rule, the SNS functions in actions requiring quick responses while the PSNS is initiated in actions that don't require immediate response.