The autonomic nervous system (ANS) is a branch of the peripheral nervous system which exerts control over smooth muscles and glands. As the name suggests, the ANS is responsible for the largely unconscious regulation of bodily functions including blood pressure (BP), heart rate (HR), temperature, digestion, the pupillary response, and sexual arousal. The two main branches of the ANS are the sympathetic nervous system (SNS; traditionally referred to as the “fight or flight” system) and the parasympathetic nervous system (PNS; traditionally referred to as the “rest and digest” system). It is the balance between the activity of these two systems that maintains homeostasis at rest and ensures the appropriate adaptive responses are initiated in response to external stressors.
Makovac E, Tracy LM, Koenig J (2020). Autonomic nervous system activity in itch and pain. In G Yosipovitch, HH Andersen, & L Arendt-Nielsen (Eds.) Itch and Pain: Similarities, Interactions, and Differences. Wolters Kluwer Health, Inc: Pennsylvania, USA. p. 127-135.