Parasympathetic Nervous System
The Parasympathetic system is the branch of the Autonomic Nervous System (ANS) responsible for the body’s ability to recuperate and return to a balanced state (known as "homeostasis") after experiencing pain or stress. While the Sympathetic system is also known as the "fight or flight" response, the Parasympathetic is often called "relax and renew."
Interestingly, the Parasympathetic functions in opposition to the Sympathetic nervous system. When the sympathetic system activates in response to some sort of stressor, the parasympathetic reacts in turn to bring the body back to a state of equilibrium. The Parasympathetic system is consistently active at a low level, but levels of activity increase when it is necessary to bring the body back to a balanced state from a state of elevated Sympathetic activity.
The primary Parasympathetic nerve is the vagus nerve, also known as cranial nerve X. When active, the Parasympathetic system slows down heart rate, dilates blood vessels, activates digestion, and stores energy. Unlike the Sympathetic system, the Parasympathetic response does not necessarily perform all of these functions at once, but selectively, as needed.