The autonomic nervous system is divided into two subsystems, the sympathetic and the parasympathetic, which work in tandem, either in a synergistic or an antagonistic way.
Sympathetically Speaking (SNS): Fight or Flight
The sympathetic system [SNS] is responsible for providing responses and energy needed to cope with stressful situations such as fear or extremes of physical activity. In response to such stress, the sympathetic system: raises blood pressure, heart rate, and the blood supply to the skeletal muscles at the expense of the gastrointestinal tract, kidneys, and skin; dilates both the pupils and the bronchioles, providing improved vision and oxygenation; and generates needed energy by stimulating glycogenolysis in the liver and lipolysis in adipose tissue. In general, it serves to stimulate organs and to mobilize energy.
Parasympathetic R&R (PNS): Everyday Living
Between stressful situations, the body needs to rest, recover, and gain new energy. These tasks are under the control of the parasympathetic system [PNS], which lowers the heart rate and blood pressure, diverts blood to the skin and the gastrointestinal tract, contracts the pupils and bronchioles to normal states, stimulates salivary gland secretion, and accelerates peristalsis. The parasympathetic system influences organs toward restoration and the saving of energy.
Enteric Mysteries: The Inner Brain (ENS)
Some anatomists refer to a third, or enteric, system primarily situated in the intestinal walls. It can be modulated by both sympathetic and parasympathetic nerve fibers which are connected to plexuses in several layers of the walls. However, the enteric nervous system is capable of operating on its own, even after having been severed from input from the SNS and PNS. This is why the enteric nervous system is sometimes referred to as a ‘second brain.’ (Hospital Practice, The Enteric Nervous System: A Second Brain, Michael D. Gershon, MD, Columbia University) ()
The enteric nervous system [ENS] regulates secretions of the intestinal glands, regeneration of the intestinal epithelium, and intestinal motility. The ENS is sometimes considered the third part of the autonomic nervous system. 1 In some modalities, this is known as the inner brain.