Responsible for managing involuntary glands, involuntary reflexes and involuntary muscles in the body, the autonomic nervous system (ANS) is an umbrella comprised of three distinct nervous systems: the parasympathetic system, the sympathetic system, and the enteric system. All three of these nervous systems rely on special autonomic neurons that regulate essential functions by delivering signals from the brain to muscles and glands that need to operate without conscious effort. ANS nerve cells, also called motor neurons, mainly control glandular secretions, digestive system organs, heart muscles and various smooth muscles. Motor neurons often take their cue from dangers or changes in the environment, triggering the brain to release response signals.
Regulated by nerves in the spinal cord and the brain’s medulla region, the parasympathetic division of the nervous system orchestrates de-stimulation of the body. It works with digestive system organs such as the pancreas, liver and stomach as well as sensory organs such as the nose and eyes to slow the heartbeat, constrict pupils, and create a relaxed and subdued mood so that the gastrointestinal system can break down and absorb food. The effects of the parasympathetic nervous system, which also include the relaxation of the sphincter muscles, can last until food and liquids pass through the intestines and bladder for waste excretion. This part of the autonomic nervous system is often referred to as the “rest and digest” system.
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Designed to stimulate the body in moments of excitement, aggression or fear, functions of the sympathetic division of the nervous system are the antithesis of the parasympathetic system. The sympathetic nervous system, controlled by neurons in the lumbar section and the thoracic section of the spinal cord, causes rapid heart rhythm, reduced digestion and elevated blood pressure. Commonly called the “flight or fight” part of the autonomic nervous system, this system relies on the release of chemicals such as norepinephrine and acetylcholine to send the body into a heightened and hyperactive state.
The enteric part of the autonomic nervous system is often called the intrinsic nervous system and exclusively caters to the digestive system where it is entirely located. Served by neurons in the membranes of the esophagus, intestines, pancreas and other gastrointestinal organs, the enteric nervous system provides assistance to digestive functions handled by the parasympathetic part of the nervous system while also providing digestive defenses for the body. Defensive digestive functions of the enteric system include triggering vomiting or diarrhea when harmful bacteria or viruses have entered the digestive tract and must be expelled. The enteric part of the autonomic nervous system also controls the amount of blood flowing to the digestive region and how abdominal muscles constrict to move food through the digestive tract.