Autonomic nervous system
The autonomic nervous system (ANS) (or visceral nervous system) is the part of the peripheral nervous system that acts as a control system, maintaining homeostasis in the body. These maintenance activities are primarily performed without conscious control or sensation. The ANS has far reaching effects, including: heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition (the discharge of urine), and sexual arousal. Whereas most of its actions are involuntary, some ANS functions work in tandem with the conscious mind, such as breathing. Its main components are its sensory system, motor system (comprised of the parasympathetic nervous system and sympathetic nervous system), and the enteric nervous system.
Components
Sympathetic nervous system
Promotes a "fight or flight" response, corresponds with arousal and energy generation, inhibits digestion:
Function
- Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.
- Blood flow to skeletal muscles, the lung is not only maintained, but enhanced (by as much as 1200%, in the case of skeletal muscles).
- Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.
- Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
- Dilates pupils and relaxes the lens, allowing more light to enter the eye.
Parasympathetic nervous system
Promotes a "rest and digest" response; promotes calming of the nerves return to regular function, and enhances digestion.
Function
- Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut.
- The parasympathetic nervous system can also constrict the bronchiolar diameter when the need for oxygen has diminished.
- During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.
- The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis, so, in keeping with the rest and digest functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.
- Is also involved in erection of genitals, via the pelvic splanchnic nerves 2–4.