The autonomic nervous system (ANS) is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS). The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.
The subdivisions of the autonomic nervous system
In the autonomic nervous system, preganglionic neurons connect the CNS to the ganglion.
Sympathetic and parasympathetic divisions typically function in opposition to each other. This opposition is often viewed as complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake.
The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Many think of sympathetic as fight or flight and parasympathetic as rest and digest or feed and breed.
However, many instances of sympathetic and parasympathetic activity cannot be ascribed to fight or rest situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus.
Another example is the constant, second-to-second modulation of heart rate by sympathetic and parasympathetic influences as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis.
Some functions of the SNS include diverting blood flow away from the gastrointestinal (GI) tract and skin via vasoconstriction, enhancing blood flow to skeletal muscles and the lungs, dilating the bronchioles of the lung to allow for greater oxygen exchange, and increasing heart rate.
The PSNS typically functions in contrast to the SNS by dilating the blood vessels leading to the GI tract, causing constriction of the pupil and contraction of the ciliary muscle to the lens to enable closer vision, and stimulating salivary gland secretion, in keeping with the rest and digest functions.