<P> The sympathetic ganglia here, are found in two chains: the pre-vertebral and pre-aortic chains . The activity of autonomic ganglionic neurons is modulated by "preganglionic neurons" located in the central nervous system . Preganglionic sympathetic neurons are located in the spinal cord, at the thorax and upper lumbar levels . Preganglionic parasympathetic neurons are found in the medulla oblongata where they form visceral motor nuclei; the dorsal motor nucleus of the vagus nerve; the nucleus ambiguus, the salivatory nuclei, and in the sacral region of the spinal cord . </P> <P> Sympathetic and parasympathetic divisions typically function in opposition to each other . But this opposition is better termed 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 . The sympathetic system is often considered the "fight or flight" system, while the parasympathetic system is often considered the "rest and digest" or "feed and breed" system . </P> <P> 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 . In general, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis . Some typical actions of the sympathetic and parasympathetic systems are listed below . </P> <P> Promotes a fight - or - flight response, corresponds with arousal and energy generation, and inhibits digestion </P>

The autonomic nervous system is divided into two parts