Function of the<unk>  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. 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 a usually antagonistic fashion, to achieve homeostasis. Higher organisms maintain their integrity via homeostasis which relies on negative feedback regulation which, in turn, typically depends on the<unk> . Some typical actions of the sympathetic and parasympathetic<unk>  systems are listed below. + Target organ/system Parasympathetic Sympathetic Digestive system Increase peristalsis and amount of secretion by digestive glands Decrease activity of digestive system Liver No effect Causes glucose to be released to blood Lungs Constricts bronchioles Dilates bronchioles Urinary bladder/ Urethra Relaxes sphincter Constricts sphincter Kidneys No effects Decrease urine output Heart Decreases rate Increase rate Blood vessels No effect on most blood vessels Constricts blood vessels in viscera; increase BP Salivary and Lacrimal glands Stimulates; increases production of saliva and tears Inhibits; result in dry mouth and dry eyes Eye (iris) Stimulates constrictor muscles; constrict pupils Stimulate dilator muscle; dilates pupils Eye (ciliary muscles) Stimulates to increase bulging of lens for close vision Inhibits; decrease bulging of lens; prepares for distant vision Adrenal Medulla No effect Stimulate medulla cells to secrete epinephrine and norepinephrine Sweat gland of skin No effect Stimulate sudomotor function to produce perspiration Sympathetic<unk>  system Promotes a fight-or-flight response, corresponds with arousal and energy generation, and inhibits digestion *Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction *Blood flow to skeletal muscles and the lungs is enhanced (by as much as 1200% in the case of skeletal muscles) *Dilates bronchioles of the lung through circulating epinephrine, which allows for greater alveolar oxygen exchange *Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for enhanced blood flow to skeletal muscles *Dilates pupils and relaxes the ciliary muscle to the lens, allowing more light to enter the eye and enhances far vision *Provides vasodilation for the coronary vessels of the heart *Constricts all the intestinal sphincters and the urinary sphincter *Inhibits peristalsis *Stimulates orgasm The pattern of innervation of the sweat gland—namely, the postganglionic sympathetic nerve fibers—allows clinicians and researchers to use sudomotor function testing to assess dysfunction of the<unk> s, through electrochemical skin conductance. Parasympathetic<unk>  system The parasympathetic<unk>  system has been said to promote a "rest and digest" response, promotes calming of the nerves return to regular function, and enhancing digestion. Functions of nerves within the parasympathetic<unk>  system include: *Dilating blood vessels leading to the GI tract, increasing the blood flow. *Constricting the bronchiolar diameter when the need for oxygen has diminished *Dedicated cardiac branches of the vagus and thoracic spinal accessory nerves impart parasympathetic control of the heart (myocardium) *Constriction of the pupil and contraction of the ciliary muscles, facilitating accommodation and allowing for closer vision *Stimulating salivary gland secretion, and accelerates peristalsis, mediating digestion of food and, indirectly, the absorption of nutrients *Sexual. Nerves of the peripheral<unk>  system are involved in the erection of genital tissues via the pelvic splanchnic nerves 2–4. They are also responsible for stimulating sexual arousal. Enteric<unk>  system The enteric<unk>  system is the intrinsic<unk>  system of the gastrointestinal system. It has been described as "the Second Brain of the Human Body". Its functions include: *Sensing chemical and mechanical changes in the gut *Regulating secretions in the gut *Controlling peristalsis and some other movements Neurotransmitters A flow diagram showing the process of stimulation of adrenal medulla that makes it release adrenaline, that further acts on adrenoreceptors, indirectly mediating or mimicking sympathetic activity. thumb At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine), along with other cotransmitters such as ATP, to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla: *Acetylcholine is the preganglionic neurotransmitter for both divisions of the<unk> , as well as the postganglionic neurotransmitter of parasympathetic neurons. Nerves that release acetylcholine are said to be cholinergic. In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter to stimulate muscarinic receptors. *At the adrenal medulla, there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act on nicotinic receptors. Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream, which acts on adrenoceptors, thereby indirectly mediating or mimicking sympathetic activity. A full table is found at Table of neurotransmitter actions in the<unk> .
Autonomic nervous system
Parasympathetic nervous system
Somatosensory system
Endocrine system
Sympathoadrenal system
Cranial nerves
Coronary circulation
Cardiac cycle