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 nervous 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 nervous 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 nervous system main article: enteric nervous system the enteric nervous system is the intrinsic nervous 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. 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 ans, 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 ans. autonomic nervous system and the immune system recent studies indicate that ans activation is critical for regulating the local and systemic immune-inflammatory responses and may influence acute stroke outcomes. therapeutic approaches modulating the activation of the ans or the immune-inflammatory response could promote neurologic recovery after stroke. history the specialised system of the autonomic nervous system was recognised by galen. in 1665, thomas willis used the terminology, and in 1900, john newport langley used the term, defining the two divisions as the sympathetic and parasympathetic nervous systems. caffeine effects caffeine is a bioactive ingredient found in commonly consumed beverages such as coffee, tea, and sodas. short-term physiological effects of caffeine include increased blood pressure and sympathetic nerve outflow. habitual consumption of caffeine may inhibit physiological short-term effects. consumption of caffeinated espresso increases parasympathetic activity in habitual caffeine consumers; however, decaffeinated espresso inhibits parasympathetic activity in habitual caffeine consumers. it is possible that other bioactive ingredients in decaffeinated espresso may also contribute to the inhibition of parasympathetic activity in habitual caffeine consumers. caffeine is capable of increasing work capacity while individuals perform strenuous tasks. in one study, caffeine provoked a greater maximum heart rate while a strenuous task was being performed compared to a placebo. this tendency is likely due to caffeine's ability to increase sympathetic nerve outflow. furthermore, this study found that recovery after intense exercise was slower when caffeine was consumed prior to exercise. this finding is indicative of caffeine's tendency to inhibit parasympathetic activity in non-habitual consumers. the caffeine-stimulated increase in nerve activity is likely to evoke other physiological effects as the body attempts to maintain homeostasis. the effects of caffeine on parasympathetic activity may vary depending on the position of the individual when autonomic responses are measured. one study found that the seated position inhibited autonomic activity after caffeine consumption (75 mg); however, parasympathetic activity increased in the supine position. this finding may explain why some habitual caffeine consumers (75 mg or less) do not experience short-term effects of caffeine if their routine requires many hours in a seated position. it is important to note that the data supporting increased parasympathetic activity in the supine position was derived from an experiment involving participants between the ages of 25 and 30 who were considered healthy and sedentary. caffeine may influence autonomic activity differently for individuals who are more active or elderly.