s: greater splanchnic nerves t5, t6, t7, t8, t9, sometimes t10 duodenum ps: vagus nerves s: greater splanchnic nerves t5, t6, t7, t8, t9, sometimes t10 jejunum and ileum ps: posterior vagal trunks s: greater splanchnic nerves t5, t6, t7, t8, t9 spleen s: greater splanchnic nerves t6, t7, t8 gallbladder and liver ps: vagus nerve s: celiac plexus right phrenic nerve t6, t7, t8, t9 colon ps: vagus nerves and pelvic splanchnic nerves s: lesser and least splanchnic nerves t10, t11, t12 (proximal colon) l1, l2, l3, (distal colon) pancreatic head ps: vagus nerves s: thoracic splanchnic nerves t8, t9 appendix nerves to superior mesenteric plexus t10 bladder ps: pelvic splanchnic nerves to detrusor muscles s: inferior hypogastric nerves s2-s4 kidneys and ureters ps: vagus nerve s: thoracic and lumbar splanchnic nerves t11, t12 function further information: parasympathetic rebound sensation the afferent fibers of the autonomic nervous system, which transmit sensory information from the internal organs of the body back to the central nervous system, are not divided into parasympathetic and sympathetic fibers as the efferent fibers are.: 34–35 instead, autonomic sensory information is conducted by general visceral afferent fibers. general visceral afferent sensations are mostly unconscious visceral motor reflex sensations from hollow organs and glands that are transmitted to the cns. while the unconscious reflex arcs normally are undetectable, in certain instances they may send pain sensations to the cns masked as referred pain. if the peritoneal cavity becomes inflamed or if the bowel is suddenly distended, the body will interpret the afferent pain stimulus as somatic in origin. this pain is usually non-localized. the pain is also usually referred to dermatomes that are at the same spinal nerve level as the visceral afferent synapse. vascular effects heart rate is largely controlled by the heart's internal pacemaker activity. considering a healthy heart, the main pacemaker is a collection of cells on the border of the atria and vena cava called the sinoatrial node. heart cells have the ability to generate electrical activity independent of external stimulation. as a result, the cells of the node spontaneously generate electrical activity that is subsequently conducted throughout the heart, resulting in a regular heart rate. in absence of any external stimuli, sinoatrial pacing contributes to maintain the heart rate in the range of 60-100 beats per minute (bpm). at the same time, the two branches of the autonomic nervous system act in a complementary way increasing or slowing the heart rate. in this context, the vagus nerve acts on sinoatrial node slowing its conduction thus actively modulating vagal tone accordingly. this modulation is mediated by the neurotransmitter acetylcholine and downstream changes to ionic currents and calcium of heart cells. the vagus nerve plays a crucial role in heart rate regulation by modulating the response of sinoatrial node; vagal tone can be quantified by investigating heart rate modulation induced by vagal tone changes. as a general consideration, increased vagal tone (and thus vagal action) is associated with a diminished and more variable heart rate. the main mechanism by which the parasympathetic nervous system acts on vascular and cardiac control is the so-called respiratory sinus arrhythmia (rsa). rsa is described as the physiological and rhythmical fluctuation of heart rate at the respiration frequency, characterized by heart rate increase during inspiration and decrease during expiration. sexual activity another role that the parasympathetic nervous system plays is in sexual activity. in males, the cavernous nerves from the prostatic plexus stimulate smooth muscles in the fibrous trabeculae of the coiled helicine arteries of penis to relax and allow blood to fill the two corpora cavernosa and the corpus spongiosum of the penis, making it rigid to prepare for sexual activity. upon emission of ejaculate, the sympathetics participate and cause peristalsis of the ductus deferens and closure of the internal urethral sphincter to prevent semen from entering the bladder. at the same time, parasympathetics cause peristalsis of the urethral muscle, and the pudendal nerve causes contraction of the bulbospongiosus (skeletal muscle is not via pn), to forcibly emit the semen. during remission the penis becomes flaccid again. in the female, there is erectile tissue analogous to the male yet less substantial that plays a large role in sexual stimulation. the pn cause release of secretions in the female that decrease friction. also in the female, the parasympathetics innervate the fallopian tubes, which helps peristaltic contractions and movement of the oocyte to the uterus for implantation. the secretions from the female genital tract aid in sperm migration. the pn (and sn to a lesser extent) play a significant role in reproduction. receptors the parasympathetic nervous system uses chiefly acetylcholine (ach) as its neurotransmitter, although peptides (such as cholecystokinin) can be used. the ach acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors. most transmissions occur in two stages: when stimulated, the preganglionic neuron releases ach at the ganglion, which acts on nicotinic receptors of postganglionic neurons. the postganglionic neuron then releases ach to stimulate the muscarinic receptors of the target organ. types of muscarinic receptors the five main types of muscarinic receptors: the m1 muscarinic receptors (chrm1) are located in the neural system. the m2 muscarinic receptors (chrm2) are located in the heart, and act to bring the heart back to normal after the actions of the sympathetic nervous system: slowing down the heart rate, reducing contractile forces of the atrial cardiac muscle, and reducing conduction velocity of the sinoatrial node and atrioventricular node. they have a minimal effect on the contractile forces of the ventricular muscle due to sparse innervation of the ventricles from the parasympathetic nervous system. the m3 muscarinic receptors (chrm3) are located at many places in the body, such as the endothelial cells of blood vessels, as well as the lungs causing bronchoconstriction. the net effect of innervated m3 receptors on blood vessels is vasodilation, as acetylcholine causes endothelial cells to produce nitric oxide, which diffuses to smooth muscle and results in vasodilation. they are also in the smooth muscles of the gastrointestinal tract, which help in increasing intestinal motility and dilating sphincters. the m3 receptors are also located in many glands that help to stimulate secretion in salivary glands and other glands of the body. they are also located on the detrusor muscle and urothelium of the bladder, causing contraction. the m4 muscarinic receptors: postganglionic cholinergic nerves, possible cns effects the m5 muscarinic receptors: possible effects on the cns types of nicotinic receptors in vertebrates, nicotinic receptors are broadly classified into two subtypes based on their primary sites of expression: muscle-type nicotinic receptors (n1) primarily for somatic motor neurons; and neuronal-type nicotinic receptors (n2) primarily for autonomic nervous system. relationship to sympathetic nervous system sympathetic and parasympathetic divisions typically function in opposition to each other. the sympathetic division typically functions in actions requiring quick responses. the parasympathetic division functions with actions that do not require immediate reaction. a mnemonic to summarize the functions of the parasympathetic nervous system is ssludd (sexual arousal, salivation, lacrimation, urination, digestion and defecation). clinical significance the functions promoted by activity in the parasympathetic nervous system are associated with our day-to-day living. the parasympathetic nervous system promotes digestion and the synthesis of glycogen, and allows for normal function and behavior. parasympathetic action helps in digestion and absorption of food by increasing the activity of the intestinal musculature, increasing gastric secretion, and relaxing the pyloric sphincter. it is called the “rest and digest” division of the ans. the parasympathetic nervous system decreases respiration and heart rate and increases digestion. stimulation of the parasympathetic nervous system results in: constriction of pupils decreased heart rate and blood pressure constriction of bronchial muscles increase in digestion increased production of saliva and mucus increase in urine secretion history the terminology ‘parasympathetic nervous system’ was introduced by john newport langley in 1921. he was the first person who put forward the concept of psns as the second division of the autonomic nervous system.