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<unk>  and sympathetic fibers as the efferent fibers are. 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 exhibit automaticity which is 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<unk>  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<unk>  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,<unk> s 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<unk> s 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<unk>  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 () are located in the neural system. * The M2 muscarinic receptors () 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<unk> . * The M3 muscarinic receptors () 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<unk>  divisions typically function in opposition to each other. The sympathetic division typically functions in actions requiring quick responses. The<unk>  division functions with actions that do not require immediate reaction. A mnemonic to summarize the functions of the<unk>  is SSLUDD (sexual arousal, salivation, lacrimation, urination, digestion and defecation).
Parasympathetic nervous system
Autonomic nervous system
Somatosensory system
Sympathoadrenal system
Central pattern generator
Central chemoreceptors
Neurovascular bundle
Neural pathway