Blue = parasympathetic
Red = sympathetic

The autonomic nervous system (ANS) (or visceral nervous system) is the part of the nervous system that controls homeostasis or the constancy of the « milieu intérieur » (the content of tissues in gasses, ions and nutrients). It does so mostly by controlling cardiovascular, digestive and respiratory functions. Salivation, perspiration, diameter of the pupils, micturition - (the discharge of urine), and erection are also controlled by the ANS. Many of the activities of the ANS are involuntary. However, breathing, for example, can be in part consciously controlled. Indeed, although breathing is a purely homeostatic function in aquatic vertebrates, in land vertebrates it accomplishes much more than oxygenating the blood: it is essential to sniff a prey or a flower, to blow a candle, to bark or sing. This example, among others, illustrates that the so-called “autonomic nervous system” is not truly autonomous. It is anatomically and functionally linked to the rest of the nervous system and a strict delineation is impossible. Download high resolution version (479x700, 60 KB)From Grays Anatomy This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... Download high resolution version (479x700, 60 KB)From Grays Anatomy This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. ... It has been suggested that Reactive homeostasis be merged into this article or section. ... For the play Breath by Samuel Beckett, see Breath (play). ...

The ANS is nevertheless a classical term, still widely used throughout the scientific and medical community. Its most useful definition could be: the sensory and motor neurons that innervate the viscera. These neurons form reflex arcs that pass through the lower brainstem or medulla oblongata. This explains that when the central nervous system (CNS) is damaged experimentally or by accident above that level, a vegetative life is still possible, whereby cardiovascular, digestive and respiratory functions are adequately regulated. In anatomy, the viscera are the internal organs of an animal, in particular the internal organs of the head, thorax and abdomen. ... Patellar reflex. ... The brain stem is the stalk of the brain below the cerebral hemispheres. ... The medulla oblongata is the lower portion of the brainstem. ...

Anatomy

The reflex arcs of the ANS comprise a sensory (or afferent) arm, and a motor (or efferent, or effector) arm. The latter alone is represented on the figure.

Sensory neurons

The sensory arm is made of “primary visceral sensory neurons” found in the peripheral nervous system (PNS), in “cranial sensory ganglia”: the geniculate, petrosal and nodose ganglia, appended respectively to cranial nerves VII, IX and X. These sensory neurons monitor the levels of carbon dioxide, oxygen and sugar in the blood, arterial pressure and the chemical composition of the stomach and gut content. (They also convey the sense of taste, a conscious perception). Blood oxygen and carbon dioxide are in fact directly sensed by the carotid body, a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion. The general visceral afferent fibers (or sympathetic afferent fibers), conduct sensory impulses from the viscera through the rami communicantes and posterior roots to the spinal cord. ... The geniculate ganglion (from Latin genu, for knee) is an L-shaped collection of fibers and sensory neurons of the facial nerve located in the facial canal of the head. ... The inferior ganglion of the glossopharyngeal nerve (petrous ganglion) is larger than the superior ganglion and is situated in a depression in the lower border of the petrous portion of the temporal bone. ... The nodose ganglion (ganglion of the trunk; inferior ganglion of vagus nerve) is cylindrical in form, of a reddish color, and 2. ... It has been suggested that this article or section be merged with Cranial nerve nucleus. ... The carotid body is a small cluster of chemoreceptors and supporting cells located near the bifurcation of the carotid artery. ... The inferior ganglion of the glossopharyngeal nerve (petrous ganglion) is larger than the superior ganglion and is situated in a depression in the lower border of the petrous portion of the temporal bone. ...

Primary sensory neurons project (synapse) onto “second order” or relay visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS), that integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the area postrema, that detects toxins in the blood and the cerebrospinal fluid and is essential for chemically induced vomiting and conditional taste aversion (the memory that ensures that an animal which has been poisoned by a food never touches it again). All these visceral sensory informations constantly and unconsciously modulate the activity of the motor neurons of the ANS The solitary nucleus and tract are structures in the brainstem. ... The Area postrema is a part of the brain. ... Cerebrospinal fluid (CSF), Liquor cerebrospinalis, is a clear bodily fluid that occupies the subarachnoid space in the brain (the space between the skull and the cerebral cortex—more specifically, between the arachnoid and pia layers of the meninges). ...

Motor neurons

Motor neurons of the ANS are also located in ganglia of the PNS, called “autonomic ganglia”. They belong to three categories with different effects on their target organs (see below “Function”): sympathetic, parasympathetic and enteric.

Sympathetic ganglia are located in two sympathetic chains close to the spinal chord: the prevertebral and pre-aortic chains. Parasympathetic ganglia, in contrast, are located in close proximity to the target organ: the submandibular ganglion close to salivatory glands, paracardiac ganglia close to the heart etc… Enteric ganglia, which as their name implies innervate the digestive tube, are located inside its walls and collectively contain as many neurons as the entire spinal chord, including local sensory neurons, motor neurons and interneurons. It is the only truly autonomous part of the ANS and the digestive tube can function surprisingly well even in isolation. For that reason the enteric nervous system has been called “the second brain”. The submandibular ganglion (or submaxillary ganglion in older texts) is of small size and is fusiform in shape. ... The enteric nervous system (ENS) is an interdependent part of the autonomic nervous system. ...

The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” (also called improperly but classically "visceral motoneurons") located in the central nervous system. Preganglionc sympathetic neurons are in the spinal chord, at thoraco-lumbar levels. Preganglionic parasympathetic neurons are in the medulla oblongata (forming visceral motor nuclei: the dorsal motor nucleus of the vagus nerve (dmnX), the nucleus ambiguus, and salivatory nuclei) and in the sacral spinal chord. Enteric neurons are also modulated by input from the CNS, from preganglionic neurons located, like parasympathetic ones, in the medulla oblongata (in the dmnX). The dorsal nucleus of the vagus nerve (or posterior motor nucleus of vagus) is a cranial nerve nucleus for the vagus nerve that arises from the floor of the fourth ventricle. ... The nucleus ambiguus (literally ambiguous nucleus) is a region of histologically disparate cells located just dorsal (posterior) to the inferior olivary nucleus in the lateral portion of the upper (rostral) medulla. ...

The feedback from the sensory to the motor arm of visceral reflex pathways is provided by direct or indirect connections between the nucleus of the solitary tract and visceral motoneurons.

Function

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. Consider sympathetic as "fight or flight" and parasympathetic as "rest and digest".

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. More generally, 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:

Sympathetic nervous system

• Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.
• Blood flow to skeletal muscles, the lung is not only maintained, but enhanced (by as much as 1200%, in the case of skeletal muscles).
• Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.
• Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
• Dilates pupils and relaxes the lens, allowing more light to enter the eye.

Grays FIG. 838– The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses. ... It has been suggested that this article or section be merged with Epidermis (skin). ... A top-down view of skeletal muscle Skeletal muscle is a type of striated muscle, attached to the skeleton. ... The lungs flank the heart and great vessels in the chest cavity. ... Heart rate is a term used to describe the frequency of the cardiac cycle. ... Contractility is one of the factors which affect myocardial performance. ... Myocyte is the technical term for a muscle cell. ...

Parasympathetic nervous system

• Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut.

• The parasympathetic nervous system can also constrict the bronchiolar diameter when the need for oxygen has diminished.

• During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.

• The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis, so, in keeping with the rest and digest functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.

• Is also involved in erection of genitals, via the pelvic splanchnic nerves 2-4.

It has been suggested that Parasympatholytic be merged into this article or section. ... In much of the digestive tract, muscles contract in sequence to produce a peristaltic wave which forces food (called bolus while in the esophagus and chyme below the esophagus) along the alimentary canal. ... Pelvic splanchnic nerves are splanchnic nerves that arise from sacral spinal nerves S2, S3, S4 to provide parasympathetic innervation to the hindgut. ...

Neurotransmitters and pharmacology

At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine) to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla: Norepinephrine, known as noradrenaline outside the USA, is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... The adrenergic receptors (or adrenoceptors) are a class of G_protein coupled receptors that is the target of catecholamines. ...

• At sweat glands, the neurotransmitter is acetylcholine, which acts on muscarinic receptors.
• At the adrenal cortex, there is no postsynapic neuron. Instead the presynaptic neuron releases acetylcholine to act on nicotinic receptors.
• Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream which will act on adrenoceptors, producing a widespread increase in sympathetic activity.

In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter, to stimulate muscarinic receptors. Sweating (also called perspiration or sometimes transpiration) is the loss of a watery fluid, consisting mainly of sodium chloride and urea in solution, that is secreted by the sweat glands in the skin of mammals. ... Amanita muscaria from which muscarine was isolated Acetylcholine - natural agonist of muscarinic and nicotinic receptors. ... Layers of cortex. ... Nicotinic Receptors form ion channels present in the plasma membrane of cells. ... In mammals, the adrenal glands are the triangle-shaped endocrine glands that sit atop the kidneys. ... Epinephrine (INN) or adrenaline (BAN) is a hormone and a neurotransmitter. ... Amanita muscaria from which muscarine was isolated Acetylcholine - natural agonist of muscarinic and nicotinic receptors. ...

The following table reviews the actions of these neurotransmitters as a function of their receptors.

Grays FIG. 838– The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses. ... An adrenergic is a drug, or other substance, which has effects similar to, or the same as, epinephrine (adrenaline). ... Anatomy and Physiology of the A.N.S. In contrast to the voluntary nervous system, the involuntary or autonomic nervous system is responsible for homeostasis, maintaining a relatively constant internal environment by controlling such involuntary functions as digestion, respiration, and metabolism, and by modulating blood pressure. ... Muscarinic receptors are those membrane bound acetylcholine receptors that are more sensitive to muscarine than to nicotine. ... Diagram of the human circulatory system. ... Cardiac output is the volume of blood being pumped by the heart, in particular a ventricle in a minute. ... The sinoatrial node (abbreviated SA node, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. ... Chronotropic effects (from chrono-, meaning time) are those that change the heart rate. ... Cardiac muscle is a type of involuntary mononucleated, or uninucleated, striated muscle found exclusively within the heart. ... Inotropic: Affecting the force of muscle contraction. ... Atria may refer to: Atria is an alternative spelling for the Etruscan city that is now Adria in the Veneto region of Northern Italy. ... The atrioventricular node (abbreviated AV node) is the tissue between the atria and the ventricles of the heart, which conducts the normal electrical impulse from the atria to the ventricles. ... Vascular smooth muscle refers to the particular type of smooth muscle found within, and composing the majority of the wall of blood vessels. ... A 250 ml bag of newly collected platelets. ... A mast cell (or mastocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. ... This article or section does not cite its references or sources. ... The Respiratory System Among four-legged animals, the respiratory system generally includes tubes, such as the bronchi, used to carry air to the lungs, where gas exchange takes place. ... Smooth muscle is a type of non-striated muscle, found within the walls of hollow organs; such as blood vessels, bladders, uteri. ... The bronchioles are the first airway branches that no longer contain cartilage. ... The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. ... The human eye The pupil is the central transparent area (showing as black). ... A human eye. ... Grays FIG. 872- The choroid and iris. ... For the Physics term GUT, please refer to Grand unification theory The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and... The salivary glands produce saliva, which keeps the mouth and other parts of the digestive system moist. ... α-Amylase Amylase (EC 3. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... Lacrimal apparatus of the right eye. ... The kidneys are bean-shaped excretory organs in vertebrates. ... Renin, also known as angiotensinogenase, is a circulating enzyme (EC 3. ... Parietal cells (also called oxyntic cells) are cells located in the stomach epithelium. ... The liver is an organ in some animals, including mammals (and therefore humans), birds, and reptiles. ... Glycogenolysis is the catabolism of glycogen (requiring removal of glucose unit from glycogen and addition of phosphate) thus producing glucose 1-phosphate, and subsequently reconfigured (C-1 -> C-6) to yield glucose 6-phosphate, a potent reaction intermediary leading to glucose available to the blood and brain, pyruvic acid (yet... Gluconeogenesis is the generation of glucose from non-sugar carbon substrates like pyruvate, lactate, glycerol, and amino acids (primarily alanine and glutamine). ... Adipose tissue is an anatomical term for loose connective tissue composed of energy in the form of fat, although it also cushions and insulates the body. ... Lipolysis is the breakdown of fat stored in fat cells. ... For the Physics term GUT, please refer to Grand unification theory The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and... Smooth muscle is a type of non-striated muscle, found within the walls of hollow organs; such as blood vessels, bladders, uteri. ... For the Physics term GUT, please refer to Grand unification theory The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and... From late Latin sphincter, from Greek sphinkter, band, contractile muscle, from sphingein, to bind tight. ... For the Physics term GUT, please refer to Grand unification theory The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and... A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... For the Physics term GUT, please refer to Grand unification theory The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and... This article or section does not cite its references or sources. ... The pancreas is an organ in the digestive and endocrine system that serves two major functions: exocrine (producing pancreatic juice containing digestive enzymes) and endocrine (producing several important hormones, including insulin). ... This page is a candidate to be moved to Wiktionary. ... In mammals, the adrenal glands are the triangle-shaped endocrine glands that sit atop the kidneys. ... Nicotinic acetylcholine receptors, or nAChRs, are ionotropic receptors that form ion channels in cells plasma membranes. ... Adrenaline redirects here. ... The urinary system is the organ system that produces, stores, and carries urine. ... A bladder is a pouch or other flexible enclosure with waterproof or gasproof walls. ... Transverse section of ureter. ... Look up Sphincter in Wiktionary, the free dictionary. ... A reproductive system is the ensembles and interactions of organs and/or substances within an organism that strictly pertain to reproduction. ... The uterus or womb is the major female reproductive organ of most mammals, including humans. ... A sex organ, or primary sexual characteristic, narrowly defined, is any of those parts of the body (which are not always bodily organs according to the strict definition) which are involved in sexual reproduction and constitute the reproductive system in an complex organism; namely: Male: penis (notably the glans penis... An integument is an outer protective covering such as the feathers or skin of an animal or rind or shell. ... Sweating (also called perspiration or sometimes transpiration) is the loss of a watery fluid, consisting mainly of sodium chloride and urea in solution, that is secreted by the sweat glands in the skin of mammals. ... A hair follicle, showing its Arrector pili muscle attached on the right. ...

See also

Wikimedia Commons has media related to:

Nervous system

• Reflex arc
• Central pattern generator

Image File history File links Commons-logo. ... Wikimedia Commons logo by Reid Beels The Wikimedia Commons (also called Commons or Wikicommons) is a repository of free content images, sound and other multimedia files. ... A reflex arc is the neural pathway mediating a reflex. ... A central pattern generator is a network of neurons (or even a single neuron) which is able to exhibit rhythmic behavior in the absence of sensory input. ...

External links

• Overview at arizona.edu

Further reading

• The Second Brain, Michael D. Gershon, HarperCollins Publishers, New York (1998)