| Latin | divisio autonomica systematis nervosi peripherici |
|---|
The autonomic nervous system (ANS or visceral nervous system or involuntary nervous system) is the part of the peripheral nervous system that acts as a control system, functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal. Most autonomous functions are involuntary but a number of ANS actions can work alongside some degree of conscious control. Everyday examples include breathing, swallowing, and sexual arousal, and in some cases functions such as heart rate.
Within the brain, the ANS is located in the medulla oblongata in the lower brainstem. The medulla's major ANS functions include respiration (the respiratory control centre, or "rcc"), cardiac regulation (the cardiac control centre, or "ccc"), vasomotor activity (the vasomotor centre or "vmc"), and certain reflex actions (such as coughing, sneezing, vomiting and swallowing). These then subdivide into other areas and are also linked to ANS subsystems and nervous systems external to the brain. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving ANS regulatory input from the limbic system to do so.
The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS) which operate independently in some functions and interact co-operatively in others. In many cases the two have "opposite" actions where one activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterisation is that the sympathetic nervous system is a "quick response mobilising system" and the parasympathetic is a "more slowly activated dampening system", but even this has exceptions, such as in sexual arousal and orgasm where both play a role. The enteric nervous system is also sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.
ANS functions can generally be divided into sensory (afferent) and motor (efferent) subsystems. Within both there are inhibitory and excitatory synapses between neurons. Relatively recently, a third subsystem of neurons that have been named 'non-adrenergic and non-cholinergic' neurons (because they use nitric oxide as a neurotransmitter) have been described and found to be integral in autonomic function, particularly in the gut and the lungs.
ANS innervation is divided into sympathetic nervous system and parasympathetic nervous system divisions. The sympathetic division has thoracolumbar “outflow”, meaning that the neurons begin at the thoracic and lumbar (T1-L2/3) portions of the spinal cord. The parasympathetic division has craniosacral “outflow”, meaning that the neurons begin at the cranial nerves (CN 3, CN7, CN 9, CN10) and sacral (S2-S4) spinal cord.
The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first, neuron will begin at the “outflow” and will synapse at the postganglionic, or second, neuron’s cell body. The postganglionic neuron will then synapse at the target organ.
The sympathetic division (thoracolumbar outflow) consists of cell bodies in the lateral horn of the spinal cord (intermediolateral cell columns) from T1 to L2/3. These cell bodies are GVE (general visceral efferent) neurons and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:
These ganglia provide the postganglionic neurons from which innervation of target organs follows. Examples of splanchnic (visceral) nerves are:
These all contain afferent (sensory) nerves as well, known as GVA (general visceral afferent) neurons.
The parasympathetic division (craniosacral outflow) consists of cell bodies from one of two locations: the brainstem (Cranial Nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:
These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:
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 and smell which, unlike most functions of the ANS, is 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. 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 or conditional taste aversion (the memory that ensures that an animal which has been poisoned by a food never touches it again). All this visceral sensory information constantly and unconsciously modulates the activity of the motor neurons of the ANS.
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 cord: the prevertebral and pre-aortic chains. Parasympathetic ganglia, in contrast, are located in close proximity to the target organ: the submandibular ganglion close to salivary 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 cord, 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 activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” (also called, improperly but classically, "visceral motoneurons") located in the central nervous system. Preganglionic sympathetic neurons are in the spinal cord, 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 the salivatory nuclei), and in the sacral spinal cord. Enteric neurons are also modulated by input from the CNS and from preganglionic neurons located, like parasympathetic ones, in the medulla oblongata (in the dmnX).
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 the visceral motor neurons.
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. 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.
Promotes a "fight or flight" response, corresponds with arousal and energy generation, and inhibits digestion.
Promotes a "rest and digest" response, promotes calming of the nerves return to regular function, and enhances digestion.
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:
The following table reviews the actions of these neurotransmitters as a function of their receptors.
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| cardiac output | β1, (β2): increases | M2: decreases |
| SA node: heart rate (chronotropic) | β1, (β2): increases | M2: decreases |
| Atrial cardiac muscle: contractility (inotropic) | β1, (β2): increases | M2: decreases |
| at AV node | β1: increases conduction increases cardiac muscle automaticity |
M2: decreases conduction Atrioventricular block |
| Ventricular cardiac muscle | β1, (β2): increases contractility (inotropic) increases cardiac muscle automaticity |
--- |
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| vascular smooth muscle in general | α1: contracts; β2: relaxes | M3: relaxes |
| renal artery | α1: constricts | --- |
| larger coronary arteries | α1 and α2: constricts | --- |
| smaller coronary arteries | β2:dilates | --- |
| arteries to viscera | α: constricts | --- |
| arteries to skin | α: constricts | --- |
| arteries to brain | α1: constricts | --- |
| arteries to erectile tissue | α1: constricts | M3: dilates |
| arteries to salivary glands | α: constricts | M3: dilates |
| hepatic artery | β2: dilates | --- |
| arteries to skeletal muscle | β2: dilates | --- |
| Veins | α1 and α2: constricts β2: dilates |
--- |
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| platelets | α2: aggregates | --- |
| mast cells - histamine | β2: inhibits | --- |
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| smooth muscles of bronchioles | β2: relaxes (major contribution) α1: contracts (minor contribution) |
M3: contracts |
The bronchioles have no sympathetic innervation, but are instead affected by circulating adrenaline
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| Pupil dilator muscle | α1: Dilates (causes mydriasis) |
|
| Iris sphincter muscle | - | M3: contracts (causes miosis) |
| Ciliary muscle | β2: relaxes (causes long-range focus) |
M3: contracts (causes short-range focus) |
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| salivary glands: secretions | β: stimulates viscous, amylase secretions α1: stimulates potassium secretions |
M3: stimulates watery secretions |
| lacrimal glands (tears) | β: stimulates protein secretion | --- |
| juxtaglomerular apparatus of kidney | β1: renin secretion | --- |
| parietal cells | --- | M1: Gastric acid secretion |
| liver | α1, β2: glycogenolysis, gluconeogenesis | --- |
| adipose cells | β1, β3: stimulates lipolysis | --- |
| GI tract (smooth muscle) motility | α1, α2, β2: decreases | M3, (M1): increases |
| sphincters of GI tract | α1, α2, β2: contracts | M3: relaxes |
| glands of GI tract | no effect | M3: secretes |
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| pancreas (islets) | α2: decreases insulin secretion from beta cells, increases glucagon secretion from alpha cells | M3: increases secretion of both insulin and glucagon. |
| adrenal medulla | N (nicotinic ACh receptor): secretes epinephrine and norepinephrine | --- |
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| Detrusor urinae muscle of bladder wall | β2, β3: relaxes | M3: contracts |
| internal urethral sphincter | α1: contracts | M3: relaxes |
| Target | Sympathetic (adrenergic) | Parasympathetic (muscarinic) |
| uterus | α1: contracts (pregnant) β2: relaxes (non-pregnant) |
--- |
| genitalia | α1: contracts (ejaculation) | M3: erection |
| Target | Sympathetic (muscarinic and adrenergic) | Parasympathetic |
| sweat gland secretions | M: stimulates (major contribution); α1: stimulates (minor contribution) | --- |
| arrector pili | α1: stimulates | --- |
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