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Autonomic nervous system

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Autonomic nervous system

"Nerves with a Skeletal Cross-Section" by?Ebers

?

?

Autonomic nervous system innervation.

Details

Identifiers

Latin

Autonomici systematis nervosi

MeSH

D001341

TA98

A14.3.00.001

TA2

6600

FMA

9905

Anatomical terminology

[edit on Wikidata]

The?autonomic nervous system?(ANS), formerly the?vegetative nervous system, is a division of the?peripheral nervous system?that supplies smooth muscle and glands,

（so, the peripheral nervous system is composed of smooth muscle and glands as basic elements,?all autonomic organs are built up in smooth muscle and gland.）

and thus influences the function of?internal organs.[1]?The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the?heart rate,?digestion,?respiratory rate,?pupillary response,?urination, and?Sexual arousal.[2]?This system is the primary mechanism in control of the?fight-or-flight response.

The autonomic nervous system is regulated by integrated?reflexes?through the?brainstem?to the?spinal cord?and?organs. Autonomic functions include?control of respiration,?cardiac regulation?(the cardiac control center),?vasomotor?activity (the?vasomotor center), and certain?reflex actions?such as?coughing,?sneezing,?swallowing?and?vomiting. Those are then subdivided into other areas and are also linked to autonomic subsystems and the peripheral nervous system. The?hypothalamus, just above the?brain stem, acts as an integrator for autonomic functions, receiving autonomic?regulatory?input from the?limbic system.[3]

The autonomic nervous system has three branches: the?sympathetic nervous system, the?parasympathetic nervous system?and the?enteric nervous system.[4][5][6][7]?Some textbooks do not include the enteric nervous system as part of this system.[8]?The sympathetic nervous system is often considered the "fight or flight" system, while the parasympathetic nervous system is often considered the "rest and digest" or "feed and breed" system. In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more slowly activated?dampening?system", but even this has exceptions, such as in?Sexual arousal?and?orgasm, wherein both play a role.[3]

There are?inhibitory?and?excitatory?synapses?between?neurons. A third subsystem of neurons have been named as?non-noradrenergic, non-cholinergic transmitters?(because they use?nitric oxide?as a?neurotransmitter) and are integral in autonomic function, in particular in the?gut?and the?lungs.[9]

Although the ANS is also known as the visceral nervous system, the ANS is only connected with the motor side.[10]?Most autonomous functions are involuntary but they can often work in conjunction with the?somatic nervous system?which provides voluntary control.

Contents

·?1Structure

·?1.1Sympathetic division

·?1.2Parasympathetic division

·?1.3Sensory neurons

·?1.4Innervation

·?1.5Motor neurons

·?2Function

·?2.1Sympathetic nervous system

·?2.2Parasympathetic nervous system

·?2.3Enteric nervous system

·?2.4Neurotransmitters

·?3History

·?4Caffeine effects

·?5See also

·?6References

·?7External links

Structure[edit]

?

Autonomic nervous system, showing?splanchnic nerves?in middle, and the vagus nerve as "X" in blue. The heart and organs below in list to right are regarded as viscera.

The autonomic nervous system is divided into the?sympathetic nervous system?and?parasympathetic nervous system. The sympathetic division emerges from the?spinal cord?in the?thoracic?and?lumbarareas, terminating around L2-3. The parasympathetic division has craniosacral “outflow”, meaning that the neurons begin at the?cranial nerves?(specifically the?oculomotor nerve,?facial nerve,?glossopharyngeal nerve?and?vagus nerve) and?sacral?(S2-S4) spinal cord.

The autonomic nervous system 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.

Sympathetic division[edit]

Main article:?Sympathetic nervous system

The sympathetic nervous system consists of cells with bodies in the?lateral grey column?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:

·?Paravertebral ganglia?(3) of the sympathetic chain (these run on either side of the vertebral bodies)

1.?cervical ganglia?(3)

2.?thoracic ganglia?(12) and rostral?lumbar ganglia?(2 or 3)

3.?caudal lumbar ganglia and?sacral ganglia

·?Prevertebral ganglia?(celiac ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion)

·?Chromaffin cells?of the?adrenal medulla?(this is the one exception to the two-neuron pathway rule: the synapse is directly efferent onto the target cell bodies)

These ganglia provide the postganglionic neurons from which innervation of target organs follows. Examples of?splanchnic (visceral) nerves?are:

·?Cervical cardiac nerves and thoracic visceral nerves, which synapse in the sympathetic chain

·?Thoracic splanchnic nerves?(greater, lesser, least), which synapse in the prevertebral ganglia

·?Lumbar splanchnic nerves, which synapse in the prevertebral ganglia

·?Sacral splanchnic nerves, which synapse in the inferior hypogastric plexus

These all contain afferent (sensory) nerves as well, known as?GVA (general visceral afferent) neurons.

Parasympathetic division[edit]

Main article:?Parasympathetic nervous system

The parasympathetic nervous system consists of cells with bodies in 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:

·?Parasympathetic ganglia?of the head: Ciliary (Cranial nerve III), Submandibular (Cranial nerve VII), Pterygopalatine (Cranial nerve VII), and Otic (Cranial nerve IX)

·?In or near the wall of an organ innervated by the Vagus (Cranial nerve X) or?Sacral nerves?(S2, S3, S4)

These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:

·?The postganglionic parasympathetic splanchnic (visceral) nerves

·?The?vagus nerve, which passes through the thorax and abdominal regions innervating, among other organs, the heart, lungs, liver and stomach

Sensory neurons[edit]

Main article:?Sensory neuron

The sensory arm is composed 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” 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 that 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.

Innervation[edit]

Autonomic nerves travel to organs throughout the body. Most organs receive parasympathetic supply by the?vagus nerve?and sympathetic supply by?splanchnic nerves. The sensory part of the latter reaches the?spinal column?at certain?spinal segments. Pain in any internal organ is perceived as?referred pain, more specifically as pain from the?dermatome?corresponding to the spinal segment.[11]

?

Autonomic nervous supply to organs in the?human body?edit

Organ

Nerves[12]

Spinal column?origin[12]

stomach

·?PS:?anterior?and?posterior vagal trunks

·?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

kidneys?and?ureters

·?PS:?vagus nerve

·?S:?thoracic?and?lumbar splanchnic nerves

T11,?T12

Motor neurons[edit]

Main article:?Motor neuron

Motor neurons of the autonomic nervous system are found in ‘’autonomic ganglia’’. Those of the parasympathetic branch are located close to the target organ whilst the ganglia of the sympathetic branch are located close to the spinal cord.

The sympathetic ganglia here, are found in two chains: the pre-vertebral and pre-aortic chains. The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” located in the central nervous system. Preganglionic sympathetic neurons are located in the spinal cord, at the thorax and upper lumbar levels. Preganglionic parasympathetic neurons are found in the medulla oblongata where they form visceral motor nuclei; the dorsal motor nucleus of the vagus nerve; the nucleus ambiguus, the?salivatory nuclei, and in the sacral region of the spinal cord.

Function[edit]

?

Function of the autonomic nervous system?[13]

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. In general, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve?homeostasis. Higher organisms maintain their integrity via homeostasis which relies on negative feedback regulation which, in turn, typically depends on the autonomic nervous system.[14]?Some typical actions of the sympathetic and?parasympathetic nervous systems?are listed below.[15]

Target organ/system

Parasympathetic

Sympathetic

Digestive system

Increase peristalsis and amount of secretion by digestive glands

Decrease activity of digestive system

Liver

No effect

Causes glucose to be released to blood

Lungs

Constricts bronchioles

Dilates bronchioles

Urinary bladder/ Urethra

Relaxes sphincter

Constricts sphincter

Kidneys

No effects

Decrease urine output

Heart

Decreases rate

Increase rate

Blood vessels

No effect on most blood vessels

Constricts blood vessels in viscera; increase BP

Salivary and Lacrimal glands

Stimulates; increases production of saliva and tears

Inhibits; result in dry mouth and dry eyes

Eye (iris)

Stimulates constrictor muscles; constrict pupils

Stimulate dilator muscle; dilates pupils

Eye (ciliary muscles)

Stimulates to increase bulging of lens for close vision

Inhibits; decrease bulging of lens; prepares for distant vision

Adrenal Medulla

No effect

Stimulate medulla cells to secrete epinephrine and norepinephrine

Sweat gland of skin

No effect

Stimulate to produce perspiration

Sympathetic nervous system[edit]

Main article:?Sympathetic nervous system

Promotes a?fight-or-flight response, corresponds with arousal and energy generation, and inhibits digestion

·?Diverts blood flow away from the?gastro-intestinal?(GI) tract and?skin?via?vasoconstriction

·?Blood flow to?skeletal muscles?and the?lungs?is enhanced (by as much as 1200% in the case of skeletal muscles)

·?Dilates?bronchioles?of the lung through circulating?epinephrine, which allows for greater?alveolar?oxygen exchange

·?Increases?heart rate?and the?contractility?of cardiac cells (myocytes), thereby providing a mechanism for enhanced blood flow to skeletal muscles

·?Dilates?pupils?and relaxes the?ciliary muscle?to the lens, allowing more light to enter the eye and enhances far vision

·?Provides?vasodilation?for the?coronary vessels?of the?heart

·?Constricts all the intestinal?sphincters?and the urinary sphincter

·?Inhibits?peristalsis

·?Stimulates?orgasm

Parasympathetic nervous system[edit]

Main article:?Parasympathetic nervous system

The parasympathetic nervous system has been 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:[citation needed]

·?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[edit]

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".[16]?Its functions include:

·?Sensing chemical and mechanical changes in the gut

·?Regulating secretions in the gut

·?Controlling?peristalsis?and some other movements

Neurotransmitters[edit]

Main articles:?Table of neurotransmitter actions in the ANS?and?Non-noradrenergic, non-cholinergic transmitter

?

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.

History[edit]

The specialised system of the autonomic nervous system was recognised by?Galen. In 1665, Willis used the terminology, and in 1900, Langley used the term, defining the two divisions as the sympathetic and parasympathetic nervous systems.[17]

Caffeine effects[edit]

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.[18]

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.[19]

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.[20]