13sympathetic nervous system

8/9/2019 13Sympathetic Nervous System

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Sympathetic nervous system

Thesympathetic nervous systemis one of the two main

divisions of theautonomic nervous system, the other be-

ing theparasympathetic nervous system.[1][2] The auto-

nomic nervous system functions to regulate the bodys un-

conscious actions. The sympathetic nervous systems pri-

mary process is to stimulate the bodysfight-or-flight re-

sponse. It is, however, constantly active at a basic level to

maintain homeostasis.[3] The sympathetic nervous system

is described as being complementary to the parasympa-

thetic nervous system which stimulates the body to rest-

and-digest orfeed and breedThe name of this system can be traced to the concept of

sympathy, in the sense of connection between parts,

first used medically by Galen.[4] In the 18th century,

Winslowapplied the term specifically to nerves.[5]

1 Structure

There are two kinds of neurons involved in the trans-

mission of any signal through the sympathetic sys-

tem: pre-ganglionic and post-ganglionic. The shorterpreganglionic neuronsoriginate from the thoracolumbar

region of the spinal cord (levels T1L2, specifically)

and travel to a ganglion, often one of the paravertebral

ganglia, where they synapse with a postganglionic neu-

ron. From there, the longpostganglionic neuronsextend

across most of the body.[6]

At the synapses within the ganglia, preganglionic neu-

rons release acetylcholine, a neurotransmitter that ac-

tivates nicotinic acetylcholine receptors on postgan-

glionic neurons. In response to this stimulus post-

ganglionic neuronswith two important exceptions

release norepinephrine, which activatesadrenergic recep-tors on the peripheral target tissues. The activation of tar-

get tissue receptors causes the effects associated with the

sympathetic system.[7]

The two exceptions mentioned above are postganglionic

neurons of sweat glands and chromaffin cells of the

adrenal medulla. Postganglionic neurons of sweat glands

release acetylcholine for the activation ofmuscarinic re-

ceptors. Chromaffin cells of the adrenal medulla are anal-

ogous to post-ganglionic neurons; the adrenal medulla

develops in tandem with the sympathetic nervous sys-

tem and acts as a modified sympathetic ganglion. Within

this endocrine gland, pre-ganglionic neurons synapse withchromaffin cells, stimulating the chromaffin to release

norepinephrine and epinephrine directly into the blood.[8]

1.1 Organization

The sympatheticnervous system extends fromthe thoracicto lum-

barvertebraeand has connections with the thoracic, abdominal,

and pelvic plexuses.

Sympathetic nerves arise from near the middle of the

spinal cordin the intermediolateral nucleusof thelateral

grey column, beginning at the first thoracic vertebra of

the vertebral column and are thought to extend to the

second or third lumbar vertebra. Because its cells be-

gin in the thoracic and lumbar regions of the spinal cord,

the sympathetic nervous system is said to have a thora-

columbar outflow. Axonsof these nerves leave the spinal

cord through theanterior root. They pass near the spinal

(sensory) ganglion, where they enter the anterior rami of

the spinal nerves. However, unlike somatic innervation,

they quickly separate out throughwhite ramiconnectors

(so called from the shiny white sheaths ofmyelinaroundeach axon) that connect to either the paravertebral (which

lie near the vertebral column) or prevertebral (which lie

1
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2 2 FUNCTION

near the aortic bifurcation) gangliaextending alongside

the spinal column.

To reach target organs and glands, the axons must

travel long distances in the body, and, to accomplish

this, many axons relay their message to a second cell

throughsynaptic transmission. The ends of the axonslink across a space, thesynapse, to thedendritesof the

second cell. The first cell (the presynaptic cell) sends a

neurotransmitter across the synaptic cleft where it acti-

vates the second cell (the postsynaptic cell). The message

is then carried to the final destination.

Presynaptic nerves axons terminate in either the

paravertebral gangliaor prevertebral ganglia. There are

four different ways an axon can take before reaching its

terminal. In all cases, the axon enters the paravertebral

ganglion at the level of its originating spinal nerve. After

this, it can then either synapse in this ganglion, ascend to a

more superior or descend to a more inferior paravertebralganglion and synapse there, or it can descend to a prever-

tebral ganglion and synapse there with the postsynaptic

cell.

The postsynaptic cell then goes on to innervate the tar-

geted end effector (i.e. gland, smooth muscle, etc.). Be-

cause paravertebral and prevertebral ganglia are relatively

close to the spinal cord, presynaptic neurons are generally

much shorter than their postsynaptic counterparts, which

must extend throughout the body to reach their destina-

tions.

A notable exception to the routes mentioned above is

the sympathetic innervation of the suprarenal (adrenal)medulla. In this case, presynaptic neurons pass through

paraverterbral ganglia, on through prevertebral ganglia

and then synapse directly with suprarenal tissue. This tis-

sue consists of cells that have pseudo-neuron like qualities

in that when activated by the presynaptic neuron, they will

release their neurotransmitter (epinephrine) directly into

the blood stream.

In the sympathetic nervous system and other compo-

nents of the peripheral nervous system, these synapses

are made at sites called ganglia. The cell that sends its

fiber is called a preganglionic cell, while the cell whose

fiber leaves the ganglion is called a postganglioniccell.As mentioned previously, the preganglionic cells of the

sympathetic nervous system are located between the first

thoracic segment and third lumbar segments of the spinal

cord. Postganglionic cells have their cell bodies in the

ganglia and send their axons to target organs or glands.

The ganglia include not just the sympathetic trunks but

also thecervical ganglia(superior,middleand inferior),

which sends sympathetic nerve fibers to the head and tho-

rax organs, and theceliacandmesenteric ganglia(which

send sympathetic fibers to the gut).

1.2 Information transmission

Messages travel through the sympathetic nervous sys-

tem in a bidirectional flow. Efferentmessages can trig-

ger changes in different parts of the body simultane-

ously. For example, the sympathetic nervous system

can accelerateheart rate; widenbronchialpassages; de-crease motility (movement) of the large intestine; con-

strict blood vessels; increaseperistalsisin theesophagus;

causepupillary dilation, piloerection (goose bumps) and

perspiration (sweating); and raise blood pressure.

The first synapse (preganglionic neuron to postganglionic

neuron) is mediated by nicotinicreceptors activated by

acetylcholine (a neurotransmitter). The target synapse

of the postganglionic neuron is mediated by adrenergic

receptorsand is activated by either noradrenaline (nore-

pinephrine) or adrenaline (epinephrine). An exception is

with sweat glands, which receive sympathetic innervation

but have muscarinicacetylcholine receptors, which arenormally characteristic of the parasympathetic nervous

system. Another exception is with certain deep muscle

blood vessels, which dilate (rather than constrict) with an

increase in sympathetic tone, this is because of the pres-

ence of more beta2 receptors, rather than alpha1, which

are more frequently found on other vessels.

2 Function

The sympathetic nervous system is responsible for up-and down-regulating in many homeostatic mechanisms in

living organisms. Fibers from the SN innervate tissues in

almost every organ system, providing at least some regu-

latory function to things as diverse aspupildiameter,gut

motility, andurinarysystem output and function.[12] It is

perhaps best known for mediating the neuronal and hor-

monal stress response commonly known as the fight-or-

flight response. This response is also known as sympatho-

adrenal responseof the body, as the preganglionicsym-

pathetic fibers that end in the adrenal medulla (but also all

other sympathetic fibers) secrete acetylcholine, which ac-

tivates the great secretion of adrenaline (epinephrine) andto a lesser extent noradrenaline (norepinephrine) from

it. Therefore, this response that acts primarily on the

cardiovascular systemis mediated directly via impulses

transmitted through the sympathetic nervous system and

indirectly via catecholaminessecreted from the adrenal

medulla.

Some evolutionary theorists suggest that the sympathetic

nervous system operated in early organisms to maintain

survival as the sympathetic nervous system is responsible

for priming the body for action.[13] One example of this

priming is in the moments before waking, in which sym-

pathetic outflow spontaneously increases in preparation

for action.

Sympathetic nervous system stimulation causes vasocon-
https://en.wikipedia.org/wiki/Catecholamineshttps://en.wikipedia.org/wiki/Cardiovascular_systemhttps://en.wikipedia.org/wiki/Adrenal_medullahttps://en.wikipedia.org/wiki/Preganglionichttps://en.wikipedia.org/wiki/Urinary_systemhttps://en.wikipedia.org/wiki/Gut_motilityhttps://en.wikipedia.org/wiki/Gut_motilityhttps://en.wikipedia.org/wiki/Pupilhttps://en.wikipedia.org/wiki/Muscarinichttps://en.wikipedia.org/wiki/Epinephrinehttps://en.wikipedia.org/wiki/Adrenergic_receptorshttps://en.wikipedia.org/wiki/Adrenergic_receptorshttps://en.wikipedia.org/wiki/Nicotinichttps://en.wikipedia.org/wiki/Sweatinghttps://en.wikipedia.org/wiki/Goose_bumpshttps://en.wikipedia.org/wiki/Pupillary_dilationhttps://en.wikipedia.org/wiki/Esophagushttps://en.wikipedia.org/wiki/Peristalsishttps://en.wikipedia.org/wiki/Large_intestinehttps://en.wikipedia.org/wiki/Motilityhttps://en.wikipedia.org/wiki/Bronchialhttps://en.wikipedia.org/wiki/Heart_ratehttps://en.wikipedia.org/wiki/Efferent_nerve_fiberhttps://en.wikipedia.org/wiki/Mesenteric_ganglionhttps://en.wikipedia.org/wiki/Celiac_ganglionhttps://en.wikipedia.org/wiki/Inferior_cervical_ganglionhttps://en.wikipedia.org/wiki/Middle_cervical_ganglionhttps://en.wikipedia.org/wiki/Superior_cervical_ganglionhttps://en.wikipedia.org/wiki/Cervical_gangliahttps://en.wikipedia.org/wiki/Postganglionichttps://en.wikipedia.org/wiki/Prevertebral_gangliahttps://en.wikipedia.org/wiki/Paravertebral_gangliahttps://en.wikipedia.org/wiki/Neurotransmitterhttps://en.wikipedia.org/wiki/Dendriteshttps://en.wikipedia.org/wiki/Synapsehttps://en.wikipedia.org/wiki/Synaptic_transmissionhttps://en.wikipedia.org/wiki/Ganglion


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3

striction of most blood vessels, including many of those

in the skin, the digestive tract, and the kidneys. This oc-

curs as a result of activation of alpha-1 adrenergic recep-

tors by norepinephrine released by post-ganglionic sym-

pathetic neurons. These receptors exist throughout the

vasculature of the body but are inhibited and counter-

balanced by beta-2 adrenergic receptors (stimulated byepinephrine release from the adrenal glands) in the skele-

tal muscles, the heart, the lungs, and the brain during a

sympatho-adrenal response. The net effect of this is a

shunting of blood away from the organs not necessary to

the immediate survival of the organism and an increase

in blood flow to those organs involved in intense physical

activity.

2.1 Sensation

The afferent fibers of the autonomic nervous system,

which transmit sensory information from the internal or-

gans of the body back to the central nervous system (or

CNS), are not divided into parasympathetic and sympa-

thetic fibers as the efferent fibers are.[14] Instead, auto-

nomic sensory information is conducted bygeneral vis-

ceral afferent fibers.

General visceral afferent sensations are mostly uncon-

scious visceral motor reflex sensations from hollow or-

gans and glands that are transmitted to the CNS. While

the unconsciousreflex arcsnormally are undetectable, in

certain instances they may send pain sensations to the

CNS masked asreferred pain. If theperitoneal cavitybe-

comes inflamed or if the bowel is suddenly distended, the

body will interpret the afferent pain stimulus assomatic

in origin. This pain is usually non-localized. The pain is

also usually referred todermatomesthat are at the same

spinal nerve level as the visceral afferentsynapse.

2.2 Relationship with the parasympathetic

nervous system

Together with the other component of the autonomic ner-

vous system, the parasympathetic nervous system, the

sympathetic nervous system aids in the control of mostof the bodys internal organs. Reaction to stress as

in the flight-or-fight response is thought to counter-

act theparasympathetic system, which generally works to

promote maintenance of the body at rest. The compre-

hensive functions of both the parasympathetic and sym-

pathetic nervous systems are not so straightforward, but

this is a useful rule of thumb.[3][15]

3 Clinical significance

Sympathicotonia is a stimulated[16] condition of the sym-pathetic nervous system, marked byvascular spasm,[17]

elevated blood pressure,[17] andgoose bumps.[17]

4 See also

Epinephrine

History of catecholamine research

Norepinephrine

Sympathetic ganglia

Sympathetic trunk

5 References

[1] Dorlands (2012). Dorlands Illustrated Medical Dictio-

nary(32nd ed.). Elsevier Saunders. p. 1862. ISBN 978-

1-4160-6257-8.

[2] Pocock, G and Richards, C (2006). Human PhysiologyThe Basis of Medicine (Third ed.). Oxford University

Press. p. 63. ISBN 978-0-19-856878-0.

[3] Brodal, Per (2004). The Central Nervous System: Structure

and Function (3 ed.). Oxford University Press US. pp.

369396.ISBN 0-19-516560-8.

[4] Sympathy, Sympathetic. Evolution of a Concept and Rel-

evance to Current Understanding of Autonomic Disorders

(2013)

[5] Olry, R. (1996). Winslows contribution to our un-

derstanding of the cervical portion of the sympathetic

nervous system. J Hist Neurosci 5 (2): 1906.doi:10.1080/09647049609525666.PMID 11619046.

[6] Drake, Richard L.; Vogl, Wayne; Mitchell, Adam W.M.,

eds. (2005). Grays Anatomy for Students (1 ed.). Else-

vier. pp. 7684.ISBN 0-443-06612-4.

[7] Rang, H.P.; Dale, M.M.; Ritter, J.M.; Flower, R.J.

(2007). Rang and Dales Pharmacology (6 ed.). Elsevier.

p. 135.ISBN 0-443-06911-5.

[8] Silverthorn, Dee Unglaub (2009). Human Physiology:

An Integrated Approach(4 ed.). Pearson/Benjamin Cum-

mings. pp. 379386. ISBN 0-321-54130-8.

[9] Unless specified otherwise in the boxes, the source is:

Moore, Keith L.; Agur, A. M. R. (2002). Essential Clin-

ical Anatomy(2nd ed.). Lippincott Williams & Wilkins.

p. 199.ISBN 978-0-7817-5940-3.

[10] Berger, Michael P. Hlastala; Albert J. (2001). Physiology

of respiration (2. ed. ed.). Oxford [u.a.]: Oxford Univ.

Press. p. 177.ISBN 0195138465.

[11] Jnig, Wilfrid (2006). The integrative action of the au-

tonomic nervous system : neurobiology of homeostasis.

Cambridge: UK. pp. 132135.ISBN 9780521845182.

[12] Moro, C; Tajouri, L; Chess-Williams, R (January 2013).

Adrenoceptor function and expression in bladder urothe-lium and lamina propria. Urology. 81 (1): 211.e17.

doi:10.1016/j.urology.2012.09.011.PMID 23200975.
https://www.ncbi.nlm.nih.gov/pubmed/23200975https://en.wikipedia.org/wiki/PubMed_Identifierhttps://dx.doi.org/10.1016%252Fj.urology.2012.09.011https://en.wikipedia.org/wiki/Digital_object_identifierhttps://en.wikipedia.org/wiki/Special:BookSources/9780521845182https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttps://en.wikipedia.org/wiki/Special:BookSources/0195138465https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttps://en.wikipedia.org/wiki/Special:BookSources/978-0-7817-5940-3https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?id=PSGWpeMmKC4Chttp://books.google.com/books?id=PSGWpeMmKC4Chttps://en.wikipedia.org/wiki/Special:BookSources/0-321-54130-8https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?id=32dHPgAACAAJ&dq=0321541308http://books.google.com/books?id=32dHPgAACAAJ&dq=0321541308https://en.wikipedia.org/wiki/Special:BookSources/0-443-06911-5https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?id=mtNy9DgYCIgC&dq=0443069115https://en.wikipedia.org/wiki/Special:BookSources/0-443-06612-4https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?cd=1&id=SJBqAAAAMAAJhttps://www.ncbi.nlm.nih.gov/pubmed/11619046https://en.wikipedia.org/wiki/PubMed_Identifierhttps://dx.doi.org/10.1080%252F09647049609525666https://en.wikipedia.org/wiki/Digital_object_identifierhttp://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=11619046http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=11619046http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=11619046http://www.neurology.org/cgi/content/meeting_abstract/80/1_MeetingAbstracts/S57.005http://www.neurology.org/cgi/content/meeting_abstract/80/1_MeetingAbstracts/S57.005https://en.wikipedia.org/wiki/Special:BookSources/0-19-516560-8https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.co.uk/books?id=WdATFQ0YUrMC&pg=PA369#v=onepage&q=&f=falsehttp://books.google.co.uk/books?id=WdATFQ0YUrMC&pg=PA369#v=onepage&q=&f=falsehttps://en.wikipedia.org/wiki/Special:BookSources/978-0-19-856878-0https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttps://en.wikipedia.org/wiki/Special:BookSources/978-1-4160-6257-8https://en.wikipedia.org/wiki/Special:BookSources/978-1-4160-6257-8https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttps://en.wikipedia.org/wiki/Sympathetic_trunkhttps://en.wikipedia.org/wiki/Sympathetic_gangliahttps://en.wikipedia.org/wiki/Norepinephrinehttps://en.wikipedia.org/wiki/History_of_catecholamine_researchhttps://en.wikipedia.org/wiki/Epinephrinehttps://en.wikipedia.org/wiki/Goose_bumpshttps://en.wikipedia.org/wiki/Hypertensionhttps://en.wikipedia.org/wiki/Vascular_spasmhttps://en.wikipedia.org/wiki/Parasympathetic_nervous_systemhttps://en.wikipedia.org/wiki/Stress_(biology)https://en.wikipedia.org/wiki/Autonomic_nervous_systemhttps://en.wikipedia.org/wiki/Autonomic_nervous_systemhttps://en.wikipedia.org/wiki/Synapsehttps://en.wikipedia.org/wiki/Dermatomeshttps://en.wikipedia.org/wiki/Somatic_nervous_systemhttps://en.wikipedia.org/wiki/Peritoneal_cavityhttps://en.wikipedia.org/wiki/Referred_painhttps://en.wikipedia.org/wiki/Painhttps://en.wikipedia.org/wiki/Reflex_arcshttps://en.wikipedia.org/wiki/Central_nervous_systemhttps://en.wikipedia.org/wiki/General_visceral_afferent_fibershttps://en.wikipedia.org/wiki/General_visceral_afferent_fibershttps://en.wikipedia.org/wiki/Autonomic_nervous_system


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4 5 REFERENCES

[13] Robert Ornstein (1992). The Evolution of Consciousness:

of Darwin, Freud, and Cranial Fire: The Origins of the

Way We Think. New York: Simon & Schuster. ISBN

0-671-79224-5.

[14] Moore, K.L., & Agur, A.M. (2007). Essential Clinical

Anatomy: Third Edition. Baltimore: Lippincott Williams

& Wilkins. 34-35. ISBN 978-0-7817-6274-8

[15] Sherwood, Lauralee (2008). Human Physiology: From

Cells to Systems(7 ed.). Cengage Learning. p. 240.ISBN

0-495-39184-0.

[16] thefreedictionary.comCiting: Dorlands Medical Dictio-

nary for Health Consumers. 2007

[17] thefreedictionary.com Citing: The American Heritage

Medical Dictionary Copyright 2007
http://medical-dictionary.thefreedictionary.com/sympathicotoniahttp://medical-dictionary.thefreedictionary.com/sympathicotoniahttps://en.wikipedia.org/wiki/Special:BookSources/0-495-39184-0https://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?id=gOmpysGBC90C&pg=PT268#v=onepage&q=&f=falsehttp://books.google.com/books?id=gOmpysGBC90C&pg=PT268#v=onepage&q=&f=falsehttps://en.wikipedia.org/wiki/Special:BookSources/9780781762748https://en.wikipedia.org/wiki/Special:BookSources/0-671-79224-5https://en.wikipedia.org/wiki/International_Standard_Book_Number


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http://creativecommons.org/licenses/by-sa/3.0/http://localhost/var/www/apps/conversion/tmp/scratch_8//en.wikipedia.org/wiki/User:Tkgd2007http://localhost/var/www/apps/conversion/tmp/scratch_8//en.wikipedia.org/wiki/User:Equazcionhttp://localhost/var/www/apps/conversion/tmp/scratch_8//en.wikipedia.org/wiki/File:Question_book.pnghttp://upload.wikimedia.org/wikipedia/en/9/99/Question_book-new.svghttp://upload.wikimedia.org/wikipedia/commons/7/77/Gray838.pnghttp://tango.freedesktop.org/The_Peoplehttp://tango.freedesktop.org/Tango_Desktop_Projecthttp://upload.wikimedia.org/wikipedia/en/f/f2/Edit-clear.svghttp://localhost/var/www/apps/conversion/tmp/scratch_8//commons.wikimedia.org/wiki/Special:Contributions/Dsmurathttp://localhost/var/www/apps/conversion/tmp/scratch_8//commons.wikimedia.org/wiki/User_talk:Dsmurathttp://localhost/var/www/apps/conversion/tmp/scratch_8//commons.wikimedia.org/wiki/User:Dsmurathttp://localhost/var/www/apps/conversion/tmp/scratch_8//commons.wikimedia.org/wiki/File:Ambox_scales.svghttp://upload.wikimedia.org/wikipedia/commons/b/b4/Ambox_important.svghttp://en.wikipedia.org/wiki/Sympathetic%2520nervous%2520system?oldid=644135319

13sympathetic nervous system

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