Editing Somatic nervous system

{{Short description|Part of the peripheral nervous system}}
{{More citations needed|date=March 2015}}
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{{Infobox anatomy
| Name        = Somatic nervous system
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| Image       = File:Somatic Nervous System Image.svg
| Caption     = 1. ([[Human brain|Brain]]) [[Precentral gyrus]]: the origin of nerve signals initiating movement.
2. (Cross section of [[Spinal cord]]) [[Corticospinal tract]]: Mediator of message from brain to [[skeletal muscle]]s.

3. [[Axon]]: the [[efferent nerve fiber]] that carries the command to [[Muscle contraction|contract muscles]].
4. [[Neuromuscular junction]]:  [[skeletal muscle#Skeletal muscle cells|muscle cells]] are stimulated to contract at this intersection
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| part_of = [[Peripheral nervous system]]
|synonym=Voluntary nervous system}}

The '''somatic nervous system''' ('''SNS'''), also known as '''voluntary nervous system''', is a part of the [[peripheral nervous system]] (PNS) that links [[brain]] and [[spinal cord]] to [[skeletal muscle]]s under conscious control, as well as to [[sensory receptor]]s in the skin.<ref>{{Cite web|date=2018-10-09|title=Somatic nervous system|url=https://qbi.uq.edu.au/brain/brain-anatomy/peripheral-nervous-system/somatic-nervous-system|access-date=2021-04-22|website=qbi.uq.edu.au|language=en}}</ref><ref>{{Cite book|url=https://www.ncbi.nlm.nih.gov/books/NBK279390/|title=How does the nervous system work?|date=2016-08-19|publisher=Institute for Quality and Efficiency in Health Care|language=en}}</ref> The other part complementary to the somatic nervous system is the [[autonomic nervous system]] (ANS). 

The somatic nervous system consists of [[nerve]]s carrying [[afferent nerve fiber]]s, which relay sensation from the body to the [[central nervous system]] (CNS), and nerves carrying [[efferent nerve fiber]]s, which relay motor commands from the CNS to stimulate [[muscle contraction]].<ref name="Akinrodoye_2022">{{cite journal | vauthors = Akinrodoye MA, Lui F | title = Neuroanatomy, Somatic Nervous System | date = 2022 | pmid = 32310487 | url = https://www.ncbi.nlm.nih.gov/books/NBK556027/ | website = StatPearls | publisher = StatPearls Publishing | access-date = 12 December 2022 }}</ref> Specialized nerve fiber ends called [[sensory receptors]] are responsible for detecting information both inside and outside the body.

The ''a-'' of ''afferent'' and the ''e-'' of ''efferent'' correspond to the [[prefix]]es ''ad-'' (to, toward) and ''ex-'' (out of).

== Structure ==
There are 43 segments of nerves in the human body.<ref name="Rea_2014">{{Cite book|chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128008980000191|chapter=Introduction to the Nervous System|date=2014-01-01|publisher=Academic Press|isbn=978-0-12-800898-0|language=en|doi=10.1016/b978-0-12-800898-0.00019-1|title=Clinical Anatomy of the Cranial Nerves| vauthors = Rea P |pages=xv-xxix}}</ref> With each segment, there is a pair of sensory and motor nerves. 31 segments of nerves are in the spinal cord and 12 are in the brain stem.<ref name="Rea_2014" /> [[Interneuron]]s also known as '''association neurons''' are present throughout the central nervous system forming links between the sensory and motor fibres.<ref name="SEER2024">{{cite web |title=Nerve Tissue {{!}} SEER Training |url=https://training.seer.cancer.gov/anatomy/nervous/tissue.html |website=training.seer.cancer.gov |access-date=19 August 2024}}</ref>
Thus the somatic nervous system consists of two parts:
* [[Spinal nerve]]s: They are [[mixed nerve]]s that carry sensory information into and motor commands out of the spinal cord.<ref name="Felten_2016">{{Citation| vauthors = Felten DL, O'Banion MK, Maida MS | chapter = 9 - Peripheral Nervous System|date= January 2016 | veditors = Felten DL, O'Banion MK, Maida MS | title = Netter's Atlas of Neuroscience | edition = Third |pages=153–231 |place=Philadelphia |publisher=Elsevier |language=en |doi=10.1016/b978-0-323-26511-9.00009-6 |isbn=978-0-323-26511-9}}</ref> The spinal nerves serve as a bridge between the environment and the central nervous system (CNS). These neurons work together to transfer autonomic, sensory, and motor impulses from the spinal cord to the body's other systems. The spinal nerves are arranged into 31 pairs according to the regions of the spinal cord. To be more precise, there are eight pairs of [[cervical nerves]] (C1–C8), twelve pairs of [[thoracic nerves]] (T1–T12), five pairs of [[lumbar nerves]] (L1–L5), five pairs of [[sacral nerves]] (S1–S5), and one pair of [[coccygeal nerves]]. Peripheral nerves are included in the category of peripheral nervous system.<ref>{{cite book | vauthors = Kaiser JT, Lugo-Pic JG | chapter = Neuroanatomy, Spinal Nerves |date=2024 | title = StatPearls | chapter-url = http://www.ncbi.nlm.nih.gov/books/NBK542218/ |access-date=2024-01-26 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31194375 }}</ref>
* [[Cranial nerves]]: They are the nerve fibers that carry information into and out of the brain stem.<ref name="Rea_2014" /> They include smell, eye muscles, mouth, taste, ear, neck, shoulders, and tongue.<ref name="Felten_2016" /> Partially innervating the head and neck structures are the cranial nerves, which supply afferent and efferent functions. Neural processes connected to certain brainstem nuclei and cortical regions make up cranial nerves, in contrast to [[spinal nerves]], which have neural fibers originating from the spinal grey matter as their roots.<ref>{{cite book | vauthors = Sonne J, Lopez-Ojeda W | chapter = Neuroanatomy, Cranial Nerve |date=2024 | title = StatPearls | chapter-url = http://www.ncbi.nlm.nih.gov/books/NBK470353/ |access-date=2024-01-26 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=29261885 }}</ref> The cranial nerves VIII (vestibulocochlear), II (optic), and I (olfactory) are regarded as entirely afferent. The exclusively efferent cranial nerves are XI (spinal accessory), XII (hypoglossal), VI (abducens), IV (trochlear), and III (oculomotor). The remaining cranial nerves, X (vagus), IX (glossopharyngeal), VII (facial), and V (trigeminal), have mixed sensory and motor functions.<ref>{{cite journal | vauthors = Traylor KS, Branstetter BF | title = Cranial Nerve Anatomy | journal = Neuroimaging Clinics of North America | volume = 32 | issue = 3 | pages = 565–576 | date = August 2022 | pmid = 35843663 | doi = 10.1016/j.nic.2022.04.004 | series = Neuroimaging Anatomy, Part 1: Brain and Skull | s2cid = 250568029 }}
</ref> Anatomically, cranial nerves are numbered from I to XII, which indicates their sequential origin from the caudal to the ventral brainstem. Alternatively, they can be postulated in groups based on the developmental functions they perform (sensory, motor, mixed).

== Function ==
{{Unreferenced section|date=May 2021}}
The somatic nervous system's '''principal goal''' is to facilitate the organs and striated muscles of the central nervous system so that we can carry out our daily responsibilities.

The [[primary motor cortex]], or [[precentral gyrus]], is home to the higher motor neurons that make up the basic motor pathway. These neurons transmit signals to the lower motor neurons in the spinal cord through axons known as the corticospinal tract. These impulses move to the [[neuromuscular junction]] (NMJ) of skeletal muscle via peripheral axons after synapsing with the [[lower motor neurons]] through the ventral horn of the spinal cord. A signal that travels to the NMJ, which innervates muscles, is produced by the release of acetylcholine by upper motor neurons. [[Acetylcholine]] binds to [[nicotinic acetylcholine receptors]] of alpha-motor neurons.<ref name="Akinrodoye_2022" />

The somatic nervous system controls all [[voluntary muscle|voluntary muscular]] systems within the body, and the process of voluntary  [[reflex arc]]s.<ref name="Openstax Anatomy & Physiology attribution">{{cite book| vauthors = Betts JG, Desaix P, Johnson E, Johnson JE, Korol O, Kruse D, Poe B, Wise J, Womble MD, Young KA | display-authors = 6 |title=Anatomy & Physiology|url=https://openstax.org/books/anatomy-and-physiology/pages/14-introduction |location=Houston |publisher=OpenStax CNX |isbn=978-1-947172-04-3 |date=July 16, 2023 |at=Introduction:The somatic nervous system}}</ref>

The basic route of [[nerve signal]]s within the [[efferent nerve fiber|efferent]] somatic nervous system involves a sequence that begins in the upper [[Cell (biology)|cell bodies]] of [[motor neuron]]s ([[upper motor neuron]]s) within the [[Brodmann area 4|precentral gyrus]] (which approximates the [[primary motor cortex]]). Stimuli from the precentral gyrus are transmitted from upper motor neurons, down the [[corticospinal tract]], to [[lower motor neuron]]s ([[alpha motor neurons]]) in the [[brainstem]] and [[Anterior horn of spinal cord|ventral horn]] of the [[spinal cord]]: upper motor neurons release a [[neurotransmitter]] called [[glutamate]] from their axon terminal knobs, which is received by [[glutamate receptor]]s on the lower motor neurons: from there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic [[receptor (biochemistry)|receptor]]s ([[nicotinic acetylcholine receptor]]s) of muscles, thereby relaying the stimulus to contract muscle fibers.

===Reflex arcs===
A [[reflex arc]] is a [[neural circuit]] that creates a more or less automatic link between a sensory input and a specific motor output.  Reflex circuits vary in complexity—the simplest spinal reflexes are mediated by a two-element chain, of which in the human body there is only one, also called a monosynaptic reflex (there is only one synapse between the two neurones taking part in the arc: sensory and motor). The singular example of a monosynaptic reflex is the [[patellar reflex]]. The next simplest reflex arc is a three-element chain, beginning with sensory neurons, which activate [[interneuron]]s inside of the spinal cord, which then activate motor neurons.  Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex ("knee jerk") activated by tapping the patellar tendon, contribute to ordinary behavior.

==Clinical Significance==
A medical condition known as [[peripheral neuropathy]] affects the somatic nervous system's peripheral nerve fibers.
They can be divided into congenital and acquired disorders based on the causes. They can also be categorized based on whether the [[myelin sheath]](demyelinating neuropathy) or axons (axonal neuropathy) have the predominant disease. There is a wide range of causes for axonal peripheral neuropathy, most of which are toxic-metabolic in origin and include group B vitamin deficiencies and diabetes.
Demyelinating neuropathies do not vary with length. They are frequently immune-mediated, which causes a more widespread involvement of sensorimotor function and an early loss of deep tendon reflexes. When joint position and vibratory sensory loss are present, sensory participation is more selective.

Defects in the [[central nervous system]], peripheral nervous system, or muscle itself are the cause of numerous congenital illnesses of sensory and motor function.
Owing to the vast territory encompassed by the somatic nerve system, these ailments may manifest as localized in nature, or as broad and systemic. [[Charcot-Marie-Tooth disease]], [[Myasthenia gravis]], and [[Guillain–Barré syndrome]] are a few instances of them.<ref>{{cite book | vauthors = Waxenbaum JA, Reddy V, Varacallo M | chapter = Anatomy, Autonomic Nervous System |date=2024 | title = StatPearls | chapter-url= http://www.ncbi.nlm.nih.gov/books/NBK539845/ |access-date=2024-01-26 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=30969667 }}</ref>

=== Charcot-Marie-Tooth (CMT) ===
The Charcot-Marie-Tooth (CMT) disease group comprises diverse hereditary illnesses that manifest as chronic, progressive neuropathy that affects both the motor and sensory neurons.<ref>{{cite journal | vauthors = Szigeti K, Lupski JR | title = Charcot-Marie-Tooth disease | journal = European Journal of Human Genetics | volume = 17 | issue = 6 | pages = 703–710 | date = June 2009 | pmid = 19277060 | pmc = 2947101 | doi = 10.1038/ejhg.2009.31 }}
</ref>

=== Myasthenia Gravis (MG) ===
An autoimmune neurological condition called myasthenia gravis (MG) is typified by impaired neuromuscular junction communication.<ref>{{cite journal | vauthors = Dresser L, Wlodarski R, Rezania K, Soliven B | title = Myasthenia Gravis: Epidemiology, Pathophysiology and Clinical Manifestations | journal = Journal of Clinical Medicine | volume = 10 | issue = 11 | pages = 2235 | date = May 2021 | pmid = 34064035 | pmc = 8196750 | doi = 10.3390/jcm10112235 | doi-access = free }}</ref>

=== Guillain-Barré syndrome (GBS) ===
A rare but dangerous post-infectious immune-mediated neuropathy is Guillain-Barré syndrome (GBS). It is brought on by an autoimmune reaction that destroys peripheral nervous system nerves, leading to symptoms including tingling, weakness, and numbness that can become paralysis.<ref>{{cite book | vauthors = Nguyen TP, Taylor RS | chapter = Guillain-Barre Syndrome |date=2024 | title = StatPearls | chapter-url= http://www.ncbi.nlm.nih.gov/books/NBK532254/ |access-date=2024-01-26 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=30335287 }}</ref>

==Signs of Somatic Nervous System Problems==
Depending on whether the damage is to the motor nerves, which regulate movement, or the sensory nerves, which affect the senses, the symptoms of a somatic nervous system problem can differ.<ref>{{Cite web |title=Somatic Nervous System: What It Is & Function |url=https://my.clevelandclinic.org/health/body/23291-somatic-nervous-system |access-date=2024-01-26 |website=Cleveland Clinic |language=en}}</ref>

Damage to the motor nerves shows as:
* Loss of movement control
* Spasms or cramps in muscles
* Tremors or twitching
* Wasting of muscles ([[muscle atrophy]])

The following signs could be present if the sensory system is damaged:
* Inability to feel things you touch
* Numbness or tingling
* Sharp or burning pain in the damaged area

==Other animals==
In [[invertebrates]], depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory.  For [[vertebrate]]s, however, the response of a [[skeletal striated muscle]] fiber to a neurotransmitter – always [[acetylcholine]] (ACh) – can only be excitatory.

== See also ==
*[[Autonomic nervous system]]
*[[Enteric nervous system]]
*[[Nervous system]]

== References ==
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{{Nervous system}}
{{Human systems and organs}}
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[[Category:Somatic nervous system| ]]
[[Category:Sensory systems]]
[[Category:Peripheral nervous system]]