Editing Peripheral nervous system (section)

== Structure ==
The peripheral nervous system can be divided into a somatic and an autonomic division, which are part of the [[somatic nervous system]] and the [[autonomic nervous system]], respectively. The somatic nervous system is under voluntary control, and transmits signals from the brain to end organs such as [[muscle]]s. The [[sensory nervous system]] is part of the somatic nervous system and transmits signals from senses such as [[taste]] and touch (including fine touch and gross touch) to the spinal cord and brain. The autonomic nervous system is a "self-regulating" system which influences the function of organs outside voluntary control, such as the [[heart rate]], or the functions of the [[digestion|digestive system]].

===Somatic nervous system===
{{More citations needed section|date=May 2020}}
{{See also|List of nerves of the human body}}

The [[somatic nervous system]] includes the [[sensory nervous system]] (ex. the [[somatosensory system]]) and consists of sensory nerves and somatic nerves, and many nerves which hold both functions.

In the [[head]] and [[neck]], [[cranial nerve]]s carry somatosensory data. There are twelve cranial nerves, ten of which originate from the [[brainstem]], and mainly control the functions of the anatomic structures of the head with some exceptions.  One unique cranial nerve is the [[vagus nerve]], which receives sensory information from organs in the [[thorax]] and [[abdomen]]. The other unique cranial nerve is the [[accessory nerve]] which is responsible for innervating the [[sternocleidomastoid muscle|sternocleidomastoid]] and [[trapezius muscle]]s, neither of which are located exclusively in the head.

For the rest of the body, [[spinal nerve]]s are responsible for somatosensory information. These arise from the [[spinal cord]]. Usually these arise as a web ("plexus") of interconnected nerves roots that arrange to form single nerves. These nerves control the functions of the rest of the body. In humans, there are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. These nerve roots are named according to the spinal vertebrata which they are adjacent to. In the cervical region, the spinal nerve roots come out ''above'' the corresponding vertebrae (i.e., nerve root between the skull and 1st cervical vertebrae is called spinal nerve C1). From the thoracic region to the coccygeal region, the spinal nerve roots come out ''below'' the corresponding vertebrae. This method creates a problem when naming the spinal nerve root between C7 and T1 (so it is called spinal nerve root C8). In the lumbar and sacral region, the spinal nerve roots travel within the dural sac and they travel below the level of L2 as the cauda equina.

====Cervical spinal nerves (C1–C4)====
{{Further|Cervical plexus}}
The first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that serve the neck and back of head.

Spinal nerve C1 is called the [[suboccipital nerve]], which provides motor innervation to muscles at the base of the [[skull]].
C2 and C3 form many of the nerves of the neck, providing both sensory and motor control. These include the [[greater occipital nerve]], which provides sensation to the back of the head, the [[lesser occipital nerve]], which provides sensation to the area behind the [[ear]]s, the [[greater auricular nerve]] and the [[lesser auricular nerve]].

The [[phrenic nerve]] is a nerve essential for our survival which arises from nerve roots C3, C4 and C5. It supplies the [[thoracic diaphragm]], enabling [[breathing]]. If the spinal cord is transected above C3, then spontaneous breathing is not possible.{{citation needed|date=May 2020}}

====Brachial plexus (C5–T1)====
{{Further|Brachial plexus}}
The last four cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form the [[brachial plexus]], or [[plexus brachialis]], a tangled array of nerves, splitting, combining and recombining, to form the nerves that subserve the upper-limb and upper back. Although the brachial plexus may appear tangled, it is highly organized and predictable, with little variation between people. See [[brachial plexus injuries]].

==== Lumbosacral plexus (L1–Co1) ====
The [[anterior]] divisions of the [[lumbar nerves]], [[sacral nerves]], and [[coccygeal nerve]] form the [[lumbosacral plexus]], the first lumbar nerve being frequently joined by a branch from the [[twelfth thoracic]]. For descriptive purposes this plexus is usually divided into three parts:
[[File:3D Medical Animation of Lumbosacral Plaxus.jpg|alt=3D Medical Animation still shot of Lumbosacral Plexus|thumb|3D Medical Animation still shot of Lumbosacral Plexus]]
* [[lumbar plexus]]
* [[sacral plexus]]
* [[Pudendal plexus (nerves)|pudendal plexus]]

===Autonomic nervous system===
The [[autonomic nervous system]] (ANS) controls involuntary responses to regulate physiological functions.<ref name=":0">{{Cite journal|last=Laight|first=David|date=September 2013|title=Overview of peripheral nervous system pharmacology|journal=Nurse Prescribing|volume=11|issue=9|pages=448–454|doi=10.12968/npre.2013.11.9.448|issn=1479-9189}}{{Dead link|date=May 2020 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> The brain and spinal cord of the [[central nervous system]] are connected with organs that have smooth muscle or cardiac muscle, such as the heart, bladder, and other cardiac, exocrine, and endocrine related organs, by ganglionic neurons.<ref name=":0" /> The most notable physiological effects from autonomic activity are pupil constriction and dilation, and salivation of saliva.<ref name=":0" /> The autonomic nervous system is always activated, but is either in the sympathetic or parasympathetic state.<ref name=":0" /> Depending on the situation, one state can overshadow the other, resulting in a release of different kinds of [[neurotransmitters]].<ref name=":0" />

==== Sympathetic nervous system ====
The [[sympathetic system]] is activated during a "fight or flight" situation in which mental stress or physical danger is encountered.<ref name=":0" /> Neurotransmitters such as [[norepinephrine]], and [[epinephrine]] are released,<ref name=":0" /> which increases heart rate and blood flow in certain areas like muscle, while simultaneously decreasing activities of non-critical functions for survival, like digestion.<ref name=":1" /> The systems are independent to each other, which allows activation of certain parts of the body, while others remain rested.<ref name=":1" />

==== Parasympathetic nervous system ====
Primarily using the neurotransmitter [[acetylcholine]] (ACh) as a mediator, the [[parasympathetic system]] allows the body to function in a "rest and digest" state.<ref name=":1" /> Consequently, when the parasympathetic system dominates the body, there are increases in salivation and activities in digestion, while heart rate and other sympathetic response decrease.<ref name=":1" /> Unlike the sympathetic system, humans have some voluntary controls in the parasympathetic system. The most prominent examples of this control are urination and defecation.<ref name=":1" />

==== Enteric nervous system ====
There is a lesser known division of the autonomic nervous system known as the [[enteric nervous system]].<ref name=":1">{{Cite journal|last=Matic|first=Agnella Izzo|date=2014|title=Introduction to the Nervous System, Part 2: The Autonomic Nervous System and the Central Nervous System|url=http://eds.b.ebscohost.com/eds/pdfviewer/pdfviewer?sid=3ecac6d9-040d-48a2-87cb-cb214913ff07%40sessionmgr103&vid=&hid=114|journal=AMWA Journal: American Medical Writers Association Journal (AMWA J)|issn=1075-6361}}{{Dead link|date=May 2020 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Located only around the digestive tract, this system allows for local control without input from the sympathetic or the parasympathetic branches, though it can still receive and respond to signals from the rest of the body.<ref name=":1" /> The enteric system is responsible for various functions related to gastrointestinal system.<ref name=":1" />