The peripheral nervous system can be divided into asomatic division and anautonomic division. Each of these can further be differentiated into a sensory and a motor sector.[4] In the somatic nervous system, thecranial nerves are part of the PNS with the exceptions of theolfactory nerve and epithelia and theoptic nerve (cranial nerve II) along with theretina, which are considered parts of the central nervous system based on developmental origin. The second cranial nerve is not a true peripheral nerve but a tract of thediencephalon.[5]Cranial nerve ganglia, as with allganglia, are part of the PNS.[6] The autonomic nervous system exerts involuntary control oversmooth muscle andglands.[7]
The peripheral nervous system can be divided into a somatic and an autonomic division, which are part of thesomatic nervous system and theautonomic nervous system, respectively. The somatic nervous system is under voluntary control, and transmits signals from the brain to end organs such asmuscles. Thesensory nervous system is part of the somatic nervous system and transmits signals from senses such astaste 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 theheart rate, or the functions of thedigestive system.
In thehead andneck,cranial nerves carry somatosensory data. There are twelve cranial nerves, ten of which originate from thebrainstem, and mainly control the functions of the anatomic structures of the head with some exceptions. One unique cranial nerve is thevagus nerve, which receives sensory information from organs in thethorax andabdomen. The other unique cranial nerve is theaccessory nerve which is responsible for innervating thesternocleidomastoid andtrapezius muscles, neither of which are located exclusively in the head.
For the rest of the body,spinal nerves are responsible for somatosensory information. These arise from thespinal 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 outabove 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 outbelow 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.
Thephrenic nerve is a nerve essential for our survival which arises from nerve roots C3, C4 and C5. It supplies thethoracic diaphragm, enablingbreathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible.[citation needed]
The last four cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form thebrachial plexus, orplexus 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. Seebrachial plexus injuries.
Theautonomic nervous system (ANS) controls involuntary responses to regulate physiological functions.[8] The brain and spinal cord of thecentral 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.[8] The most notable physiological effects from autonomic activity are pupil constriction and dilation, and salivation of saliva.[8] The autonomic nervous system is always activated, but is either in the sympathetic or parasympathetic state.[8] Depending on the situation, one state can overshadow the other, resulting in a release of different kinds ofneurotransmitters.[8]
Thesympathetic system is activated during a "fight or flight" situation in which mental stress or physical danger is encountered.[8] Neurotransmitters such asnorepinephrine, andepinephrine are released,[8] which increases heart rate and blood flow in certain areas like muscle, while simultaneously decreasing activities of non-critical functions for survival, like digestion.[9] The systems are independent to each other, which allows activation of certain parts of the body, while others remain rested.[9]
Primarily using the neurotransmitteracetylcholine (ACh) as a mediator, theparasympathetic system allows the body to function in a "rest and digest" state.[9] 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.[9] Unlike the sympathetic system, humans have some voluntary controls in the parasympathetic system. The most prominent examples of this control are urination and defecation.[9]
There is a lesser known division of the autonomic nervous system known as theenteric nervous system.[9] 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.[9] The enteric system is responsible for various functions related to gastrointestinal system.[9]
Diseases of the peripheral nervous system can be specific to one or more nerves, or affect the system as a whole.
Any peripheral nerve or nerve root can be damaged, called amononeuropathy. Such injuries can be because of injury or trauma, orcompression. Compression of nerves can occur because of a tumour mass or injury. Alternatively, if a nerve is in an area with a fixed size it may be trapped if the other components increase in size, such ascarpal tunnel syndrome andtarsal tunnel syndrome. Common symptoms ofcarpal tunnel syndrome include pain and numbness in the thumb, index and middle finger. In peripheral neuropathy, the function one or more nerves are damaged through a variety of means. Toxic damage may occur because of diabetes (diabetic neuropathy), alcohol, heavy metals or other toxins; some infections; autoimmune and inflammatory conditions such asamyloidosis andsarcoidosis.[8] Peripheral neuropathy is associated with a sensory loss in a "glove and stocking" distribution that begins at the peripheral and slowly progresses upwards, and may also be associated with acute and chronic pain. Peripheral neuropathy is not just limited to the somatosensory nerves, but the autonomic nervous system too (autonomic neuropathy).[8]
^Campbell biology. Lisa A. Urry, Michael L. Cain, Steven Alexander Wasserman, Peter V. Minorsky, Rebecca B. Orr, Neil A. Campbell (12th ed.). New York, NY. 2021.ISBN978-0-13-518874-3.OCLC1119065904.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
