Peripheral nervous system | |
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![]() The Human Nervous System. Blue is PNS while yellow is CNS.
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Identifiers | |
TA | A14.2.00.001 |
FMA | 9903 |
Anatomical terms of neuroanatomy |
The peripheral nervous system (PNS) is the part of the nervous system that consists of the nerves and ganglia on the outside of the brain and spinal cord.[1] The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs, essentially serving as a communication relay going back and forth between the brain and spinal cord with the rest of the body.[2] Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain barrier, which leaves it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system; some textbooks also include sensory systems. The cranial nerves are part of the PNS with the exception of cranial nerve II, the optic nerve, along with the retina. The second cranial nerve is not a true peripheral nerve but a tract of the diencephalon.[3] Cranial nerve ganglia originate in the CNS. However, the remaining twelve cranial nerve axons extend beyond the brain and are therefore considered part of the PNS.[4] Autonomic nervous system is an involuntary control of smooth muscle.[5]
Structure
The Sensory-Somatic Nervous System
Ten out of the twelve cranial nerves originate from the brainstem, and mainly control the functions of the anatomic structures of the head with some exceptions. The nuclei of cranial nerves I and II lie in the forebrain and thalamus, respectively, and are thus not considered to be true cranial nerves. CN X (10) receives visceral sensory information from the thorax and abdomen, and CN XI (11) is responsible for innervating the sternocleidomastoid and trapezius muscles, neither of which being exclusively in the head.
Spinal nerves take their origins from the spinal cord. They 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. 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. It is important to note that 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)
The first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that subserve 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 ears, the greater auricular nerve and the lesser auricular nerve. See occipital neuralgia. The phrenic nerve arises from nerve roots C3, C4 and C5. It innervates the diaphragm, enabling breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible. See myelopathy
Brachial plexus (C5–T1)
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–L4)
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:
The Autonomic Nervous System
The autonomic nervous system controls involuntary responses to regulate physiological functions.[5] The brain and spinal cord from central nervous system are connected with organs that have smooth muscle, such as heart, bladder, and other cardiac, exocrine, and endocrine related organs, by ganglionic neurons.[5] The most notable physiological effects from autonomic activity are pupil constriction and dilation, and salivation of saliva.[5] The autonomic nervous system is always activated, but is either in the sympathetic or parasympathetic state.[5] Depending on the situation, one state can overshadows the other, resulting in a release of different kinds of neurotransmitters.[5] There is a lesser known division of the autonomic nervous system known as the enteric nervous system.[6] 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.[6] The enteric system is responsible for various functions related to gastrointestinal system.[6]
The Sympathetic Nervous System
The sympathetic system is activated during a “fight or flight” situation in which great mental stress or physical danger is encountered.[5] Neurotransmitters such as noradrenaline (NA), norepinephrine (NE), and adrenaline (Ad) are released,[5] which increases heart rate and blood flow in certain areas like muscle, while simultaneously decreasing activities of non-critical functions for survival, like digestion.[6] The systems are independent to each other, which allows activation of certain parts of the body, while others remain rested.[6]
The 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.[6] 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.[6] Unlike the sympathetic system, humans have some voluntary controls in the parasympathetic system. The most prominent examples of this control are urination and defecation.[6]
Pathology
Peripheral Neuropathy is one of the most common forms of disease within the Peripheral Nervous System.[7] Damage or malfunction in peripheral nerves generates not only acute and chronic pain, but also changes in sensory pathways.[7] In response to such nerve damage, restorative processes, including Inflammation and other proinflammatory processes, create hyperexcitability within the sensory neurons, called peripheral sensitization.[7] These peripheral nerve fibers are then able to associate with nociceptive fibers and are then able to elicit pain.[7] This sensitization pathway is not functioning during either normal tissue operation or lack of inflammation unless the sensory neurons are been permanently damaged.[7] The pain that stems from Peripheral Neuropathy stems from NMDA receptors and the release of the neurotransmitter glutamate from sensory neurons in the brain. [7] Release of glutamate and coupled neurotransmitters that connect neurokinin receptors causes release of magnesium and calcium ions while simultaneously activating NMDA receptors.[7] This causes postsynaptic cells to send a painful signal to the brain.[7] Upon interaction with calcium, the NMDA receptors become more sensitive and the neuronal threshold is lowered.[7] Mononeuropathy is a subset under the broad context of Peripheral Neuropathy and occurs only when a single nerve outside of the Central Nervous System is affected.[7] The most common forms of mononeuropathy are when physical stress is placed on a particular nerve, known more specifically as compression neuropathy.[7] One of the most common forms of mononeuropathy is carpal tunnel syndrome.[7] Common symptoms of carpal tunnel syndrome include pain and numbness in the thumb, index and middle finger.[7] One of the major causes of carpal tunnel syndrome is demyelination of axons through mechanical stress.[7] A lack of myelin can prevent nervous transmission and eventually, development of an endoneural edema.[7] This culminates in the degeneration of axons and release of pro inflammatory molecules, causing pain and redness.[7] Other subsets of peripheral neuropathy include polyneuropathy which causes pain in similar areas on both sides of the body, diabetic neuropathy or nerve damage associated with diabetes mellitus, and autonomic neuropathy which consists of peripheral nerve damage that directly affects the autonomic nervous system.[7]
See also
- Spinal nerves
- Cranial nerves
- Autonomic nervous system
- Connective tissue in the peripheral nervous system
- Preferential motor reinnervation
References
- ^ "peripheral nervous system" at Dorland's Medical Dictionary
- ^ http://www.mayoclinic.com/health/brain/BN00033&slide=6
- ^ Board Review Series: Neuroanatomy, 4th Ed., Lippincott Williams & Wilkins, Maryland 2008, p. 177. ISBN 978-0-7817-7245-7.
- ^ James S. White (21 March 2008). Neuroscience. McGraw-Hill Professional. pp. 1–. ISBN 978-0-07-149623-0. Retrieved 17 November 2010.
- ^ a b c d e f g h Laight, David (September 2013). "Overview of peripheral nervous system pharmacology". Nurse Prescribing. ISSN 1479-9189.
- ^ a b c d e f g h Matic, Agnella Izzo (2014). "Introduction to the Nervous System, Part 2: The Autonomic Nervous System and the Central Nervous System". AMWA Journal: American Medical Writers Association Journal (AMWA J). ISSN 1075-6361.
- ^ a b c d e f g h i j k l m n o p q Vanotti, Alessandra; Osio, Maurizio; Mailland, Enrico; Nascimbene, Caterina; Capiluppi, Elisa; Mariani, Claudio (2007). "Overview on Pathophysiology and Newer Approaches to Treatment of Peripheral Neuropathies". CNS Drugs. ISSN 1172-7047.
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