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Peripheral nervous system

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The peripheral nervous system (PNS) resides or extends outside the central nervous system (CNS), which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. [1]

General classificationEdit

The peripheral nervous system can be classified either by direction of neurons or by function.

By directionEdit

There are three types of directions of the neurons:

By functionEdit

By function, the peripheral nervous system is divided into the somatic nervous system, autonomic nervous system and the enteric nervous system. The somatic nervous system is responsible for coordinating the body movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control. The autonomic nervous system is then split into the sympathetic division, parasympathetic division, and enteric division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of adrenaline in the system. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and genitourinary systems. The role of the enteric nervous system is to manage every aspect of digestion, from the esophagus to the stomach, small intestine and colon.

Naming of specific nerves Edit

Nervous system diagram

The Human Nervous System. Blue is PNS while red is CNS.

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 is 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)Edit

Further information: Cervical plexus

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)Edit

Further information: 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 arm 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

Before forming three cordsEdit

The first nerve off the brachial plexus, or plexus brachialis, is the dorsal scapular nerve, arising from C5 nerve root, and innervating the rhomboids and the levator scapulae muscles. The long thoracic nerve arises from C5, C6 and C7 to innervate the serratus anterior. The brachial plexus first forms three trunks, the superior trunk, composed of the C5 and C6 nerve roots, the middle trunk, made of the C7 nerve root, and the inferior trunk, made of the C8 and T1 nerve roots. The suprascapular nerve is an early branch of the superior trunk. It innervates the suprascapular and infrascapular muscles, part of the rotator cuff. The trunks reshuffle as they traverse towards the arm into cords. There are three of them. The lateral cord is made up of fibers from the superior and middle trunk. The posterior cord is made up of fibers from all three trunks. The medial cord is composed of fibers solely from the medial trunk.

Lateral cordEdit

The lateral cord gives rise to the following nerves:

Posterior cordEdit

The posterior cord gives rise to the following nerves:

Medial cordEdit

The medial cord gives rise to the following nerves:

NeurotransmittersEdit

The main neurotransmitters of the peripheral nervous system are acetylcholine and noradrenaline. However, there are several other neurotransmitters as well, jointly labeled Non-noradrenergic, non-cholinergic (NANC) transmitters. Examples of such transmitters include non-peptides: ATP, GABA, dopamine, NO, and peptides: neuropeptide Y, VIP, GnRH, Substance P and CGRP. [2]


See also Edit

ReferencesEdit

  1. Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health, 132-144, Englewood Cliffs, New Jersey, USA: Prentice Hall.
  2. Pharmacology, (Rang, Dale, Ritter & Moore, ISBN 0443071454, 5:th ed., Churchill Livingstone 2003). Page 132.

Further readingEdit


Nervous system

Brain - Spinal cord - Central nervous system - Peripheral nervous system - Somatic nervous system - Autonomic nervous system - Sympathetic nervous system - Parasympathetic nervous system


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