- Peripheral nervous system
The peripheral nervous system (PNS) resides or extends outside the
central nervous system(CNS), which consists of the brainand 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 boneor by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous systemand the autonomic nervous system. [cite book
last = Maton
first = Anthea
coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
title = Human Biology and Health
publisher = Prentice Hall
date = 1993
location = Englewood Cliffs, New Jersey, USA
pages = 132-144
isbn = 0-13-981176-1]
The peripheral nervous system can be classified either by direction of
neuronsor by function.
There are three types of directions of the
Sensory systemby sensory neurons, between the sensory and motor neurons. However, there are relay neurons in the CNS as well .
By function, the peripheral nervous system is divided into the
somatic nervous system, autonomic nervous systemand the enteric nervous system.The somatic nervous systemis 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 systemis 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 genitourinarysystems. 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
Ten out of the twelve
cranial nervesoriginate 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. The naming convention for spinal nerves is to name it after the vertebraimmediately above it. Thus the fourth thoracic nerve originates just below the fourth thoracic vertebra. This convention breaks down in the cervical spine. The first spinal nerve originates above the first cervical vertebra and is called C1. This continues down to the last cervical spinal nerve, C8. There are only 7 cervical vertebrae and 8 cervical spinal nerves.
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 nervewhich 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 nervewhich provides sensation to the back of the head, the lesser occipital nervewhich provides sensation to the area behind the ears, the greater auricular nerveand the lesser auricular nerve. See occipital neuralgia.The phrenic nervearises 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 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 cords
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 scapulaemuscles.The long thoracic nervearises 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 nerveis 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 arminto 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.
The lateral cord gives rise to the following nerves:
lateral pectoral nerve, C5, C6 and C7 to the pectoralis majormuscle, or musculus pectoralis major.
musculocutaneous nervewhich innervates the biceps muscle
median nerve, partly. The other part comes from the medial cord. See below for details.
The posterior cord gives rise to the following nerves:
upper subscapular nerve, C7 and C8, to the subscapularismuscle, or musculus supca of the rotator cuff.
lower subscapular nerve, C5 and C6, to the teres majormuscle, or the musculus teres major.
thoracodorsal nerve, C6, C7 and C8, to the latissimus dorsimuscle, or musculus latissimus dorsi.
axillary nerve, which supplies sensation to the shoulder and motor to the deltoidmuscle or musculus deltoideus, and the teres minormuscle, or musculus teres minor, also of the rotator cuff.
radial nerve, or nervus radialis, which innervates the triceps brachiimuscle, the brachioradialismuscle, or musculus brachioradialis,, the extensor muscles of the fingers and wrist ( extensor carpi radialismuscle), and the extensor and abductor muscles of the thumb. See radial nerve injuries.
The medial cord gives rise to the following nerves:
median pectoral nerve, C8 and T1, to the pectoralis muscle
medial brachial cutaneous nerve, T1
medial antebrachial cutaneous nerve, C8 and T1
median nerve, partly. The other part comes from the lateral cord. C7, C8 and T1 nerve roots. The first branch of the median nerve is to the pronator teresmuscle, then the flexor carpi radialis, the palmaris longusand the flexor digitorum superficialis. The median nerve provides sensation to the anterior palm, the anterior thumb, index fingerand middle finger. It is the nerve compressed in carpal tunnel syndrome.
ulnar nerveoriginates in nerve roots C7, C8 and T1. It provides sensation to the ring and pinky fingers. It innervates the flexor carpi ulnarismuscle, the flexor digitorum profundusmuscle to the ring and pinky fingers, and the intrinsic muscles of the hand (the interosseous muscle, the lumbrical muscles and the flexor pollicus brevismuscle). This nerve traverses a groove on the elbow called the cubital tunnel, also known as the funny bone. Striking the nerve at this point produces an unpleasant sensation in the ring and little finger.
neurotransmittersof the peripheral nervous system are acetylcholineand 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 Pand CGRP. Pharmacology, (Rang, Dale, Ritter & Moore, ISBN 0443071454, 5:th ed., Churchill Livingstone 2003). Page 132. ]
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