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  Peripheral Nervous System, Biology tutorial

Introduction:

The peripheral nervous system or PNS comprises of the nerves and ganglia outside of the brain and spinal cord.

The major function of the PNS is to join the central nervous system (CNS) to the limbs and organs. Dissimilar the CNS, the PNS is not protected through the bone of skull and spine or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is categorized into the somatic nervous system and the autonomic nervous system; a few textbooks as well comprise sensory systems.

General classification:

1) By Functions:

The peripheral nervous system is functionally and also structurally categorized into the somatic nervous system and autonomic nervous system. The somatic nervous system is accountable for coordinating the body movements and as well for receiving the external stimuli. It is the system which regulates activities which are beneath conscious control. The autonomic nervous system is then divided into the sympathetic division, parasympathetic division and enteric division. The sympathetic nervous system responds to the impending danger and is accountable for the rise of one's heartbeat and blood pressure, among other physiological changes, all along with the sense of excitement one feels due to the rise of adrenaline in the system. The parasympathetic nervous system, on the other hand, is evident if a person is resting and feels relaxed and is responsible for such things as the constriction of the pupil, the slowing of heart, dilation of the blood vessels, and the stimulation of the digestive and genitourinary systems. The role of enteric nervous system is to manage each and every feature of digestion, from the esophagus to the stomach, small intestine and colon.

2) By direction:

There are two kinds of neurons, carrying nerve impulses in various directions. These two groups of neurons are:

i) The sensory neurons are afferent neurons that relay nerve impulses toward the central nervous system.

ii) The motor neurons are efferent neurons that relay nerve impulses away from the central nervous system.

The peripheral nervous system is functionally and also structurally categorized into the somatic nervous system and autonomic nervous system. The somatic nervous system is accountable for coordinating the body movements, and as well for receiving the external stimuli. It is the system which regulates activities that are beneath conscious control. The autonomic nervous system is then divided into the sympathetic division, parasympathetic division and enteric division.

The sympathetic nervous system responds to the imminent danger, and is accountable for the mount of one's heartbeat and blood pressure, among other physiological changes, all along with the sense of excitement one feels due to the rise of adrenaline in the system. The parasympathetic nervous system, on the other hand, is obvious if a person is resting and feels relaxed and is accountable for such things as the constriction of the pupil, the slowing of the heart, the dilation of blood vessels and the stimulation of digestive and genitourinary systems. The role of enteric nervous system is to manage each and every feature of digestion, from the esophagus to the stomach, small intestine and colon.

Specific nerves and plexi:

Ten out of the twelve cranial nerves invent from the brainstem and basically control the functions of the anatomic structures of the head by some exceptions. The nuclei of cranial nerves I and II lie in the forebrain and thalamus, correspondingly, and are therefore not considered being true cranial nerves. CN X (10) receives visceral sensory information from the thorax and abdomen, and CN XI (11) is accountable for innervating the sternocleidomastoid and trapezius muscles, neither of which is completely in the head. Spinal nerves take their origins from the spinal cord. They control the functions of the rest of body. In humans, there are around 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. In the cervical area, the spinal nerve roots come out above the corresponding vertebrae (that is, nerve root among the skull and first cervical vertebrae is termed as spinal nerve C1).

From the thoracic area to the coccygeal area, the spinal nerve roots come out beneath the corresponding vertebrae. It is significant to note that this process makes a problem when naming the spinal nerve root between C7 and T1 (so it is termed as spinal nerve root C8). In the lumbar and sacral area, the spinal nerve roots for travel in the dural sac and they travel beneath the level of L2 as the cauda equina.

1) Cervical spinal nerves (C1-C4):

The first four cervical spinal nerves, C1 through C4, split and recombine to generate diversity of nerves which sub serve the neck and back of head. Spinal nerve C1 is termed as the suboccipital nerve that gives motor innervations to muscles at the base of the skull. C2 and C3 form most of the nerves of the neck, giving both sensory and motor control. These comprise the greater occipital nerve that provides sensation to the back of the head, the lesser occipital nerve that gives sensation to the area behind the ears, the greater auricular nerve and the lesser auricular nerve. The phrenic nerve occurs from nerve roots C3, C4 and C5. This innervates the diaphragm, enabling breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible.

2) Brachial plexus:

The final four cervical spinal nerves, C5 via 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 which sub serve the arm and upper back. However the brachial plexus might appear tangled, it is highly organized and predictable by little variation among people.

3) Neurotransmitter:

The key neurotransmitters of the peripheral nervous system are acetylcholine and noradrenaline. Though, there are some other neurotransmitters as well, both labeled Non-noradrenergic, non-cholinergic (NANC) transmitters. Illustrations of such transmitters comprise non-peptides: ATP, GABA, dopamine, NO, and peptides: neuropeptide Y, VIP, GnRH, Substance P and CGRP.

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