Peripheral Nervous System
The peripheral nervous system (PNS) is one component of the overall nervous system network in the human body. The other component is the central nervous system (CNS). The two are interconnected, and function to both sense and react to environments inside and outside the body.
The PNS is made up of neurons; the cellular pathways for the various signals on which the proper operation of the nervous system relies. There are two types of neurons operating in the PNS. The first are the sensory neurons that run from the myriad of sensory receptors throughout the body. Sensory receptors provide the connection between the stimulus (such as heat, cold and pain) and the CNS. As well, the PNS also consists of motor neurons. These neurons connect the CNS to various muscles and glands throughout the body. These muscles and glands are also known as effectors. That is, they are the places where the responses to the stimuli are translated into action.
An example of the above is the sequence of events that occurs when a hot object, such as a frying pan, is touched. Sensory receptors detect the heat and associated pain. These signals are conveyed to the brain by the sensory neurons. Interpretation of the signals occurs in the brain. A response signal is ferried to the appropriate muscles by motor neurons. Finally the particular effectors are stimulated to perform the action, in this example, contraction of the muscles, so as to pull the hand away from the source of the heat and pain. This cycle is completed within a second.
The PNS is divided into two subsystems: the sensory-somatic nervous system and the autonomic nervous system. The sensory-somatic nervous system is the sensory gateway between the environment outside of the body and the central nervous system. Responses tend to be conscious. The autonomic nervous system is the gateway between the internal environment and the central nervous system. The control of activity, typically automatic, of many organs and muscles is the responsibility of the autonomic nervous system.
The sensory nervous system comprises twelve pairs of cranial nerves and thirty-one pairs of spinal nerves. Some pairs are exclusively sensory neurons, such as the pairs involved in smell, vision, hearing, and balance. Other pairs are strictly made up of motor neurons, such as those involved in the movement of the eyeballs, swallowing, and movement of the head and shoulders. Still other pairs consist of a sensory and a motor neuron working in tandem, such as those involved in taste, and other aspects of swallowing. All of the spinal neuron pairs are mixed; that is, they contain both sensory and motor neurons. This allows the spinal neurons to properly function as the conduit of transmission of the signals of the stimuli and the subsequent response.
The autonomic nervous system consists of both sensory and motor neurons. These connect the brain components of the central nervous system (in particular the hypothalamus and medulla oblongata) and various internal organs (such as the heart, lungs, and glands). In yet another subdivision, the autonomic nervous system consists of three subsystems; the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. The sympathetic nervous system is involved with the response of the body to emergencies--a quite general reaction commonly known as the fight or flight reaction. Release of a chemical called noradrenaline (also called norepinephrine) alternately stimulates or inhibits the functioning of a myriad of glands and muscles. Examples include the acceleration of the heartbeat, raising of blood pressure, shrinkage of the pupils of the eyes, shrinkage of the air passages in the lungs, and the redirection of blood away from the skin to muscles, brain and the heart.
The parasympathetic nervous subsystem operates to return the body to it's normal levels of function following the sudden alteration by the sympathetic nervous subsystem; the so-called "rest and digest" state. Examples include the restoration of resting heartbeat, blood pressure, pupil diameter, and flow of blood to the skin.
The enteric nervous system is made up of nerve fibers that supply the "viscera" of the body; the gastrointestinal tract, pancreas and gall bladder.
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