Peripheral Nervous System
The peripheral nervous system (PNS) consists of all parts of the nervous system, except the brain and spinal cord, which are the components of the central nervous system (CNS). The peripheral nervous system connects the central nervous system to the remainder of the body, and is the conduit through which neural signals are transmitted to and from the central nervous system. Within the peripheral nervous system, sensory neurons transmit impulses to the CNS from sensory receptors. A system of motor neurons transmit neural signals from the CNS to effectors (glands, organs, and muscles).
The peripheral nervous system is composed of nerve fibers that provide 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 is 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, meaning they are the places where the responses to the stimuli are translated into action.
The peripheral nervous system is subdivided into two subsystems: the sensory-somatic nervous system and the autonomic nervous system.
The sensory-somatic 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 sensory nervous system comprises 12 pairs of cranial nerves and 31 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: 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
The autonomic nervous system (ANS) consists of three subsystems: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. The ANS regulates the activities of cardiac muscle, smooth muscle, endocrine glands, and exocrine glands. The ANS functions involuntarily (i.e., reflexively)
In contrast to the somatic nervous system that always acts to excite muscle groups, the autonomic nervous systems can act to excite or inhibit innervated tissue. The autonomic nervous system achieves this ability to excite or inhibit activity via a dual innervation of target tissues and organs. Most target organs and tissues are innervated by neural fibers from both the parasympathetic and sympathetic systems. The systems can act to stimulate organs and tissues in opposite ways (antagonistically). For example, parasympathetic stimulation acts to decrease heart rate. In contrast, sympathetic stimulation results in increased heart rate. The systems can also act in concert to stimulate activity (e.g., both increase the production of saliva by salivary glands, but parasympathetic stimulation results in watery as opposed to viscous or thick saliva). The ANS achieves this control via two divisions of the ANS, the sympathetic nervous system and the parasympathetic nervous system.
The autonomic nervous system also differs from the somatic nervous system in the types of tissue innervated and controlled. The somatic nervous system regulates skeletal muscle tissue, while the ANS services smooth muscle, cardiac muscle, and glandular tissue.
Although the sympathetic systems share a number of common features (i.e., both contain myelinated preganglionic nerve fibers that usually connect with unmyelinated postganglionic fibers via a cluster of neural cells termed ganglia), the classification of the parasympathetic and the sympathetic systems of the ANS is based both on anatomical and physiological differences between the two subdivisions.
The sympathetic nervous system
The nerve fibers of the sympathetic system innervate smooth muscle, cardiac muscle, and glandular tissue. In general, stimulation via sympathetic fibers increases activity and metabolic rate. Accordingly, sympathetic system stimulation is a critical component of the fight or flight response.
The cell bodies of sympathetic fibers traveling toward the ganglia (preganglionic fibers) are located in the thoracic and lumbar spinal nerves. These thoraco-lumbar fibers then travel only a short distance within the spinal nerve (composed of an independent mixture of fiber types) before leaving the nerve as myelinated white fibers that synapse with the sympathetic ganglia that lie close to the side of the vertebral column. The sympathetic ganglia lie in chains that line both the right and left sides of the vertebral column, from the cervical to the sacral region. Portions of the sympathetic preganglionic fibers do not travel to the vertebral ganglionic chains, but travel instead to specialized cervical or abdominal ganglia. Other variations are also possible. For example, preganglionic fibers can synapse directly with cells in the adrenal medulla.
In contrast to the parasympathetic system, the preganglionic fibers of the sympathetic nervous system are usually short, and the sympathetic postganglionic fibers are long fibers that must travel to the target tissue. The sympathetic postganglionic fibers usually travel back to the spinal nerve via unmyelineted or gray rami before continuing to the target effector organs.
With regard to specific target organs and tissues, sympathetic stimulation of the pupil dilates the pupil. The dilation allows more light to enter the eye and acts to increase acuity in depth and peripheral perception.
Sympathetic stimulation acts to increase heart rate and increase the force of atrial and ventricular contractions. Sympathetic stimulation also increases the conduction velocity of cardiac muscle fibers. Sympathetic stimulation also causes a dilation of systemic arterial blood vessels, resulting in greater oxygen delivery.
Sympathetic stimulation of the lungs and smooth muscle surrounding the bronchi results in bronchial muscle relaxation. The relaxation allows the bronchi to expand to their full volumetric capacity and thereby allow greater volumes of air passage during respiration. The increased availability of oxygen and increased venting of carbon dioxide are necessary to sustain vigorous muscular activity. Sympathetic stimulation can also result in increased activity by glands that control bronchial secretions.
Sympathetic stimulation of the liver increases glycogenolysis and lipolysis to make energy more available to metabolic processes. Constriction of gastrointestinal sphincters (smooth muscle valves or constrictions) and a general decrease in gastrointestinal motility assure that blood and oxygen needed for more urgent needs (such as fight or flight) are not wasted on digestive system processes that can be deferred for short periods. The fight or flight response is a physical response; a strong stimulus or emergency causes the release of a chemical called nor-adrenaline (also called norepinephrine) that alternately stimulates or inhibits the functioning of a myriad of glands
Sympathetic stimulation results in renin secretion by the kidneys and causes a relaxation of the bladder. Accompanied by a constriction of the bladder sphincter, sympathetic stimulation tends to decrease urination and promote fluid retention.
Acetylcholine is the neurotransmitter most often found in the sympathetic preganglionic synapse. Although there are exceptions (e.g., sweat glands utilize acetylcholine), epinephrine (noradrenaline) is the most common neurotransmitter found in postganglionic synapses.
The parasympathetic nervous system
Parasympathetic fibers innervate smooth muscle, cardiac muscle, and glandular tissue. In general, stimulation via parasympathetic fibers slows activity and results in a lowering of metabolic rate and a concordant conservation of energy. Accordingly, the parasympathetic nervous sub-system operates to return the body to its 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 preganglionic fibers of the parasympathetic system derive from the neural cell bodies of the motor nuclei of the occulomotor (cranial nerve: III), facial (VII), glossopharyngeal (IX), and vagal (X) cranial nerves. There are also contributions from cells in the sacral segments of the spinal cord. These cranio-sacral fibers generally travel to a ganglion that is located near or within the target tissue. Because of the proximity of the ganglia to the target tissue or organ, the postganglionic fibers are much shorter.
Parasympathetic stimulation of the pupil from fibers derived from the occulomotor (cranial nerve: III), facial (VII), and glossopharyngeal (IX) nerves constricts or narrows the pupil. This reflexive action is an important safeguard against bright light that could otherwise damage the retina. Parasympathetic stimulation also results in increased lacrimal gland secretions (tears) that protect, moisten, and clean the eye.
The vagus nerve (cranial nerve: X) carries fibers to the heart, lungs, stomach, upper intestine, and ureter. Fibers derived from the sacrum innervate reproductive organs, portions of the colon, bladder, and rectum.
With regard to specific target organs and tissues, parasympathetic stimulation acts to decrease heart rate and decrease the force of contraction. Parasympathetic stimulation also reduces the conduction velocity of cardiac muscle fibers.
Parasympathetic stimulation of the lungs and smooth muscle surrounding the bronchi results in bronchial constriction or tightening. Parasympathetic stimulation can also result in increased activity by glands that control bronchial secretions.
Parasympathetic stimulation usually causes a dilation of arterial blood vessels, increased glycogen synthesis within the liver, a relaxation of gastrointestinal sphincters (smooth muscle valves or constrictions), and a general increase in gastrointestinal motility (the contractions of the intestines that help food move through the system).
Parasympathetic stimulation results in a contracting spasm of the bladder. Accompanied by a relaxation of the sphincter, parasympathetic stimulation tends to promote urination.
The chemical most commonly found in both pre- and postganglionic synapses in the parasympathetic system is the neurotransmitter acetylcholine.
The enteric nervous system
The enteric nervous system is made up of nerve fibers that supply the viscera of the body: the gastrointestinal tract, pancreas, and gallbladder.
Regulation of the autonomic nervous system
The involuntary ANS is controlled in the hypothalamus, while the somatic system is regulated by other regions of the brain (cortex). In contrast, the somatic nervous system may control motor functions by neural pathways that contain only a single axon that innervates an effector (i.e., target) muscle. The ANS is comprised of pathways that must contain at least two axons separated by a ganglia that lies in the path between the axons.
ANS reflex arcs are stimulated by input from sensory or visceral receptors. The signals are processed in the hypothalamus (or regions of the spinal cord) and target effector control is then regulated via myelinated preganglionic neurons (cranial and spinal nerves that also contain somatic nervous system neurons). Ultimately, the preganglionic neurons terminate in a neural ganglion. Direct effector control is then regulated via unmyelinated postganglionic neurons.
The principal neurotransmitters in ANS synapses are acetylcholine and norepinephrine.
General PNS disorders
General PNS disorders include loss of sensation or hyperesthesia (abnormal or pathological sensitivity). Sensations such as prickling or tingling without observable stimulus (paresthesia) or burning sensations are also abnormal.
Stabbing or throbbing pains are often due to neuralgia (e.g., trigeminal neuralgia, also known as tic douloureux). Neuritis (an inflammation of the nerve) can be caused by a number of factors, including trauma, infection (both bacterial and viral), or chemical injury.
Goldman, Cecil. Textbook of Medicine, 21st ed. New York: W. B. Saunders Co., 2000.
Guyton & Hall. Textbook of Medical Physiology, 10th ed. New York: W. B. Saunders Company, 2000.
Tortora, G. J., and S. R. Grabowski. Principles of Anatomy and Physiology, 9th ed. New York: John Wiley and Sons Inc., 2000.
Brian Douglas Hoyle, PhD