Peripheral Nervous System Health Article

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 and muscles. Examples include the acceleration of the heartbeat, raising of blood pressure, shrinkage of the pupils of the eyes, and the redirection of blood away from the skin to muscles, brain, and the heart.

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.