Nervous System, Autonomic

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Nervous System, Autonomic

Definition

The autonomic nervous system is a network of nerves that regulate involuntary control of cardiac muscle, organ smooth muscle, and glands such that basic biological processes such as digestion and breathing can occur without conscious thought.

Description

The peripheral nervous system consists of nerves that must travel outside of the brain and spinal cord in order to contact organs, glands, and muscles. Under the umbrella of the peripheral nervous system are the somatic and autonomic nervous systems. The somatic nervous system is responsible for controlling voluntary movements during activities such as walking while the autonomic nervous system regulates involuntary tasks such as food digestion. More specifically, the somatic division mediates voluntary or reflexive control of skeletal muscles while the autonomic nervous system is responsible for the involuntary and reflexive control of glands, organ smooth muscle, and cardiac muscle.

The autonomic nervous system has three components:

The enteric nervous system is the less common of the three and is responsible for coordinating the digestive functions of the gastrointestinal tract, pancreas, and gall bladder. The two other subdivisions of the autonomic nervous system, parasympathetic and sympathetic, work in concert to subconsciously control other bodily functions, such as heart rate, blood pressure, digestion, metabolism, reproduction, breathing, excretion, sweating, and temperature.

The parasympathetic and sympathetic divisions have similar organizations but are distinguishable at the anatomical, biochemical, and functional levels. Both systems are organized into a two-neuron chain. The first neuron in this chain is referred to as a preganglionic neuron and the second as a postganglionic neuron. The nucleus containing cell bodies of preganglionic neurons are found in the brain and spinal cord of the central nervous system. The preganglionic neuron extends a fiber process, known as an axon, outside of the central nervous system to make contact with the cell body of the postganglionic neuron. The place where the axon of the preganglionic neuron meets the cell body of the postganglionic neuron is called a synapse. The synapses of the autonomic nervous system are outside of the brain and spinal cord of the central nervous system in specialized structures known as autonomic ganglia.

The preganglionic neurons of the parasympathetic nervous system originate in the brainstem and sacral spinal cord. These preganglionic neurons communicate with postganglionic neurons by extending very long axons that release the neurotransmitter, acetylcholine. The synapses of the parasympathetic ganglia are usually in or near the targeted organ. The postganglionic neuron expresses protein receptors on he surface that are capable of responding to acetylcholine. The postganglionic neurons have very short axons that release acetylcholine onto the targeted organ to modulate the intrinsic activity of that particular organ. These organs include the eye, lacrimal gland, salivary gland, heart, bronchi and lungs, small intestine, stomach, gallbladder, liver, pancreas, large intestine, rectum, genitalia, blood vessels, and bladder. Each of thee targeted organs expresses acetylcholine receptors to respond to the parasympathetic nervous system.

The preganglionic neurons of the sympathetic nervous system originate in the thoracic and upper lumbar regions of the spinal cord. These preganglionic neurons send very short axons to synapse in the paravertebral or in the prevertebral ganglia. The paravertebral ganglia lie in close proximity to the spinal cord. The postganglionic neurons of the aravertebral ganglia send axons to the head, trunk, and limb regions. The other organs in the body receive inputs from the prevertebral ganglia which is further away from the spinal cord and closer to the target organ. An exception to organization is the adrenal gland which is directly contacted by preganglionic neurons of the sympathetic nervous system. Identical to the parasympathetic nervous system, the preganglionic neurons of the sympathetic nervous system communicate by releasing the neurotransmitter acetylcholine. However, the postganglionic neurons of the sympathetic nervous system differ in that they release norepinephrine onto the targeted organ. An exception to this is in the sweat glands where sympathetic postganglionic neurons release acetylcholine instead of norepinephrine. The target organs of the sympathetic nervous system include many of the same ones as the parasympathetic nervous system.

Function

The autonomic nervous system maintains internal balance (homeostasis) but also enables humans to respond to changes in the environment. This is achieved because the parasympathetic and sympathetic divisions of the systems are antagonistic. The parasympathetic and sympathetic nervous system usually have opposing effects on target organs. The predominate resting tone of an organ is established by either the sympathetic or parasympathetic system. For example, the predominate resting tone of the eye pupil is constriction, maintained by the parasympathetic nervous system. However, a fearful situation may induce pupil dilation, mediated by the sympathetic nervous system. In other words, the autonomic nervous system enables humans to deviate from normal functions to respond to changes in the environment. The parasympathetic nervous system is often referred to as "rest and digest" and the sympathetic nervous system as "fight or flight."

Each organ has a predominate resting tone that is influenced in a distinct way by the sympathetic and parasympathetic nervous systems. The sympathetic nervous system increases heart rate, while the parasympathetic slows it down. Likewise, the sympathetic system constricts blood vessels while the parasympathetic dilates them and therefore both systems influence blood pressure. The sympathetic nervous system reduces motility of the stomach and intestines while the parasympathetic increases motility. Most of the organs and glands controlled by the autonomic nervous system have this dual but opposing mechanism of regulation.

In some situations it is beneficial to override the autonomic nervous system. The postganglionic neurons and the targeted organs express protein receptors that sense and respond to the neurotransmitters acetylcholine and norepinephrine. The practice of autonomic pharmacology uses drugs to modify these receptors to override the existing setting. In this manner, dysfunctions such as high blood pressure can be treated and maintained.

Role in human health

The autonomic nervous system has a crucial role in human health because it maintains the internal balance as well as allows the individual to respond to environmental stimuli. Problems can arise when this system is over- or underactive. The role of stress on the autonomic nervous system is of serious consequence. The autonomic nervous system is designed to respond to stress but too much stress can lead to abnormal resting organ tones. This is exemplified by heart disease and high blood pressure which can be treated by drugs that block the autonomic nervous system.

Common diseases and disorders

Holmes-Adie's syndrome

This is believed to be a disorder of the autonomic nervous system characterized by loss of the ability to constrict the eye pupil. This syndrome is also referred to as tonic pupil. The presenting patient maintains a dilated pupil and has decreased reflexes. The cililary ganglion, where the parasympathetic pre- and post-ganglion fibers meet, has been observed to degenerate. This loss of the parasympathetic tone renders the patient unable to constrict the pupil in response to light and nearby objects. The underlying cause is unknown but possibilities include viral infections that induce inflammation of the ciliary ganglion.

Familial dysautonomia

Familial dysautonomia is also referred to as Riley-Day syndrome and is an inherited disorder of the autonomic nervous system. The inheritance is autosomal recessive with widespread prevalence in patients of Ashkenazi Jewish decent. It is characterized by an increase in pain sensation, decreased lacrimation, an inability to regulate temperature, excessive sweating, and hypertension. It is usually diagnosed early in life and impairs development. There is evidence that there are a decreased number of sensory and autonomic nervous system neurons. Recently, the gene has been mapped to chromosome 9 and codes for a protein called IKAP. The function of IKAP is unknown, but it is hypothesized to be involved in gene activation mechanisms.

Horner's syndrome

Horner's syndrome is characterized by a lack of sympathetic tone to one side of the face. Therefore, symptoms that present are dropping eyelids, pupil constriction, and dryness to the face. The underlying cause of this is not clear but may originate within the spinal cord due to injury or tumor formation.

Shy-Drager syndrome

Patients with Shy-Drager syndrome have general autonomic nervous system dysfunction as well as parkinsonian-like symptoms. The autonomic symptoms included a decrease in blood pressure, orthostatic hypotension, constipation, urinary incontinence, and abnormal sweating. Some patients may also develop irregular heartbeats and have difficulty breathing. The parkinsonian-like symptoms included, tremor, slowness of movement, and problems maintaining balance. A key feature of the syndrome is dizziness or fainting due to the inability to maintain blood pressure. The underlying cause of the disease is unknown but neurons in the spinal cord have been observed to degenerate.

KEY TERMS

Acetylcholine— A chemical messenger of the nervous system that is also known as a neurotransmitter.

Norepinephrine— A chemical messenger of the nervous system that is also known as a neurotransmitter.

Parasympathetic— The "rest and digest" division of the autonomic nervous system.

Sympathetic— The "fight or flight" division of the autonomic nervous system.

Resources

BOOKS

Guyton, A.C., and J.E. Hall. Medical Physiology. Philadelphia, PA: W.B. Saunders Company, 2000.

Powley, Terry L. "Central Control of AutonomicFunctions." In Fundamental Neuroscience, edited by M.J. Zigmond, F. E. Bloom, S. C. Landis, J. L. Roberts, and L. R. Squire. San Diego, CA: Academic Press, 1999, pp. 1027-1036.

PERIODICALS

Slaugenhaupt, Susan A., et. al. "Tissue-Specific Expression of a Splicing Mutation in the IKBKAP Gene Causes Familial Dysautonomia." American Journal of Human Genetics 68 (March 2001): 6803-6806.