Nervous System, Autonomic

The autonomic nervous system has three components:

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 nuclei containing cell bodies of preganglionic neurons are found in the brain and spinal cord of the central nervous system. The preganglionic neuron extends a long nerve cell projection, known as an axon, outside the central nervous system to make contact with the cell body of the postganglionic neuron. The place where the axon meets the postganglionic neuron is called a synapse. The synapses of the autonomic nervous system are outside the brain and spinal cord in specialized structures known as autonomic ganglia.

The preganglionic neurons of the parasympathetic nervous system originate in the brainstem and sacral spinal cord. These neurons communicate with postganglionic neurons by extending very long axons that release the neurotransmitter acetylcholine. The synapses are usually in or near the targeted organ. The postganglionic neuron has protein receptors on the cell surface that respond to acetylcholine. These neurons have very short axons that release acetylcholine onto the targeted organ to control the activity of that particular organ. These organs include the eyes, lacrimal gland, salivary gland, heart, bronchi and lungs, small intestine, stomach, gallbladder, liver, pancreas, large intestine, rectum, genitalia, blood vessels, and bladder. Each of these targeted organs contains 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 and send axons to the head, trunk, and limb regions. Other organs also receive inputs from the prevertebral ganglia, which is further away from the spinal cord and closer to the target organ. An exception to this organization is the adrenal gland, which is directly contacted by preganglionic neurons of the sympathetic nervous system. Identical to the parasympathetic nervous system, the sympathetic nervous system communicates by releasing the neurotransmitter acetylcholine. 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. The target organs of the sympathetic nervous system include many of the same ones as the parasympathetic nervous system.

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 systems are antagonistic. That is, they usually have opposing effects on organs. For example, at rest, the eye pupil is constricted, maintained by the parasympathetic nervous system. A fearful situation, however, may cause pupil dilation, mediated by the sympathetic nervous system. Thus, 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 resting state that is influenced by the sympathetic and parasympathetic nervous systems. For example, the sympathetic nervous system increases heart rate, whereas the parasympathetic slows it down. Likewise, the sympathetic system constricts blood vessels, but the parasympathetic dilates them; therefore, both systems influence blood pressure. The sympathetic nervous system reduces motility of the stomach and intestines, whereas 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. Drugs can modify or override the nervous systems' functioning. In this manner, dysfunctions, such as high blood pressure, can be treated and maintained.

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

Familial dysautonomia is also referred to as Riley-Day syndrome and is an inherited disorder of the autonomic nervous system, especially prevalent in individuals of Ashkenazi Jewish decent. It is characterized by an increase in pain sensation, decreased lacrimation (inability to regulate body temperature), excessive sweating, and hypertension. It is usually diagnosed early in life and impairs development. Evidence suggests that patients have a decreased number of sensory and autonomic nervous system neurons. In the 1990s, the gene was mapped to chromosome 9 and codes for a protein called IKAP. The function of IKAP is unknown, but it may be involved in activating genes.

Horner syndrome is characterized by a lack of sympathetic tone to one side of the face. Therefore, symptoms include drooping eyelids, pupil constriction, and decreased sweating on the affected side. The underlying cause of this is not clear but may originate within the spinal cord due to injury or tumor formation.

Patients with Shy-Drager syndrome have general autonomic nervous system dysfunction as well as parkinsonian-like symptoms. Symptoms included a decrease in blood pressure, orthostatic hypotension (dizziness or lightheadedness after standing suddenly), constipation, urinary incontinence, and abnormal sweating. Some patients may also develop irregular heartbeats and have difficulty breathing. The parkinsonian-like symptoms include 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 have degenerated.

The autonomic nervous system controls muscles of the internal organs and glands, affecting the heart, blood vessels, lungs, stomach, intestines, salivary glands, and sweat glands. One part of this system helps the body relax and digest food, whereas another is geared toward quick reactions in an emergency.

As a result, the autonomic nervous system has a direct impact on organs and glands during exercise, as well as during ingestion and digestion of food. This system is constantly changing in response to physical activity and environmental changes.

During exercise, the autonomic nervous system balances fluctuations in respiration, blood pressure, digestion, and circulation of blood and hormones. All activity within the autonomic nervous system is involuntary and occurs in concert with ongoing physiological activities.

The mind-body connection is important to autonomic nervous system functioning. The sympathetic portion of the autonomic nervous system responds readily to stress-related situations, and parasympathetic portions of the autonomic nervous system maintain nonstress-related bodily functions. Vital signs such as breathing, blood pressure, and heart rate will increase or decrease accordingly, as the brain interacts with the autonomic nervous system, whether at rest or in a physically active situation.

During exercise, the sympathetic portion of the autonomic nervous system is in control, increasing heart rate, heart contraction, muscle movement, and constriction of certain blood vessels. In addition, norepinephrine and epinephrine are released throughout the body, further assisting in the body's response to an exercise activity.

When at rest, the parasympathetic system takes over, allowing for a slower heart rate and dilation of blood vessels. In situations of high or long-term stress, the parasympathetic system is less engaged, so individuals feel less relaxed or rested. It is important then, that they pay attention to warning signs of stress such as insomnia, agitation, changes in appetite, muscle tension, and digestive problems. Exercise has long been known to aid in reducing stress and has the added benefit of increased confidence associated with feeling and looking physically fit. In fact, research has indicated a positive effect of regular exercise on the autonomic nervous system, along with cardiovascular and metabolic protection, and improvements in mental health, including mood and self-esteem.

Diets high in fat can create an imbalance in the autonomic nervous system. For reasons not yet clear, some chemicals in the body, such as dopamine, are affected by a high-fat diet, resulting in low levels of dopamine release and uptake and disruption of autonomic nervous system balance. Studies show that low dopamine levels promote overeating and a feeling of not being satiated after eating a meal. Studies suggest that there may be a correlation between obesity and autonomic nervous system imbalance, causing negative consequences in health and wellness, including insulin resistance. Nutrition, body weight, activity levels, and exercise play an important role in maintaining a healthy autonomic nervous system.

Susan M. Mockus, PhD
Revised by Laura Jean Cataldo, RN, EdD