How the autonomic nervous system works

Understanding what the ANS does will help you understand the body's normal response to stress or lack of stress

The human nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system contains the brain and spinal cord, and the peripheral nervous system contains the rest. One division or part of the peripheral nervous system is the autonomic nervous system.

The autonomic nervous system is automatic. This means it controls the automatic activities in the body. For instance, we cannot voluntarily increase our heart rate or slow down our digestive tract. We must rely on the autonomic nervous system.

The autonomic nervous system is divided into two branches; one contains sympathetic nerves and the other the parasympathetic nerves. The sympathetic nervous system is responsible for the body's reaction to stress; this is the "fight or flight" response. The parasympathetic nervous system supports the vegetative bodily functions, such as digestion, and strives to conserve energy.

The autonomic nervous system controls activities like heart rate or digestive tract.
The autonomic nervous system controls activities like heart rate or digestive tract. (Courtesy/

It is important to keep this in mind when attempting to determine the sympathetic and parasympathetic effects on the body. The sympathetic and parasympathetic nerves release chemicals called neurotransmitters. Neurotransmitters relay the nerve's electrical signal across the gaps created when the nerve connects to cells, organs or other nerves. These chemical neurotransmitters then attach to sympathetic or parasympathetic receptor sites on the connected nerves, cells or organs to cause an effect.

Adrenergic receptors 

It is a lock and key relationship. The lock is the receptor and the key is the neurotransmitter that unlocks the receptor causing its effect in the body. A quick example may help: Imagine you are in the back of your ambulance with a very large patient who decides to become uncooperative – in fact, he becomes downright unfriendly. Without any conscious effort on your part, the sympathetic nerves in your body increase the release of a neurotransmitter called norepinephrine inside your body.

Norepinephrine unlocks receptors in the heart causing it to beat faster and stronger. At the same time, it unlocks adrenergic receptors in your blood vessels, causing them to constrict to increase blood return to the heart so it can pump out more blood; except in the skeletal muscles where the vessels dilate to accept more blood.

In the lungs, norepinephrine is simultaneously unlocking adrenergic receptors that enlarge or dilate your bronchi and bronchioles so you can get more air in and out. This is all done with the intent of increasing oxygen and energy to the muscles in your body so you can fight or flee this potential threat in the back of your ambulance. We can do the same thing in your body by injecting a medication called epinephrine.

Again, the chemical that is released from the sympathetic nerve ending is called norepinephrine and it acts on the sympathetic receptors of other nerves, cells or organs. Sympathetic receptors can also be called adrenergic receptors. This is because the adrenal gland is also involved with the body’s response to stress.

There is an adrenal gland attached to the top of each kidney, and when these glands are stimulated they release norepinephrine and epinephrine, which circulates in the blood stream to the sympathetic or adrenergic receptors on other nerves, cells or organs to produce an effect. In fact, another name for the medication epinephrine is adrenaline.

Now we switch to the parasympathetic nerve endings where the chemical released is acetylcholine and it acts on the parasympathetic or cholinergic receptors on other nerves, cells or organs. Medicine often has two or more terms for the same thing, thus parasympathetic and cholinergic are equivalent.

Neurotransmitters and adrenergic receptors

Recall the epinephrine example above where we can introduce chemicals into the human body that act as neurotransmitters. These chemicals can be medications or poisons and will reach sympathetic or adrenergic receptors and cause sympathetic effects, or reach parasympathetic or cholinergic receptors and cause parasympathetic effects.

Outside chemicals that stimulate adrenergic receptors are called sympathomimetic because they mimic the sympathetic response of the sympathetic neurotransmitters norepinephrine and epinephrine.

The sympathomimetics can also be called sympathetic agonists. Thus the terms sympathomimetic and sympathetic agonist are interchangeable. Medicine would be much easier if we would just use one name for things!

Now the reverse – if the chemicals block adrenergic receptors, also called sympathetic receptors and thus blocks sympathetic activity, they are called sympatholytic because they lyse or break the sympathetic response. And of course they have other names: sympathetic antagonist and sympathetic blocker. So, the terms sympatholytic, sympathetic antagonist and sympathetic blocker are also interchangeable.

Now, for the parasympathetic side of the autonomic nervous system. Outside chemicals, that is medications or poisons that stimulate parasympathetic or cholinergic receptors are called parasympathomimetics because they mimic the parasympathetic response caused by the neurotransmitter acetylcholine. They can also be called parasympathetic agonists. Note the parallel with the sympathetic nervous system terms.

If the chemicals block parasympathetic receptors and thus block parasympathetic activity, they are called parasympatholytic because they lyse or break the parasympathetic activity. Again, we can also call these chemicals parasympathetic antagonists or parasympathetic blockers.

Understanding how the ANS works will help you understand the body’s normal response to stress or lack of stress, as well as how certain medications and poisons can affect your patient.

Test your knowledge: Quiz: Sympathetic and parasympathetic nervous systems

This article, originally posted Sept. 9, 2010, has been updated

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