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The human body is truly a complex compilation of systems all working together to respond to the many demands it faces. While it can be argued that all the systems are important or even critical to maintaining life, the autonomic nervous system finds itself right in the middle of all the action.
The nervous system has several subdivisions, each with specific roles. The first tier is the central nervous system versus the peripheral nervous system. The peripheral nervous system is divided into sensory and motor functions. Motor function branch is further separated into somatic, or voluntary, nervous functions and the autonomic nervous system or ANS.
The autonomic nervous system controls all the functions of the body that we do not need to think about to perform. While a conscious thought is required for a somatic function such as innervating muscles to lift an apple to your mouth, we do not need to tell our stomach and intestines to digest the apple. The autonomic nervous system kicks in and does that without further instructions from the conscious brain.
Within the ANS are the sympathetic and parasympathetic nervous systems. The sympathetic system is often referred to as the “fight or flight” response. Shock and stress on the body trigger sympathetic responses, including increased heart rate, increased force of heart contraction, constriction of some blood vessels, dilation of the pupils and opening of the airways.
As the name implies, the parasympathetic nervous system counters the sympathetic responses by allowing the body to “rest and digest.” When the body is no longer under stress, it needs to recover and rebuild energy reserves. Parasympathetic responses include lowering the rate and force of contraction of the heart, activating the digestive system, narrowing the pupils and lowering the blood pressure.
The ANS uses receptors throughout the body and chemical messengers to turn the sympathetic system on and off. Receptors can either be activated by a neurotransmitter to turn the system on or a chemical can get into the receptor to block it. When a receptor is blocked, the neurotransmitter that would normally activate the receptor cannot get in to signal the system to respond. A neurotransmitter that activates a system by matching the receptor is called an agonist, while one that blocks the receptor is called an antagonist. Epinephrine is an example of an agonist and metoprolol is an example of an antagonist.
Alpha and Beta receptors
Adrenergic receptors are the most commonly discussed receptors of the sympathetic nervous system. Adrenergic receptors are comprised of alpha and beta receptors, including beta 1 receptors and beta 2 receptors.
Table: Adrenergic Receptors
The parasympathetic system generally gets its signal to act from the Vagus nerve. Acetylcholine transmits the message from the Vagus nerve to the parasympathetic receptor. It is inhibited through parasympathetic antagonists, such as atropine or ipratropium.
Understanding the autonomic nervous system, the actions of the sympathetic and parasympathetic systems, alpha and beta receptors and the agents that activate or block their responses will help an EMS provider understand the medications that a patient may be taking. It is also key to knowing why and when medications that we may administer are indicated.
Now it is your turn to test your knowledge of the autonomic system, its divisions and how they work.
