Become a better medic. Join Today!

Instant, unlimited access to our complete EMS practice test system. Includes the National Registry Simulatorâ„¢, course content, practice tests, and personalized learning dashboard.

Category: Medical

Topic: Pharmacology

Level: AEMT

Next Unit: Pharmacodynamics

22 minute read

The autonomic nervous system controls all of the movement in your body that is automatic, like your heartbeat, the movement of food through your digestive tract, and breathing. The sympathetic and parasympathetic nervous systems usually do opposite things in the body:

  • The sympathetic nervous system prepares your body for physical and mental activity.
  • The parasympathetic nervous system is responsible for bodily functions when we are at rest: digestion, various metabolic processes, and relaxation.

The sympathetic and parasympathetic nervous systems do not always work in opposite directions; they sometimes complement each other.


The Autonomic Nervous System (ANS)

The autonomic nervous system is divided into two branches:

1) The SYMPATHETIC NERVOUS SYSTEM (The Adrenergic Pathway). The sympathetic nervous system prepares your body for physical and mental activity. It makes your heart beat faster and stronger, opens your airways so you can breathe more easily, and inhibits digestion.

Main neurotransmitters: Epinephrine and norepinephrine.

2) The PARASYMPATHETIC NERVOUS SYSTEM (The Cholinergic Pathway). It is that part of the nervous system that slows the heart, dilates blood vessels, decreases pupil size, increases digestive juices, and relaxes muscles in the gastrointestinal tract.

Main neurotransmitter: Acetylcholine (ACh).

Drugs that affect the sympathetic and parasympathetic nervous systems, therefore, affect the adrenergic and cholinergic pathways:

  • Adrenergics
  • Anti-adrenergics
  • Cholinergics
  • Anticholinergics


Sympathetic Nervous System

The sympathetic division of the ANS is referred to as the adrenergic system--nerves in which epinephrine (adrenaline) and norepinephrine (noradrenaline) are the principal transmitters.

This adrenergic part of the sympathetic nervous system is responsible for the "fight-or-flight" response and uses epinephrine and norepinephrine to create its effects in the body:

  • cardiac effects for more efficient physical responses to danger, shunting circulation and function away from the gastrointestinal tracts to the musculoskeletal system, and
  • dilating the pupils for better visualization of danger. It also
  • affects the pilomotor muscles that make hair stand on end, an evolutionary holdover for when such things made us face a challenge more menacingly.

ADRENERGIC RECEPTORS (adrenoceptors): found throughout the body. Two main types of adrenergic receptor are Alpha- and Beta-receptors.

In alpha-1, norepinephrine > epinephrine in agonist potency. in alpha-2, epinephrine > norepinephrine in agonist potency. In beta-1: norepinephrine = epinephrine in agonist potency. in beta-2: epinephrine > norepinephrine in agonist potency.


Alpha-1 and alpha-2 receptors both function in a balance but the ratio can be tipped toward the actions needed at the time.

Alpha-1 receptors: alpha-1 adrenergic receptors cause:

  • vasoconstriction in skin, mucosa, abdominal organs, shunting blood to the musculoskeletal system for efficient "fight-or-flight" response;
  • uterine contractions and sphincter contractions in GI tract and bladder; and
  • mydriasis (pupil dilation).

When alpha-1 receptors are stimulated by agonists, this results in peripheral VASOCONSTRICTION, which contributes to the shunting of blood to the more vital areas. Besides shunting, vasoconstriction shrinks the diameter of arteries to increase the blood pressure by increasing the rate of the flow of blood. Alpha-1 stimulation also increases the contractile force of the heart and lowers the rate at which it needs to pump. Stimulation also has a psychostimulant effect, increases vigilance, and decreases the requirement of sleep.

Alpha-2 receptors: alpha-2 adrenergic receptors cause:

  • peripheral vasoconstriction;
  • a decrease norepinephrine release; and causes
  • smooth muscle contraction.

The alpha-2 adrenoceptors located on the sympathetic nerve terminals inhibit the release of norepinephrine and therefore act as a feedback mechanism for modulating the release of norepinephrine via the alpha-1adrenoceptors.


Beta-adrenoceptors are activated by the catecholamines norepinephrine and epinephrine,

Beta-1 receptors: agonists have primarily cardiac effects: when stimulated by agonists, increases inotropic (strength) and chronotropic (rate) of cardiac muscle:

  • increased heart rate and
  • increased force of the heart's contraction.

Beta-2 receptors: agonists induce

  • bronchodilation,
  • peripheral vasodilation, and
  • smooth muscle (and uterine) relaxation.


The Parasympathetic Nervous System (PNS)

The parasympathetic nervous system uses chiefly acetylcholine (ACh) as its neurotransmitter.

The parasympathetic system is that part of the nervous system that slows the heart, dilates blood vessels, decreases pupil size, increases digestive juices, and relaxes muscles in the gastrointestinal tract. It also promotes digestion and the synthesis of glycogen.

While the sympathetic nervous system is associated with the "fight-or-flight" response, the parasympathetic system is responsible for the complimentary "rest-and-digest" or "feed and breed" activities that occur when the body is at rest, especially after eating, and includes

  • sexual arousal,
  • salivation,
  • lacrimation (tears),
  • urination,
  • digestion, and
  • defecation.

When the parasympathetic nervous system is stimulated, it enhances blood flow to the gastrointestinal (GI) system and the genitourinary (GU) system.

The nerves of the sympathetic nervous system, for the most part, use the thoracic and lumbar nerve tracts, while the parasympathetic nervous system uses the cranial nerves vagus, and pelvic "splanchnic" nerves from the sacrum (S2, S3, S4):

  • Cranial nerves: (specifically, nerves for eye movement, salivation, and pupil sizing).
  • Vagus nerve: with inhibitory properties to the heart and influences on the abdominal organs. 75% of all parasympathetic nerve fibers are in the vagus nerve.
  • Pelvic Splanchnic nerves: influences on the urinary bladder, peristalsis of ureters and intestines, and defecation.


Acetylcholine acts on two types of receptors, the

  • Muscarinic receptors, responsible for bringing the heart back to normal after the actions of the sympathetic nervous system; and the
  • Nicotinic cholinergic receptors, which have actions for both somatic motor and neuronal-type autonomic function.


ANS and Pharmacology

The ANS is made up of the sympathetic and parasympathetic nervous system. The primary neurotransmitters for them are:

1) Sympathetic:

  • Epinephrine and
  • norepinephrine.

2) Parasympathetic:

  • Acetylcholine (ACh).



  • Sympathomimetics are medications that stimulate the sympathetic nervous system.
  • Vasopressors act on the alpha-1, beta-1, and beta-2 adrenergic receptors to stimulate smooth muscle contraction in the blood vessels.

This causes blood vessels to become narrow and blood pressure to increase, useful in shock (hypoperfusion) and hemorrhage. Certain vasopressors may also be used for colds or allergies. As nasal decongestants, they can shrink the swollen blood vessels in the mucous membranes in the nose. Examples of different vasopressors include: --ephedrine --epinephrine --dopamine --phenylephrine --pseudoephedrine --oxymetazoline

  • Cardiac stimulators can be used to stimulate and restore the heartbeat. They’re used if your heart stops beating suddenly because of electrocution, suffocation, or drowning. When this happens, epinephrine can be injected directly into your heart to help make it start beating again.
  • Sympatholytics are medications that block action in the sympathetic nervous system.

Antiadrenergic drugs can be used to stop the adrenergic (fight-or-flight) effects in the body by tying up those receptor sites with adrenergic receptor antagonists. They are primarily prescribed for hypertension and anxiety.

Alpha-blockers (alpha-1 antagonists) can inhibit peripheral vasoconstriction. Beta-blockers (beta-1 antagonists) decrease heart rate and the force of contraction, therefore, are a class of anti-adrenergic.


  • Parasympathomimetics are medications that stimulate the parasympathetic nervous system.

They stimulate the parasympathetic nervous system in the same manner as does acetylcholine. They are used in treating Alzheimer’s Disease, Myasthenia gravis, and atony of the smooth muscle of the GI system or urinary tract. They promote urination via contraction of the detrusor muscle and relaxation of the urinary sphincter to facilitate emptying of the urinary bladder.

  • Parasympatholytics are medications that block action in the parasympathetic nervous system.

Anticholinergic drugs are used to stop the cholinergic responses and to treat many GI and GU disorders. Anticholinergics decrease cardiovascular response to vagal stimulation resulting in tachycardia. (Increases vagal tone).

Example: Atropine, used to increase heart rate in symptomatic bradycardias and higher blocks.

Anticholinergics Increase bronchodilation and decrease respiratory tract secretions and act as antispasmodics of GI tract due to decreased tone and motility

(Atrovent and atropine are two types of commonly used EMS anticholinergic medications.)



Help Us Make MedicTests Better!
We value your feedback. How can we improve this unit? You can copy/paste from the content and suggest changes.