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Category: Medical

Topic: Stress and Disease

Level: AEMT

23 minute read

ANS: The autonomic nervous system ("ANS") controls all of the movement in your body that is automatic, e.g., your heartbeat, the movement of food through your digestive tract, and breathing. It is made up of two branches

  1. SYMPATHETIC NERVOUS SYSTEM ("Adrenergic"--adrenalin, or epinephrine, and noradrenaline, or norepinephrine, as major neurotransmitters)
  2. PARASYMPATHETIC NERVOUS SYSTEM  ("Cholinergic"--acetylcholine as the major neurotransmitter)

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:
    • increased heart rate and BP,
    • shunting blood (and oxygenation) to the brain and muscles. 
  • 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.

Image result for opposites symbolIn Chinese philosophy, yin and yang ("dark-bright" or "negative-positive") describe how seemingly opposite or contrary forces may actually be complementary, interconnected, and interdependent in the natural world, and how they may give rise to each other as they interrelate to one another. This is a pretty good analogy for the way the two parts of the ANS interrelate. 



The Sympathetic Nervous System ("Fight-or-Flight")

SYMPATHETIC NERVOUS SYSTEM (The Adrenergic Pathway): 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, dilates your pupils to see danger better (mydriasis), and inhibits digestion (a side effect to shunting blood away from the GI tract so that more can be delivered to the muscles needed to escape that man-eating tiger).

The main neurotransmitters of the Sympathetic Nervous System:


The sympathetic division of the ANS is referred to as the adrenergic system--nerves in which the catecholamines--epinephrine (adrenaline) and norepinephrine (noradrenaline)--are the principal transmitters and have varying effects on the adrenergic receptors throughout the body:

  • norepinephrine > epinephrine in agonist potency; 
  • epinephrine > norepinephrine in agonist potency; and
  • norepinephrine = epinephrine in agonist potency.

FIGHT-or-FLIGHT: The 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 allowed for a more menacing presentation. 
  • anxiety and restlessness, which in limited amounts makes easier alertness and "thinking on one's feet" during challenging events.

SYMPATHETIC RECEPTORS--Adrenergic receptors (adrenoceptors) are found throughout the body. Two main types of adrenergic receptor are

  1. Alpha (types 1 and 2: alpha-1 and alpha-2) and
  2. Beta (types 1 and 2: ß1 and ß2) receptors.

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 adrenergic receptors:

  • vasoconstriction in the skin, mucosa, and abdominal organs;
  • shunting blood to the musculoskeletal system for an efficient "fight-or-flight" response;
  • uterine contractions and sphincter contractions in the GI tract and bladder;
  • 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 and increases the rate of blood flow to increase blood pressure.

Alpha-1 stimulation 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 adrenergic receptors: 

  • peripheral vasoconstriction;
  • a decrease in norepinephrine release; and
  • 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-1 adrenoceptors.

Beta ("ß") adrenoceptors are activated by the catecholamines norepinephrine and epinephrine.

ß-1 receptors: agonists have primarily cardiac effects:

  • increases inotropic (strength) and
  • chronotropic (rate) of cardiac muscle; 

that is, increased force of the heart's contraction and the heart rate.

ß-2 receptors: agonists induce

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

The Parasympathetic Nervous System ("Rest-and-Digest")

PARASYMPATHETIC NERVOUS SYSTEM (the Cholinergic Pathway): 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).

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.

And when the parasympathetic nervous system is stimulated, it

  • enhances blood flow to the gastrointestinal (GI) system and the genitourinary (GU) system.


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


Nerve Pathways of the Sympathetic Nervous System

  1. SYMPATHETIC: the nerves of the sympathetic nervous system, for the most part, use the thoracic and lumbar nerve tracts, while

  2. PARASYMPATHETIC: the parasympathetic nervous system uses the cranial nerves 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.


ANS and Pharmacology

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

  • Sympathetic: Epinephrine and norepinephrine.
  • Parasympathetic: Acetylcholine (ACh).

Therefore, medications can be used to enhance or defeat the purposes of both the Adrenergic Pathway (sympathetic) and the Cholinergic Pathway (parasympathetic):


  • SYMPATHOMIMETICS (adrenergics) are medications that stimulate the sympathetic nervous system.
  • Vasopressors act on the alpha-1, ß-1, and ß-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. (NOTE: Intracardiac injection may be out-dated--check with local protocols; instead, IV-push and CPR.)
  • SYMPATHOLYTICS (antiadrenergicsare 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 (ß-1 antagonists) decrease heart rate and the force of as a class of anti-adrenergic.


  • PARASYMPATHOMIMETICS (cholinergicsare 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 (anticholinergics) are medications that block action in the parasympathetic nervous system. Anticholinergic drugs are used to stop 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

NOTE: Atrovent and Atropine are two types of commonly used EMS anticholinergic medications.