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22. Agents and Actions of the Autonomic Nervous System: Sympathetic Nervous System

Sympathetic Nervous System

Sympathetic Nervous System [SNS] – “fight or flight”

The SNS can also be thought of as the “E division” – embarrassment, emergency, exercise, excitement


What do you want your body to do when in fight or flight situation?

  • Alertness
  • Bronchodilation – increased oxygen necessary for brain and muscles to function well
  • Blood shunted to organs and muscles – need organs and muscles to function well
  • Decreased digestion – body does not need to exert energy on digestion in emergency
  • Increased cardiac output – improved blood and oxygen delivery
  • Production of energy (fatty acid release, glycogenolysis) – important for muscles to perform
  • Prevention of waste elimination – body does not need to exert energy on elimination in emergency
  • Sweat – help maintain homeostasis

EXCEPTIONS – most blood vessels and all sweat glands only have sympathetic innervation

Sympathetic Neurons

Pre-ganglionic sympathetic neurons release acetylcholine [ACh] and post-ganglionic sympathetic neurons release norepinephrine [NE]. ACh will then go onto stimulate the post-ganglion sympathetic neurons at muscarinic receptors releasing NE and then NE will go onto stimulate adrenoceptors located on a variety of effector organs and tissue such as cardiac muscle, smooth muscle, and glands.


preganglionic neuron

EXCEPTIONS – acetylcholine [ACh] is released by post-ganglionic sympathetic neurons innervating sweat glands, which means ACh receptors (as opposed to adrenoceptors) have to be blocked in order to decrease sweating as


Norepinephrine [NE] interacts with two types of receptors:

  1. Alpha-adrenergic receptors [alpha adrenoceptors]
    1. Alpha-1 – blood vessels
    2. Alpha-2 – CNS (presynaptic)
  2. Beta-adrenergic receptors [beta adrenoceptors]
    1. Beta-1 – heart
    2. Beta-2 – lungs
    3. Beta-3 – bladder

Physiological Effects of SNS on Organs/Tissue & Respective Receptors

Organ/Tissue Receptor Subtype Physiological Effect
BLADDER – smooth muscle Beta-3 relaxes prevents urination
BLADDER – sphincter Beta-3 contracts prevents urination
EYE – ciliary muscle no SNS innervation relaxes improves far-sighted vision
EYE – pupil Beta- dilation/relaxation mydriasis
GASTROINTESTINAL – glands Beta-2 inhibits secretion prevents digestion
GASTROINTESTINAL – smooth muscle Alpha-2
relaxes smooth muscle prevents digestion
GASTROINTESTINAL – sphincter Alpha-1 contracts prevents defecation
HEART – AV node Beta-1 increased conduction
Heart – cardiac muscle Beta-1 increased contractility positive ionotropic effect – enhances blood delivery
Heart – cardiac output Beta-1 increases enhances blood delivery
Heart – SV node Beta-1 increased heart rate positive chronotropic effect – increases oxygen delivery
Lung – smooth muscle (bronchioles & trachea) Beta-2 dilation/relaxation enhances breathing
Salivary Gland Beta-2 stimulates viscous secretions
Sweat Gland stimulates sweat secretion regulates body temperature
Vasculature – smooth muscle Alpha-1
relaxes coronary arteries
shunt blood to vital organs (brain, heart, etc), enhance cardiac output


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Principles of Pharmacology - Study Guide Copyright © by Edited by Dr. Esam El-Fakahany and Becky Merkey, MEd is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.