The Autonomic Nervous System

flight The organs (the "viscera") of our body, such as the heart, stomach and intestines, are regulated by a part of the nervous system called the autonomic nervous system (ANS). The ANS is part of the peripheral nervous system and it controls many organs and muscles within the body. In most situations, we are unaware of the workings of the ANS because it functions in an involuntary, reflexive manner. For example, we do not notice when blood vessels change size or when our heart beats faster. However, some people can be trained to control some functions of the ANS such as heart rate or blood pressure.

The ANS is most important in two situations:

  1. In emergencies that cause stress and require us to
    "fight" or take "flight" (run away)


  2. In non-emergencies that allow us to "rest" and "digest".

The ANS regulates:
  • Muscles
    • in the skin (around hair follicles; smooth muscle)
    • around blood vessels (smooth muscle)
    • in the eye (the iris; smooth muscle)
    • in the stomach, intestines and bladder (smooth muscle)
    • of the heart (cardiac muscle)
  • Glands

The ANS is divided into three parts:

  • The sympathetic nervous system
  • The parasympathetic nervous system
  • The enteric nervous system.

The Sympathetic Nervous System

It is a nice, sunny are taking a nice walk in the park. Suddenly, an angry bear appears in your path. Do you stay and fight OR do you turn and run away? These are "Fight or Flight" responses. In these types of situations, your sympathetic nervous system is called into action - it uses energy - your blood pressure increases, your heart beats faster, and digestion slows down.

Notice in the picture on the left that the sympathetic nervous system originates in the spinal cord. Specifically, the cell bodies of the first neuron (the preganglionic neuron) are located in the thoracic and lumbar spinal cord. Axons from these neurons project to a chain of ganglia located near the spinal cord. In most cases, this neuron makes a synapse with another neuron (post-ganglionic neuron) in the ganglion. A few preganglionic neurons go to other ganglia outside of the sympathetic chain and synapse there. The post-ganglionic neuron then projects to the "target" - either a muscle or a gland.

Two more facts about the sympathetic nervous system: the synapse in the sympathetic ganglion uses acetylcholine as a neurotransmitter; the synapse of the post-ganglionic neuron with the target organ uses the neurotransmitter called norepinephrine. (Of course, there is one exception: the sympathetic post-ganglionic neuron that terminates on the sweat glands uses acetylcholine.)


The Parasympathetic Nervous System

It is a nice, sunny are taking a nice walk in the park. This time, however, you decide to relax in comfortable chair that you have brought along. This calls for "Rest and Digest" responses. Now is the time for the parasympathetic nervous to work to save energy - your blood pressure decreases, your heart beats slower, and digestion can start.

Notice in the picture on the left, that the cell bodies of the parasympathetic nervous system are located in the spinal cord (sacral region) and in the medulla. In the medulla, the cranial nerves III, VII, IX and X form the preganglionic parasympathetic fibers. The preganglionic fiber from the medulla or spinal cord projects to ganglia very close to the target organ and makes a synapse. This synapse uses the neurotransmitter called acetylcholine. From this ganglion, the post-ganglionic neuron projects to the target organ and uses acetylcholine again at its terminal.

Here is a summary of some of the effects of sympathetic and parasympathetic stimulation. Notice that effects are generally in opposition to each other.

The Autonomic Nervous System
Structure Sympathetic Stimulation Parasympathetic Stimulation
Iris (eye muscle) Pupil Dilation Pupil Constriction
Salivary Glands Saliva production reducedSaliva production increased
Oral/Nasal Mucosa Mucus production reducedMucus production increased
Heart Heart rate and force increasedHeart rate and force decreased
Lung Bronchial muscle relaxed Bronchial muscle contracted
Stomach Peristalsis reduced Gastric juice secreted; motility increased
Small Intestine Motility reducedDigestion increased
Large Intestine Motility reducedSecretions and motility increased
Liver Increased conversion of
glycogen to glucose
Kidney Decreased urine secretionIncreased urine secretion
Adrenal medulla Norepinephrine and
epinephrine secreted
Bladder Wall relaxed
Sphincter closed
Wall contracted
Sphincter relaxed

It should be noted that the autonomic nervous system is always working. It is NOT only active during "fight or flight" or "rest and digest" situations. Rather, the autonomic nervous system acts to maintain normal internal functions and works with the somatic nervous system.

The enteric nervous system is a third division of the autonomic nervous system that you do not hear much about. The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, and gall bladder).


 Actions of the Autonomic Nervous System
Effector Organ Autonomic Division Action
Eye : pupil sympathetic 


dilation of the pupil 

constriction of the pupil

Eye : ciliary muscle sympathetic 


allows far vision 

allows near vision

Lachrymal (tear) glands sympathetic 



secretion of tears

Salivary glands sympathetic 


vasoconstriction and secretion of mucous with a low enzyme count 

secretion of watery saliva with a high enzyme count

Heart sympathetic 


dilation of coronary arteries, increased heart rate, increased force of contraction, increased rate of pacemaker conduction 

coronary artery constriction 

slows, heart rate, reduces contraction and conduction, constricts coronary arteries

Bronchii sympathetic 



constriction and mucous secretion

Oesophagus sympathetic 



peristalsis, secretion of mucous

Stomach and Intestines sympathetic 


inhibition of peristalsis and secretion 

vasoconstriction, spinctre contraction 

peristalsis and secretion

Spleen sympathetic contraction
Adrenal medulla sympathetic adrenaline and noradrenaline secreted into the bloodstream
Liver sympathetic break down of glycogen (glyogenolysis)
Gall Bladder sympathetic 




Pancreas sympathetic inhibition of insulin secretion 

stimulation of insulin secretion

Descending colon sympathetic 



inhibition of peristalsis and secretion 

peristalsis and secretion

Sigmoid colon, rectum and anus sympathetic 


constriction of sphincter muscles 

inhibition of peristalsis and secretion 

peristalsis and secretion

Bladder sympathetic 


contraction of sphincter 

relaxation of detrusor muscle 

contraction of detrusor muscle

Penis sympathetic 




Clitoris parasympathetic erection
Uterus sympathetic contraction 


Blood vessels in:
Skin sympathetic constriction
Mucosal linings sympathetic constriction
Muscle sympathetic dilation
Kidneys sympathetic constriction
Lungs sympathetic constriction
Intracranial sympathetic slight constriction
Sweat glands
sweat glands except palm of hands sympathetic sweating
sweat glands on palms of hands sympathetic sweating
Pilomotor muscles at root of body hair sympathetic piloerection (making hair "stand on end") horripilation ("goose pimples")
Adipose tissue sympathetic lipolysis (break down of fat to release energy)

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Interactive Word Search Puzzle on the Autonomic Nervous System

More on the autonomic nervous system:
  1. Autonomic Nervous System - Detailed review
  2. Autonomic Nervous System - General Organization
  3. The Response to Stress