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Understanding the Autonomic Nervous System


Dysautonomia is an umbrella term used to describe a collection of disorders related to the dysfunction of the Autonomic Nervous System (ANS).  So to understand dysautonomia, it begins with understanding how the ANS works.

The autonomic nervous system regulates certain processes that occur automatically within the body, without a person's conscious effort to make them work.  These processes are things like breathing, heart rate, and blood pressure. 

When the autonomic nervous system doesn't work correctly (dysfunctions) it can affect any body part or process.  These disorders may be reversible or they may progressively worsen over time. 

The ANS is the part of the nervous system responsible for the function of our blood vessels, stomach, intestines, liver, kidneys, bladder, genitals, lungs, pupils, heart and sweat,, salivary and digestive glands.  It is easy to see how debilitating disorders of the ANS can be since they affect so many parts of the body. 

NEU_autonomic_nervous_system-labels.gif.da50ba28a6374029f3568bc345ae6026.gifThe autonomic nervous system has two main divisions:
  • Sympathetic

  • Parasympathetic

Both of these divisions are affected when the ANS is not working properly.  

To understand this better, it is important to understand how this works within the body.  

When a signal is received by the ANS from the body and the environment,  it responds by stimulating processes within the body, usually through the sympathetic portion of the ANS, or it inhibits a process through the parasympathetic division of the ANS.  For example;  the heat in a room gets turned up.  This is an external message that is received by the ANS within the body.  A properly functioning ANS would respond by sending a signal causing the body to sweat.  This would be a natural reaction to received stimuli. 

The processes the ANS are responsible for are:

  • Blood pressures
  • Heart rate and respiration (breathing)
  • Body temperature
  • Digestion
  • Metabolism (affects weight)
  • The balance of water & electrolytes (affecting sodium & calcium levels)
  • Production of body fluids (affecting saliva, sweat & tears)
  • Urination
  • Defecation
  • Sexual response

One of the more challenging aspects to understand is that although many organs are controlled primarily by the sympathetic or parasympathetic divisions, sometimes the two divisions can have opposite effects on the same organ.  For example, the sympathetic division increases blood pressure, and the parasympathetic division decreases it.  Overall, the two divisions work together to ensure the body responds appropriately to different situations.  (Low, Dec 2018)

The sympathetic division prepares the body for stress and emergency situations - Fight or Flight.  The fight or flight mechanism increases the heart rate and blood pressure (the force that the heart contracts) it dilates (widens) the airways making breathing easier.  It causes the body to release stored energy and muscular strength is increased.  It also causes your palms to sweat, pupils to dilate and your hair to stand on end.  It slows down the processes within the body that are less important in an emergency, like digestion and urination.  (Low, Dec 2018)

On the other hand, the parasympathetic division controls the processes in the body during ordinary situations.  Generally, it conserves and restores.  It slows the heart rate and decreases blood pressure.  It stimulates digestion causing the body to process food and eliminate waste.  It uses energy from processed food to restore and build body tissue.  

Two chemical messengers (neurotransmitters) are used to communicate within the autonomic nervous system:

  • Acetylcholine
  • Norepinephrine

Nerve fibers that secrete acetylcholine are called cholinergic fibers. Fibers that secrete norepinephrine are called adrenergic fibers. Generally, acetylcholine has parasympathetic (inhibiting) effects and norepinephrine has sympathetic (stimulating) effects. However, acetylcholine has some sympathetic effects. For example, it sometimes stimulates sweating or makes the hair stand on end. (Low, Dec 2018)

The role of cholinergic and adrenergic fibers continues to be studied in dysautonomia research and particularly the role they play in the ANS function of POTS patients. 


Dysautonomia is the dysfunction of the autonomic nervous system.  Because the ANS does not respond correctly to the messages it receives, it results in the misfiring of the processes described above.  The causes of dysautonomia are not always known.  It is important to understand that although there are patients who develop dysautonomia disorders as their primary illness, there are many more who develop dysautonomia disorders secondary to other illnesses that cause the dysfunction of the ANS, such as Parkinson's Disease, EDS, Spinal Cord trauma or Brain Injury, and many others.

Follow this link to read more about the mechanisms within the body associated with various types of dysautonomia disorders,  Explore the Information Resources section of this site to learn more about specific disorders related to the dysfunction of the ANS. 


Phillip Low, MD, Professor of Neurology, College of Medicine, Mayo Clinic; Consultant, Department of Neurology, Mayo Clinic updated December 2018   Merck Manual, Consumer Edition, full text


Image:  https://www.merckmanuals.com/home/brain,-spinal-cord,-and-nerve-disorders/autonomic-nervous-system-disorders/overview-of-the-autonomic-nervous-system


Edited by edriscoll

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A wonderfully concise description of the autonomic nervous system, its overall structure and function.

Thank you for this.

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