The Autonomic Nervous System


The autonomic nervous system (ANS) is the division of the nervous system that supplies the internal organs, including the blood vessels, stomach, intestine, liver, kidneys, bladder, genitals, lungs, pupils, heart, and sweat, salivary, and digestive glands. The ANS is responsible to changes that happen within the body without voluntary control (in other words, “autonomously).

The ANS regulates important bodily functions, including:

  • Blood pressure
  • Heart and breathing rates
  • Body temperature
  • Digestion
  • Metabolism (thus affecting body weight)
  • The balance of water and electrolytes (such as sodium and calcium)
  • The production of body fluids (saliva, sweat, and tears)
  • Urination
  • Defecation
  • Sexual response

Dividing the Autonomic Nervous system

To understand how the ANS works, you have to understand how it is organised. The ANS consists of two divisions:

  1. The sympathetic nervous system (SNS)
  2. The parasympathetic nervous system (PNS

The two divisions arise from different areas of the spinal cord; neurons of the SNS emerge from the thoracic or lumbar regions of the spinal cord whilst neurons of the PNS arise from cranial or sacral regions.

The SNS can be thought of as the pathways through which we respond to danger, or the “fight or flight” response. The actions of the PNS are broadly categorised by healing mechanisms, digestion/excretion and sexual activity, or the “rest & digest” response. 

Sometimes the two divisions have opposite effects on the same organ. The SNS tends to have a stimulatory effect on structures within the body, the PNS tends to have an inhibitory effect. For example, the sympathetic division increases blood pressure, and the parasympathetic division decreases it. Overall, the two divisions work together to ensure that the body responds appropriately to different situations.

Sympathetic response (“fight or flight”)

The sympathetic nervous system is responsible for the body’s rapid involuntary response to dangerous or stressful situations. A flash flood of hormones boosts the body’s alertness and heart rate, sending extra blood to the muscles and away from less essential organs, like the gut. Breathing rate increases, fresh oxygen is delivered to the brain, and glucose is made more readily available for a quick energy boost. This response occurs so quickly that people often don’t realize it’s taken place, according to Harvard Medical School. For instance, a person may jump from the path of a moving car before they fully register that it’s hurtling toward them.

The parasympathetic nervous system has the role of de-stressing the nervous system once the threat of danger has passed, according to the Clinical Anatomy of the Cranial Nerves, published in 2014 by Academic Press. To counter the fight-or-flight response, this system encourages the body to “rest and digest.” Blood pressure, breathing rate and hormone flow return to normal levels as the body settles into homeostasis, or equilibrium, once more.

The sympathetic and parasympathetic nervous systems work together closely to maintain this baseline and normal body function. Even in the absence of an external threat, we rely on the collaboration of the two divisions of the ANS to mediate bodily functions, e.g. the control of hormones . 

Click here to read about how the sympathetic nervous system is relevant in Chronic Fatigue Syndrome/Myalgic Encephalopathy (CFS/ME).