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The Autonomic Nervous System

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While sobering, these figures are perhaps unsurprising since there is now one new case of dementia diagnosed every 4 s, or 77 million cases per year (2). More than 300 million people are now living with depression, an increase of more than 18% between 2005 and 2015. The parasympathetic nervous system (PNS) controls homeostasis and the body at rest and is responsible for the body's 'rest and digest' function. The sympathetic nervous system (SNS) controls the body's responses to a perceived threat and is responsible for the 'fight or flight' response. The PNS and SNS are part of the autonomic nervous system (ANS), which is responsible for the involuntary.

The autonomic nervous system (ANS) regulates visceral functions, i.e. functions of the internal organs such as the heart, stomach and intestines. The ANS is part of the peripheral nervous system and also has control over some muscles within the body. The functions of the ANS are involuntary and reflexive, e.g. the beating of the heart, expansion or contraction of blood vessels or pupils, etc. — which is why we are seldom conscious of it. The parasympathetic and sympathetic nervous systems, along with the enteric nervous system make up the ANS.

What is the parasympathetic nervous system?

The parasympathetic nervous system is part of the autonomic nervous system. It originates in the spinal cord and the medulla and controls homeostasis, or the maintenance of the body's systems. The parasympathetic nervous system controls the 'rest and digest' functions of the body.

What is the sympathetic nervous system?

The sympathetic nervous system, also part of the autonomic nervous system, originates in the spinal cord; specifically in the thoracic and lumbar regions. It controls the body's 'fight or flight' responses, or how the body reacts to perceived danger.

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Sympathetic vs Parasympathetic Responses

With sympathetic nervous responses, the body speeds up, tenses up and becomes more alert. Functions that are not essential for survival are shut down. Following are the specific reactions of sympathetic nervous system:

• increase in the rate and constriction of the heart
• dilation of bronchial tubes in the lungs and pupils in the eyes
• contraction of muscles
• release of adrenaline from the adrenal gland
• conversion of glycogen to glucose to provide energy for the muscles.
• shut down of processes not critical for survival
• decrease in saliva production: the stomach does not move for digestion, nor does it release digestive secretions.
• decrease in urinary output
• sphincter contraction.

The parasympathetic nervous system counterbalances the sympathetic nervous system. It restores the body to a state of calm. The specific responses are:

• decrease in heart rate
• constriction of bronchial tubes in the lungs and pupils in the eyes
• relaxation of muscles
• saliva production: the stomach moves and increases secretions for digestion.
• increase in urinary output
• sphincter relaxation.

How it Works

The parasympathetic nervous system is a slower system and moves along longer pathways. Preganglionic fibers from the medulla or spinal cord project ganglia close to the target organ. They create a synapse, which eventually creates the desired response.

The sympathetic nervous system is a faster system as it moves along very short neurons. When the system is activated, it activates the adrenal medulla to release hormones and chemical receptors into the bloodstreams. The target glands and muscles get activated. Once the perceived danger is gone, the parasympathetic nervous system takes over to counterbalance the effects of the sympathetic nervous system's responses.

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References

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