Monday, April 11, 2011

Sympathetic Nervous System vs. Parasympathetic Nervous System

Sympathetic nervous system promotes a "fight or flight" response, corresponds with arousal and energy generation, and inhibits digestion. It diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.

Blood flow to the skeletal muscles and the lungs is not only maintained, but enhanced (by as much as 1200% in the case of skeletal muscles).

Bronchioles of the lung dilate, which allows for greater alveolar oxygen exchange.

Heart rate and the contractility of cardiac cells (myocytes) increases, thereby providing a mechanism for the enhanced blood flow to skeletal muscles.

This dilates pupils and relaxes the lens, allowing more light to enter the eye. And can provide vasodilation for the coronary vessels of the heart.

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Generally it inhibits peristalsis.


Parasympathetic nervous system promotes a "rest and digest" response, promotes calming of the nerves return to regular function, and enhances digestion.

Blood vessels leading to the GI tract are dilated thus increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut.

The parasympathetic nervous system can also constrict the bronchiolar diameter when the need for oxygen has diminished.

Dedicated cardiac branches of the vagus nerve and thoracic spinal accessory nerve impart parasympathetic control of the heart or myocardium.

During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.

The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis. In keeping with the rest and digestive functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.

It is also involved in erection of genitals, via the specific pelvic nerves.

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