Whenever a human or other vertebrate is under stress, a fight or flight response is activated. Sympathetic nerves are a critical element in the response, and their arousal prepares the vertebrate to stand its ground and fight or take flight and run away from danger.
Autonomic Nervous System
The nerves that communicate between the brain stem and the body's internal organs comprise the autonomic nervous system. This system regulates internal body processes that require no conscious awareness, such as the rate of heart contractions, breathing and stomach acid secretion. The autonomic nervous system is made up of two parts: the sympathetic nervous system, which responds to stressful situations, and the parasympathetic nervous system, which generally relaxes the body once the danger has passed.
Sympathetic Nervous System
When exposed to a stressful situation, the sympathetic nervous system is aroused. When this occurs, the body undergoes the following changes: the pupils dilate, the flow of saliva is inhibited, the bronchi of the lungs relax, the heartbeat accelerates, digestive activity is inhibited, the bladder relaxes and glucose is released by the liver for extra energy. All of these responses reflect the arousal of the sympathetic system and its role in preparing the body to respond to alarm or stress.
When the nerve endings of the sympathetic system are aroused, the body releases the hormone epinephrine. Epinephrine is also known as adrenaline. Adrenaline stimulates the heart to beat faster and harder, and it causes the blood vessels to constrict, thereby creating a rise in blood pressure. Faster and stronger heart contractions and a higher blood pressure to push the blood through the vessles more rapidly readies the body to fight or flee from danger.
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