19.6:
Sympathetic Activation
The sympathetic nervous system triggers numerous physiological changes to prepare the body for the fight-or-flight response.
During rigorous physical activity, the sympathetic nervous system redirects blood flow from the skin and abdominal organs to the skeletal muscles, reducing the activity of the digestive and urinary systems.
It also regulates the heart rate by constricting and dilating blood vessels in response to changes in blood pressure. It further promotes the release of renin from the kidneys, an enzyme that controls blood pressure.
Sympathetic activation increases the metabolic rate and glycogen breakdown in the liver and muscles to meet the energy needs of the body during emergencies.
In stressful situations, the sympathetic nervous system increases mental alertness by stimulating the reticular activating system in the brain.
The sympathetic nervous system also maintains a stable body temperature.
When body temperature rises, sympathetic nerves dilate skin blood vessels and activate sweat production to release the excess heat. Conversely, when body temperature drops, skin blood vessels constrict to retain heat.
19.6:
Sympathetic Activation
The sympathetic division can influence tissues and organs by releasing norepinephrine at peripheral synapses and distributing epinephrine and norepinephrine through the bloodstream. In times of crisis or stress, sympathetic activation occurs, which is regulated by sympathetic centers in the hypothalamus. As a result, sympathetic activation prepares the body for physical exertion, rapid ATP production, and heightened alertness, allowing individuals to respond effectively to challenging or threatening situations.
Physiology of Sympathetic Activation
During sympathetic activation, various physiological changes take place. The individual experiences increased alertness due to stimulation of the reticular activating system, leading to feeling "on edge." There is a surge of energy and euphoria, often accompanied by a temporary insensitivity to pain and a disregard for danger. The cardiovascular and respiratory centers in the brainstem become more active, resulting in increased heart rate, blood pressure, breathing rate, and depth of respiration.
Muscle tone is elevated, and the person may appear tense and may even begin to shiver. The mobilization of energy reserves occurs through the release of lipids by adipose tissue and the accelerated breakdown of glycogen in muscle and liver cells. These changes, along with the peripheral alterations, prepare the individual to cope with the stressful situation.
Sympathetic stimulation has longer-lasting and more widespread effects compared to parasympathetic stimulation. This effect is due to the extensive divergence of sympathetic postganglionic axons, allowing for simultaneous activation of multiple tissues. Additionally, norepinephrine lingers in the synaptic cleft longer than acetylcholine, and the release of epinephrine and norepinephrine from the adrenal medullae intensifies and prolongs the sympathetic responses.