Last updated: Tue, Mar 11, 2025
The sympathetic nervous system normally works at a low level. The sympathetic nerves fire at a low rate to maintain homeostasis. Sympathetic activity increases with stress, whether physical or emotional. Fright, fear, and pain are serious stressors that can cause the hypothalamus to activate the entire sympathetic system strongly and all at once. This is called a mass discharge. A mass discharge dumps large amounts of epinephrine (adrenaline) and norepinephrine into the bloodstream. These hormones/neurotransmitters circulate in the bloodstream and cause constriction of most blood vessels, cause the heart to beat more strongly, reduce the activity of the gastrointestinal tract, and dilate the pupils. The mass discharge releases glucose from the liver, increases blood glucose, increases the strength of the skeletal muscles, and increases mental activity. The overall effect of a mass discharge can raise the metabolic level of the entire body by as much as 100 percent. This activation of the muscular and mental systems is called the stress response.
Stress causes the pituitary gland, which extends below the hypothalamus, to secrete a hormone into the blood that in turn causes the adrenal glands to release cortisol (hydrocortisone) into the bloodstream. Cortisol is anti-inflammatory. It increases the amount of amino acids available throughout the body and increases the production of red blood cells. While the stress response begins very quickly (at neural speeds), release of cortisol occurs only after the circulatory system has carried the corticol releasing hormome to the adrenal glands. This takes several minutes. Cortisol works to prevent inflammation from developing or, if it has already developed, it works to resolve it more quickly.