The medulla produces norepinephrine and epinephrine (adrenaline), the hormone of “fight or flight. ” Whenever there is a sudden need for energy or a response to stress, a rapid discharge of epinephrine into the bloodstream initiates a series of changes in the body that permit it to cope with the emergency. For example, someone is walking along a dark, lonely street at night, and hears a sudden, unexpected sound. The sound is transmitted from the ear to the brain, which has not expected this stimulus, and interprets it as a threat. The brain then sends out a message to the muscles: get tense, be prepared to fight or run!
Epinephrine readies the body for action by increasing the heart rate and blood pressure to improve the flow of blood to the muscles, and by opening the blood vessels of the arm and leg muscles to permit more blood to flow to them. Epinephrine also stimulates the liver to convert glycogen to sugar to raise the blood sugar level so that the muscles will have an increased supply of energy.
The hormone causes the pupils of the eyes to dilate to increase the field of vision. Other alterations occurring from blood distribution include excessive perspiration, hair standing on end, and trembling of the hands. A continued state of fight or flight readiness, resulting from high anxiety or emotion, can produce physical fatigue. For example, American astronaut Alan Shepard and his colleague Edgar Mitchell tried to climb the steep slopes of a lunar crater, their cardiac rates increased to 150 beats per minute. The moon walk had to be postponed because the astronauts were too tired to continue.
A highly sensitive person can overwork his adrenal medulla by stimulating discharge of epinephrine even in minor crises—for instance, when getting caught in a traffic jam or while waiting for service in a restaurant. Such hyper-sensitivity causes unnecessary stress and fatigue. Fight or flight symptoms also occur in diabetic patients who have
had too much insulin. An abnormal fall in the blood sugar level causes the release of epinephrine, which produces these symptoms. Along with extra sugar, the epinephrine brings the blood sugar back to normal by releasing glucose from the liver.