When large portions of the sympathetic nervous system discharge at the same time—that is, a mass discharge—this increases in many ways the ability of the body to perform vigorous muscle activity. Let us summarize these ways:
2. Increased blood flow to active muscles concurrent with decreased blood flow to organs such as the gastrointestinal tract and the kidneys that are not needed for rapid motor activity
3. Increased rates of cellular metabolism throughout the body
4. Increased blood glucose concentration
5. Increased glycolysis in the liver and in muscle
6. Increased muscle strength
7. Increased mental activity
The sum of these effects permits a person to perform far more strenuous physical activity than would otherwise be possible. Because either mental or physical stress can excite the sympathetic system, it is frequently said that the purpose of the sympathetic system is to provide extra activation of the body in states of stress: this is called the sympathetic stress response.
The sympathetic system is especially strongly activated in many emotional states. For instance, in the state of rage, which is elicited to a great extent by stimulating the hypothalamus, signals are transmitted downward through the reticular formation of the brain stem and into the spinal cord to cause massive sympathetic discharge; most aforementioned sympathetic events ensue immediately. This is called the sympathetic alarm reaction. It is also called the fight or flight reaction because an animal in this state decides almost instantly whether to stand and fight or to run. In either event, the sympathetic alarm reaction makes the animal's subsequent activities vigorous.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.