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Physiology 2130 notes for Module 1-9

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Western University
Physiology 2130
Anita Woods

3 Negative Feedback Control Systems (cont.) Here is how negative feedback controls body temperature. The set point for body temperature is 37°C (98.6°F). This is the temperature that you want your body to maintain. Your actual body temperature could be different; it could drop on a cold day to 35°C (95°F). This actual body temperature would be detected by sensors in the nervous system which would signal a control center in a specific region of the brain called the hypothalamus. The control center would notice a difference between the set point (what you want) and the actual value of 35°C (95°F). The control center would then activate organs and systems (the effector) to generate heat (the controlled variable) by shivering and conserving heat by decreasing blood flow to the skin. Once the body temperature rises back to 37°C (98.6°F), the control center would stop the shivering and would return the blood flow to the extremities. The opposite would happen if bo dy temperature increased above the set point. The sensors would detect the rise in temperature and signal the hypothalamus. Utilizing the nervous system, the hypothalamus would signal blood vessels in the skin to dilate and sweat glands to sweat. Heat would dissipate and body temperature would drop to normal. Positive Feedback Control Systems Unlike negative feedback systems where the controlled variable eventually shuts off its own production, a positive feedback control system (also called feedforward systems) is a mechanism where the controlled variable actually stimulates its own production . Positive feedback systems are self-amplifying mechanisms that can produce a very rapid change in a physiological system. As shown at right, the controlled variable is detected by the sensor that signals the control center to activate the effector to produce more of the controlled variable. In this way, large amounts of the controlled variable are produced very rapidly. Throughout the modules, we will see several positive feedback systems, including the generation of the action potential in nerve cells and the surge of luteinizing hormone that causes the ovulation of the egg from the ovary.
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