The most common homeostatic control mechanism is the negative feedback system. A negative feedback system operates to maintain a constant output parameter or steady-state, even if the output parameter is disturbed by an applied stimulus. When the steady-state is disturbed the system first detects the change in the output parameter. It then produces an opposite polarity signal (negative feedback signal) proportional to the output deviation, and feeds this signal back to the input of the system. This feedback signal changes the input and, thus, acts to correct the output deviation (Figure PG.2).
A simple physiological example of a negative feedback system is illustrated by the gamma efferent system controlling resting length in skeletal muscle to maintain posture. Here the output parameter is muscle
stretch and the input is the gamma efferent signal to the muscle. When stretch is increased, the displacement is detected by the muscle spindles, which feed back a signal to produce an increase in gamma efferent signal strength. This increases muscle tone thus acting to correct the original stretch or stimulus. Such a response is referred to as a reflex and describes the events in the simple "knee jerk'.
The analogous homeostatic reflex is designed to maintain a given physiological parameter at a constant value or in a steady-state. The physiological parameter may be a variable such as mean arterial blood pressure, which rests normally at its operating point. When a change occurs in the blood pressure, the change is detected by a baroreceptor that relays a signal to an integrating centre (vasomotor centre of the medulla). The integrating centre then transmits a signal to an effector (vascular smooth muscle), which exerts a response to oppose the original change.
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