Most homeostatic mechanisms utilize a control system known as negative feedback. (See Figure 1.4) Negative feedback works like a thermostat. A sensor responds to a variable stimulus. When that variable is outside the normal range the sensor notifies a control center, which then responds directly or triggers a response by an effector. The response has the effect of reducing or negating the original stimulus by bringing the variable back within the normal range. A negative feedback mechanism is self regulating, it turns itself off.