The activities of physiological systems are controlled—speeded up or slowed down—by actions of the nervous and endocrine systems. But to regulate these systems and maintain homeostasis, information is required. As an analogy, think of the thermostat that controls the furnace and air conditioner to regulate the temperature of a house (Figure 39.5). The desired temperature is a set point, or reference point on the thermostat. The thermostat acts as a comparator by sensing the current temperature in the house and comparing that value to the set point. Thus the sensing of the air temperature is feedback information. Any difference between the set point and feedback information results in an error signal. The error signal is converted into commands to the furnace or air conditioner, turning them on or off.

Regulatory systems such as a building thermostat or one of the many physiological regulatory systems described in this book obtain, process, and integrate information. Regulatory systems use that information to issue commands to effectors such as muscles or glands that effect changes in the internal environment. Effectors are also called controlled systems because their activities are controlled by the neural or hormonal signals from their respective regulatory systems. Important components of any regulatory system are the sensors such as light-, temperature-, and pressure-sensitive cells that provide feedback information to be compared with internal set points.

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Negative feedback is information used to counteract the influence that created an error signal. Whatever force is pushing the system away from its set point must be “negated.” In our thermostat analogy, an air temperature below the set point causes the furnace to be turned on, which then reverses the direction of change in the air temperature.

Although not as common as negative feedback, positive feedback also exists in physiological systems. Rather than returning a system to a set point, positive feedback amplifies a response (i.e., it increases the deviation from the set point). An example is sexual behavior, in which a little stimulation causes more behavior, which causes more stimulation, and so on. Positive feedback responses tend to reach a limit and terminate rapidly. The birth process is a good example. Contractions of the uterus push the baby into the birth canal, and stretching of the birth canal stimulates more and stronger contractions until the baby is delivered, at which time contractions cease.

Feedforward information is another feature of regulatory systems. Its function is to change the set point in anticipation of a change in conditions. The timer on a thermostat provides feedforward information by changing the system’s temperature set point, usually lowering it in the evening and raising it in the morning. Hearing the words “on your mark” before a race is feedforward information that raises your heart rate in anticipation of running. Feedforward information anticipates a change in the internal environment before that change occurs.