Communication within the body is essential for homeostasis. Communication is accomplished chiefly by the nervous and endocrine systems, which use neural electrical impulses or bloodborne hormones, respectively, as information carriers. The details of how these two great regulating systems operate are covered in later chapters, but the basic characteristics of control systems that promote homeostasis are explained here.
Regardless of the factor or event being regulated—the variable—all homeostatic control mechanisms have at least three interdependent components (Figure 1.4). The first component, the receptor, is some type of sensor that monitors the environment and responds to changes, called stimuli, by sending information (input) to the second component, the control center. Input flows from the receptor to the control center along the so-called afferent pathway. The control center, which determines the set point (the level or range at which a variable is to be maintained), analyzes the input it receives and then determines the appropriate response or course of action.
The third component, the effector, provides the means for the control center’s response (output) to the stimulus. Information flows from the control center to the effector along the efferent pathway. The results of the response then feed back to influence the stimulus, either depressing it (negative feedback) so that the whole control mechanism is shut off or enhancing it (positive feedback) so that the reaction continues at an even faster rate. (To help you remember the difference between “afferent” and “efferent,” you might note that information traveling along the afferent pathway approaches the control center and efferent information exits from the control center.)
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