13 - Mechanisms of control

Abstract

This chapter reviews the physiological concepts related to various control mechanisms. The fundamental concept in physiology is that of control. This includes control of the internal environment so as to permit cell and organ function to continue at an optimal level, and control in the sense of the direction of both conscious and unconscious functions on which, however indirectly, the survival of the individual and the species depends. Most control systems are termed feedback systems, because information about any changes that are occurring must be fed back to some controller that can cause an appropriate action, usually one to restore the original condition. Systems of this kind are negative feedback systems—the active change reversing the initial change. Negative feedback systems are inherently stable, always resisting change. They are typical of biological systems. The control of a central heating system provides an everyday example of a negative feedback system. Heaters are set to maintain a given temperature in a room. Any departure from the set temperature causes the heaters to operate more actively if temperature falls, or less actively if temperature rises. Positive feedback systems, with their natural instability, are much less common. The chapter describes reflex as as an involuntary response to a peripheral stimulus. Reflexes like coughing, sneezing, vomiting, and swallowing have centers in the medulla. Reflexes involving visual stimuli pass through the midbrain. Studies on men with spinal or brain injuries have demonstrated different levels of reflex control.