The Nervous System and Homeostasis — Interactions with the Internal and External Environments

A major function of the nervous system is to control the relative constancy of the internal environment of the organism. That is, to provide the right chemical environment for living processes to take place. This control of the internal environment is known as homeostasis. Any disturbances in the internal environment are monitored by sense organs: for example chemoreceptors sensitive to the partial pressure of oxygen in the arterial blood, mechanoreceptors sensitive to blood pressure, and chemoreceptors within the central nervous system itself sensitive to hydrogen ion concentration or to various hormones. The information from the sense organs is fed to the central nervous system, where it is processed and appropriate outputs sent to the effectors, muscle (striated, smooth and cardiac) and glands, to counteract the disturbance. Such a function is essentially the same as that carried out by a physical servosystem like a governor on a steam engine, or a central heating control system that measures room temperature and keeps it within certain limits by regulating the output of the heating unit (Fig. 16.1).