6.2: Main Functions for Homeostasis

The overall goal of the nervous system is to regulate the operations of parts of the body to make sure they contribute to homeostasis and a satisfactory quality of life. The nervous system regulates muscles and glands directly by sending impulses to those structures. Among the glands controlled by the nervous system are the sweat glands and salivary glands. This system regulates other parts of the body indirectly by adjusting the amounts of hormones produced by some of the endocrine glands.

Monitoring

The nervous system performs six main functions to carry out its overall goal. Three operations stem from the three steps in negative feedback systems: monitoring, communicating, and adjusting. Many of the neurons in the brain and nerves monitor conditions in and around the body. These neurons do very little if conditions are proper and fairly stable. However, they are affected by harmful conditions and are sensitive to any change in conditions. When conditions are unfavorable for the cells or when there is a change (a stimulus), the neurons respond by starting messages (nerve impulses) within themselves.

Communicating

The initiation of impulses by neurons leads to the second main function: communicating. The neurons carry impulses to other parts of the nervous system, where they are passed to other neurons, which pass them to still other neurons, and so on. Thus, many parts of the nervous system are informed that a change has occurred. They are also informed of the nature of the change, its extent, and where it is happening. For example, if an insect bites a person, that person feels that something is happening. He or she also knows that it is a bite rather than something soft brushing against the skin, has a sense of the severity of the bite, and knows where to scratch or hit to remove the insect.

Stimulating

Communicating leads to the third function: stimulating. In the case of an insect bite, the nervous system activates muscles in the arms to remove the source of irritation. Note that the nervous system does not actually perform the adjustment, which is the third step in negative feedback. It only stimulates other parts of the body to do so.

These three functions can activate responses to promote beneficial changes as well as eliminate harmful ones. For example, when neurons in the stomach sense that it is empty and brain neurons detect that the nutrient level in the blood is low, a person feels hungry. If other neurons detect the sound of someone cooking in the kitchen while other neurons detect dinner aromas, the nervous system will activate muscles so that the hungry person will go to the kitchen and obtain nourishment. When the stomach has become full and blood nutrient levels begin to rise, other neurons initiate a negative response, causing the person to stop eating.

Coordinating

Making adjustments often requires the contributions of many parts of the body, and the nervous system must stimulate them so that they all work in harmony. At the same time, parts of the body that can interfere with achieving the desired outcome must be inhibited from acting. The nervous system provides these stimulations and inhibitions through its fourth main function: coordinating. For example, to walk to the kitchen, a person must activate some muscles while inhibiting others in order to step forward with one foot at a time.

Remembering

When a person must adjust to a new situation, it may take quite a while for all the necessary impulses to reach their destinations, especially when the situation is complicated and the proper response requires the coordinated stimulation of many structures. Furthermore, sometimes mistakes are made and the wrong response occurs. This is when remembering, the fifth main function of the nervous system, becomes helpful.

By remembering, the nervous system stores information about past experiences that includes the recollection of a situation, the responses that were made, and the degree of success that was provided by each response. Then, when faced with the same situation, a person can avoid trial and error by remembering what to do. This procedure saves time and prevents costly mistakes. Simple examples include remembering the way home after traveling the route a few times and remembering the answers to test questions using information that was studied many times.

The benefits of remembering are used on an unconscious level as well. For example, when one is practicing an activity such as walking, playing an instrument, or riding a bicycle, the nervous system remembers the sequences of muscle contractions that resulted in failure or success. With enough practice, one need only consciously start the activity. One can then continue to perform well without thinking about the activity because, like a recording, the nervous system plays the rest of the program of successful commands for the muscles.

Memory also recalls situations that led to favorable or unfavorable outcomes. When the nervous system recognizes the presence of such situations, it will alert a person to proceed or take evasive action. This is why an experienced child will reach out for candy but back away from fire.

Thinking

Remembering tends to provide the same type of successful response every time a person is in the same circumstance. The more successful the same response is in the same situation, the faster and more accurately that response will occur. However, remembering does little when a person is faced with a new situation. That person must try to find the correct response by trial and error or by mentally imagining different responses and the results they might cause. Creating mental images of new courses of action and their possible outcomes is the sixth main function of the nervous system: thinking.

Thinking depends on memory to provide initial mental images and information. In thinking, a person intelligently rearranges the remembered images and information to create new images that have not been experienced before. Many alternatives can be mentally explored in a few seconds without actually trying any of them. People are thinking when they make plans, solve problems by analysis, and create mental images of things that do not occur naturally. Thinking provides the variety of acting that many people believe separates humans from other living things.

Thinking allows people to decide the best response to a new situation quickly, accurately, and without having to risk the consequences of untested attempts. It can even produce new responses to situations that have been created by people. For example, this is how people return to the Earth from a trip to the moon.