The Nervous System
Control center. Your nervous system is like a general’s headquarters in the army. It has an intelligence division to find out information, a messenger division to deliver orders to the proper units, and a general headquarters, where all but the least important decisions are made.
The general headquarters of the brain is composed of the cerebrum and cerebral cortex. These two parts of the brain control your thinking and voluntary actions. These structures and their associated nerves are called the central nervous system. This system controls your conscious actions. These parts are in control when you lift an ice cream cone to your mouth or when you think about the answer to a math problem.
The cerebellum and cerebrum are connected to two kinds of nerves. The first, afferent nerves, pick up information from the sense organs. The second, efferent nerves, relay impulses from the conscious center to the muscles. These nerve pathways enable you to sense your surroundings and carry out your activities.
Other parts of the brain control involuntary actions such as maintaining balance, focusing the eyes, and regulating internal organs. Look at the illustration to see what areas the parts of the brain control.
The spinal cord. Within the thirty-three bones of the spine is the spinal cord. This cord is the largest central nerve trunk of the body. The nerves begin at the base of the brain and branch off the spinal cord. They connect various internal organs with the brain.
Your spinal cord also takes part in body reactions. Whenever a quick but automatic reaction is required, the reaction is made in the spinal cord. It is the spinal cord, for instance, that directs your muscles to jerk your hand away from a flame or makes you lift your foot hurriedly if you step on a tack.
Your nerves are the spies and messengers of the body. They sense the temperatures, pressures, sights, sounds, and smells around you and send tiny electrical impulses to the brain or spinal cord to be interpreted. When the control center interprets the messages, it sends back an answer along other nerves to the muscles, which carry out the body’s activities.
Regulation of the body. Some nerves and parts of the brain are part of the autonomic nervous system. This system controls the internal organs. It regulates your breathing, digestion, rate of perspiration, and even the way in which you digest your food.
The autonomic system works in the same way that the central nervous system works. There are afferent nerves that are stimulated by heat, pressure, or pain; control centers in the spinal column and the base of the brain; and efferent nerves to send the messages to muscles that can carry out the necessary action.
Nerve cells. All nerve cells, whether they are in the nerves, the spinal cord, or the brain, are similar in structure. Nerve cells have a nucleus and cell body like all cells, and they have two special structures that allow them to send and receive electrical impulses. These are the dendrites and axons.
The dendrites receive impulses from a sensory organ and pass them through the cell body to the axon, which passes them to the dendrite of the next cell in the chain. Thus, a nerve fiber is like a long bucket brigade in which an impulse is transmitted from a sensory organ to a control center. However, the "bucket" travels at 300 feet per second, much faster than a bucket has ever traveled in any human bucket brigade.
These chains of nerve cells penetrate every part of the body. The nerves that make up the central nervous system, or conscious control centers, constitute the five senses and are connected to the skeletal muscles that you can control. The nerve chains that are part of the autonomic nervous system are concentrated in the organs inside the body. You seldom feel the effects of these nerves, but their activities are essential to the operation of your organs and systems.
Have you ever had perspiring palms when you were about to take a very important examination? This condition is evidence that there is a link between the sweat glands in your hand and the higher conscious control centers of your brain. Other evidence of the great complexity of the nervous system is seen in the many ways the rest of the organs react to emotional stress.
Think for Yourself
What internal organs are controlled by the autonomic nervous system? Which by the central nervous system?
The Human Body
The study of the human body. Many years ago people explained some processes in the human body by saying that a "vital spirit" within the body controlled some body functions. They used this term to explain processes that they could not understand. They believed that this vital spirit could not be explained by the physical laws of science.
Today, scientists know that the human body functions according to the laws of nature. There is no mysterious vital spirit within the body that causes the heart to beat or the brain to operate. Scientists now explain the working of the body by discovering the physical principles behind its operation.
A physiologist is a modem scientist who studies the body and the ways Its parts function. He seeks scientific explanations for the ways the parts of the body operate. He is interested in the ways the parts of the human body work together to help a human to live.
The body is the most complicated organism known. Its many parts interact in so many ways that physiologists must simplify the organisation for purposes of study. In order to understand the functions of the major organs and tissues of the body, physiologists usually divide them into six major systems. These are the skeletal-muscular system, which controls the basic shape and movement of the body; the gastrointestinal system, which controls the progress of foods within the body; the respiratory system, which controls the oxidation of food in the body; the circulatory system, which is the blood circulating mechanism; the nervous system, which is the voluntary and involuntary control center for the body; and the endocrine system, which is a chemical control system for the other systems.
These systems are linked in many ways. For example, the blood in the circulatory system carries the oxygen from the respiratory system to the muscles in the skeletal-muscular system.
The organs. Each of these six systems is composed of organs, such as the heart, liver, stomach, and brain. Most of the important organs of the body are included in one of the six systems. Each of the organs carries out a special function, but it is so related to many other organs in the body that it often depends on them to help in the process.
Thus, the lungs cannot act without the brain, because the brain sends the impulses which regulate breathing. The stomach is worth nothing without the blood to carry the food it digests to the tissues. Virtually every organ depends on other organs to help it carry out its function.
Tissues and cells. The tissues that make up the organs are likewise dependent on other tissues and organs in the body. Tissues are made of cells, the smallest bits of living matter in a human body. The cells need nourishment and a way to cast off their wastes. This nourishment is provided by various organs of the body.
The tissues of the body, like the organs and the systems, are specialized. Each of the four kinds of tissue looks different from the others, and each makes up different organs. For instance, the epithelial cells in your skin are similar to the epithelial cells in your lungs. The muscle cells in your arm are similar to the muscle cells in your stomach. Look at the illustration to see the four shapes of tissue cells. These cells make up the tissues of the body.
When you read about each of the six systems of the body, you should keep one point in mind. The explanations of the six systems are simplifications. The systems are so complicated and so interrelated that they cannot be described in a few pages. In fact, doctors are still searching for functions of many parts of the six systems of the body.
Think for Yourself
What systems have structures that are dependent on the skin? You may find that there is more than one system.
First Aid for Fainting
Causes of fainting. Cartoons often show women shrieking and fainting at the approach of a mouse, or a snake, or a singing star. It is not very often that you will have to give first aid for this sort of a condition. However, fainting can be caused by great fear or any other condition which causes blood to drain away from the brain.
Perhaps you have heard of someone who has fainted after sitting for a long time in a stuffy room. The stuffy air was probably only one of the causes. Lack of sleep, tension, fatigue, and poor diet might have been other causes.
Helping a victim. You can treat a fainting victim by laying him down and elevating his feet slightly. This will help the flow of blood to the brain. If the person is sitting and feels faint, he should place his head between his knees to help revive circulation to the brain.
Women are not the only ones who faint. Soldiers who are forced to stand at attention in the heat of the sun for long periods often faint. The heat causes the blood vessels to expand. Because of the soldier’s inactivity and the tension he is under, the blood collects in the lower parts of the body.
If a person faints from any of the
previously mentioned causes, the first-aid measures already described should revive him within a few minutes. If the person does not respond quickly, there is probably something else wrong. The alert first-aider should then check for an injury suffered during a fall or some other complication. If simple first aid does not produce a quick recovery, a doctor should be consulted at once.
