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?

FRACTURED BONES

Problem. How does the body heal fracitured bones ? How well do broken bones heal themselves?

Background. Did you know that your bones are as alive as your skin or any other tissue in your body? Most people think that the bones in the skeleton are made of dead material. A close look would reveal some different information, however. Your bones grow rapidly during your first sixteen to eighteen years. Then they harden to form your adult skeleton. The marrow within some bones forms essential parts of your blood. Broken bones heal so completely that it is difficult to find the healed fracture. This evidence certainly points to the conclusion that bones are alive, in spite of their hardness.

Late in the seventeenth century an English physician, Dr. Clopton Havers, discovered that there are thousands of tiny canals lacing the bones. These canals, called Haversian canals after their discoverer, carry the blood vessels and nerves to the interior of the bones.

Around each canal is a group of cells called osteocytes. These cells form much of the living matter in the bones. One kind of osteocyte, an osteoblast, deposits the minerals that make up bones. This hard mineral content forms about 45 per cent of the weight of the skeleton. The other kind of osteocyte, the osteoclast, dissolves the hard calcium and other minerals in the bones. This dissolving

Explanation. When a bone breaks, whether it has split, broken into many pieces, or only cracked, there is apt to be damage to the blood vessels and nerves in the bone as well as to the bony substance itself. The repair of the break necessitates a repair of these vessels and nerves as well as a new deposit of calcium mixture at the site of the break.

As soon as the bone fragments are set so that they are close to each other, the osteocytes in the area begin to repair the break. The blood vessels grow back and help to carry away the debris dissolved by the osteoclasts. The osteoblasts deposit new calcium between the bones to cement them together. The osteoclasts dissolve away the rough edges and "polish up" the job. Finally, the break is so completely healed that it is not easily distinguished from the original bone.