Physiology: The Science of the Body

by Ernest G. Martin

IN the second chapter we saw that to make our muscles act in accordance with the information brought in by the sense organs some means of communication between them is necessary; we saw, also, that this means consists of the nervous system. Now that we have learned something about both muscles and sense organs we are ready to look into the way in which communication between them is carried on. First of all, we must realize that living protoplasm does this. The nerve cells are alive and have their basic metabolism just as do all other living cells. They also have their functional metabolism; but in them, instead of taking the form of forcible motion, as in muscles, or of the manufacture of special materials, as in gland cells, it takes the form of the transmission of a disturbance from one part of the cell to another. An interesting and important fact about this transmission of disturbances is that the actual amount of functional metabolism required by it is very small. Only by the most careful measurements has it been shown that nerve cells that are functioning have a greater metabolism than those that are at rest. For a long time it was thought that a nerve cell acted very much like a telephone or a telegraph wire, transmitting some kind of a disturbance which was set up in it, but not having any active part itself in the process. We now know that the special activity of nerve cells is a form of functional metabolism, just as is the special activity of muscle cells or gland cells.

The nerve cells have to make communication between sense organs and muscles, and, as we have already seen, these are often quite a distance apart. It is necessary, therefore, that the nerve cells be long enough to reach over these distances. As a matter of fact, it is not necessary for single cells to have this great length, because it is possible for them to be arranged end to end, making a path of living protoplasm consisting not of one cell but of a chain of them. If we look at a nerve cell under the microscope we see that it is made up of a little central mass of protoplasm to which has been given the name of “cell body.” From this cell body extends a tiny thread of living protoplasm. This thread is called the axon. It is so very slender that it cannot be seen except under a powerful microscope, and yet in our own bodies and the bodies of all large animals many of these are three feet or more in length without a break. This tiny protoplasmic thread, the axon, was formed originally by growing out from the cell body. As it grew it became surrounded by a sheath, which probably gives it strength and decreases the danger of its being broken. Another thing which helps to keep the axons from being injured is that they are always in bundles. Instead of one of these very slender axons lying all by itself, it will be bound up with several hundred others; the arrangement is similar to that in a telephone cable, where a great many single wires are bound together in the large and very strong cable. The living protoplasm of the nerve cell has a gray color, so that wherever this shows we have what is commonly called gray matter. We saw a moment ago that every axon is inclosed in a sheath. Some of these sheaths are transparent, so that the gray color can be seen underneath, but most of them have a layer of white material, which makes them look white instead of gray. The bundles of axons corresponding to the telephone cables make up what we call the nerves. Nearly all nerves are white in color because of the white material in the sheaths.

The sense organs, as we have seen in Chapters VIII and IX, are some of them inside the body, others spread over the surface of the skin, and the rest in the special sense organs, like the eyes or the ears. A very complex organ, like the eye or the ear, has thousands of axons leading from it. In the case of the eye these are grouped into a large nerve leading away from it at the back, which is called the optic nerve. There is a similar large nerve leading from the ear. When any sense organ is acted upon, as when light falls in the eye or sound on the ear, it starts a disturbance in some or all of the axons leading away from it.

As we have said over and over, the purpose of the nervous system is to arouse the muscles to activity, and to guide them in that activity. We must ask next, then, how the nerves are distributed to the muscles. If we dissect the body of any animal or bird, we can find nerves passing to various parts. For example, a large nerve goes down each leg; this nerve subdivides here and there. Since we know that the nerve consists of a great many axons bundled together, we will realize that this subdivision is not a real branching, but simply a passing of some of the axons away from the main trunk along the smaller stem. Some of these smaller stems can be traced to endings in the skin; these contain the axons connecting with sense organs. Others lead directly into muscles. Some of these axons may also connect with sense organs, since, as we have already seen, every muscle has embedded in it the organs of muscle sense, but in addition any nerve that leads to a muscle contains a great many axons which pass directly to the muscle fibers. These are the axons by which the muscles are aroused to activity. It is a general rule of the nervous system that no nerve cell extends without a break from any sense organ to any muscle fiber. The axon which communicates with the sense organ belongs to one nerve cell; the axon which connects with the muscle fiber belongs to a different nerve cell. The first is called a sensory nerve cell, the second a motor nerve cell. Some idea of the appearance of these cells can be gotten from the figures on page 126.

It will be seen that the cell body of the sensory cell appears to be off on a little side branch. As a matter of fact, the branch is double, so that when a nervous disturbance passing along from the sense organ comes to the beginning of this branch it can pass up to the cell body and then out from the cell body along the second part of the branch, and so along the other part of the axon. This part of the axon is seen in the figure to have several branches; these are really branches of protoplasm and not separate axons coming off, as in the case of the nerve trunk. The use of these branches we shall see in a moment. Also at the tips of each branch there is a tiny feathering. We shall explain this presently. Let us look first at the figure of the motor nerve cell. This has a cell body and long axon, and, besides these, has a great many short protoplasmic branches sticking out in all directions from the cell body. Since a nervous disturbance to get from a sense organ to a muscle has to pass over a sensory nerve cell, and also over a motor nerve cell, evidently there will have to be some point at which it leaves the sensory cell and gets into the motor. This is accomplished by having the tiny feathering at the tip of the sensory cell interwoven with the fine processes projecting from the body of the motor cell. This arrangement we may call a nerve junction. In the whole body there are, of course, millions of these nerve junctions.