Physiology: The Science of the Body

by Ernest G. Martin

the muscles in the body, giving what is called a convulsion. There are some other poisons which may act similarly. Convulsions are not at all uncommon in young children. The way in which a convulsion is produced is by the spreading of the disturbance all over from a single sense organ. What we have, then, in the nervous system is an arrangement whereby under special conditions a nervous disturbance can pass from any sense organ to any or all of the muscles, but under ordinary conditions the disturbance spreads to a particular muscle or group of muscles only. It is evident that the nervous system would be of no use at all, if this latter arrangement did not exist. In order for our muscles to serve us, they must act in obedience to information brought in by the sense organs, and this can happen only when certain groups of muscles work in accordance with the information brought in by certain sense organs or groups of sense organs. We explain the behavior of the central nervous system by saying that there are preferred pathways through it, or, to put it in a slightly different way, when a nervous disturbance spreads over any nerve cell it extends equally over all parts of it, but does not pass with equal ease over all the nerve junctions. Some of the nerve junctions allow the disturbance to pass more readily than others, and it is this difference in the ease of passing the nerve junctions that determines which pathway the disturbance shall follow. We know no other means by which this picking out of particular paths from the huge number of possible paths could be accomplished.

What we are describing now is the simple foundation on which all our nervous activities rest. For that reason we are not taking up at this point the working of the brain, but only the direct connections between sense organs and muscles. In some very low animals the whole nervous system is made up of such simple connections.

An object O suddenly appears in front of the eye. Its image, formed on the retina R, a sense organ, starts impulses along the fibers of the sensory nerve cell s, which in turn stimulate the motor nerve cells m. These in turn stimulate the appropriate muscles of the eyelid, compelling a wink. (After Hough and Sedgwick, “The Human Mechanism.”)

A nervous activity which consists of the passage of a disturbance from a sense organ to a muscle is called a reflex. We have many examples in ourselves; if we inadvertently touch something hot, the hand is jerked away. Tickling the soles of the feet in one who is asleep will cause them to be drawn up; irritation in the throat causes us to cough, or in the nose to sneeze; the flashing of a bright light into the eye compels us to wink; all these are examples of the direct passage of nervous disturbances from sense organs to muscles. In every case the path is from the sense organ over the sensory nerve cell to some point in the central nervous system, then either by a direct nerve junction to a motor nerve cell or over one or more connecting nerve cells to a motor nerve cell and so to the appropriate muscles. These reflex actions follow the arousing of the sense organ with no more delay than is required for the passage of the disturbance over the nervous pathway. There is a delay of a small fraction of a second at every nerve junction, which makes some reflexes slower in their action than others. For example, there is a reflex known as the knee jerk; this is an outward kick which results from a sharp blow on the front of the leg just below the knee. The kick follows so closely after the blow that there cannot be more than one or at the most two nerve junctions in the pathway. The reflex of winking, on the other hand, takes several times longer, although the eye is much nearer the central nervous system than is the place on the leg which is struck in arousing the knee jerk. Since the actual length of nerve to be passed by the disturbance is much shorter in the winking than in the knee jerk, while the time for the reflex is a good deal longer, we conclude that the nerve pathway which is used in arousing winking contains a great many more nerve junctions, and therefore includes a great many more connecting cells than does the path for the knee jerk. In this particular case the reason why the pathway of winking contains so many connecting cells is that it is a brain pathway, while that for the knee jerk includes only the lower end of the spinal cord, where the arrangement of nerve cells is very much simpler.

It is important for us to get clearly in mind the working of the reflexes in order to be able to understand the more complex nervous actions which will be described in the next chapter. We need to remember that the ordinary way of starting nervous disturbances is from the sense organs. With a few exceptions, to be described later, whenever a nervous action occurs anywhere in our bodies it can be traced back, although often very indirectly, to the sense organ from which the disturbance originally came. The part played in this by the brain and by what we call our mental processes will be described in the next chapter.

Before going on to that topic we have a word to say about nervous fatigue. We mentioned the fact in Chapter VII that much of our actual feeling of fatigue is nervous rather than muscular. Not as much is known about nervous fatigue as about the fatigue that comes on when the muscles are overworked. One thing that seems pretty evident is that the place where the fatigue actually is located is in the very delicate nerve junctions. These junctions offer some resistance to the passing of nervous disturbances over them, and if they are compelled to submit too often to this passing they appear to offer still more resistance; in other words, to become fatigued. We must remember that the nerve junctions are exceedingly delicate things, consisting, as they do, of the interweaving of the almost inconceivably tiny featherings at the tips of sensory or connecting nerve cells with the equally tiny featherlike branches from the cell bodies of connecting or motor nerve cells. It is likely, also, that on account of their delicacy they are easily affected by the waste products that may be circulating about in the blood stream from the active muscles. In either case, the way to recover from nervous fatigue is simply by resting. It is not hard for the delicate nerve junctions to throw off fatigue if given a chance. The way to give them this chance is not to use them. As we shall see in the next chapter, mental processes are made up of nervous disturbances passing here and there in the brain. If we allow ourselves to be occupied too continuously with the same lines of thought, we are evidently sending nervous disturbances over the same nerve junctions over and over again. In order to give those nerve junctions a chance to rest, what we have to do is to think about something entirely different. The word that best expresses what we have in mind is “diversion.” In the strict sense diversion means a turning aside from what we have been doing to something different, and that is the best way to allow the brain to rest. The man who takes his business home with him, and dwells on it during the hours that are supposed to be set aside for rest, may be able to achieve more for the moment than if he were really to rest, although even that is doubtful; but in the long run there is no doubt that continuous efficiency depends on allowing the fatigued nerve junctions ample opportunity to recover, which means that the thoughts must be directed into entirely different channels.