The Fundamental Laws of Human Behavior
FOURTH LECTURE
Max Meyer
Table of Contents | Next | Previous
Reflex arches. Their peripheral and central points. Central sensory points and central motor points. Central points of a lower and higher order. Lettering of diagrams explained. Central sensory and central motor neurons. Reflex and instinct. Instinct a selecting and collecting agent. Overflow of a strong sensori-motor discharge into the most closely connected arches. The source of motor power different from the signal for its expenditure. What a nervous excitation can be likened to. Signaling by rods and levers or by pneumatic tubes. Velocity of signal very great, but not infinite; not dependent on intensity. A neuron likened to an electric storage element. A simple picture of a nervous process needed for our imagination of nervous function.
THE design of Figure 16 allows infinitely many modifications and elaborations without deviating from the general principle upon which it is based. This principle may be stated in the following words. Each sensory and each motor point of the body contains one of the ends of a neuron which, accordingly, may be called either a sensory neuron or a motor neuron. The other ends of each pair of a sensory and a motor neuron are connected by what may be called a connecting neuron. The whole, represented in the figure in the shape of an arch, for example, Sa S1a M1a Ma, may be called a reflex arch. Each reflex arch is composed of at least one sensory, one motor, and one connecting neuron.
(47)
The number of neurons, however, forming a reflex arch, need not be limited. Any sine in our diagram, the sensory neuron Sa S1a, for example, might actually be a chain composed of several neurons. But for our present purpose of constructing a simple diagram typical of the design of the nervous system, this possibility need not be dwelt upon. Further, several sensory neurons may run together into a common meeting point, as Sd S1d, S’d S1d, and S’‘d S1d in Figure 17. And the same may be true for several motor neurons, as M1b Mb and M1b M’b in the same figure. Nevertheless we may say, since to generalize is our present business, that each reflex arch is composed of a sensory, a connecting, and a motor neuron. We must say a word, however, as to why we have called these arches reflex arches. The term reflex has been a physiological term for centuries. The early physiologists used it to signify the great quickness and definiteness with which certain actions occur compared with the slowness and variability of others. To take an example from human life, if we bring the tip of the finger suddenly close to the eye of a stranger sitting opposite us in the street car, he surely and most quickly shuts his eye. Such an action seems to deserve the name of reflex action. It occurs, or seems to occur,— as promptly and definitely as our own features are reflected back towards us when we look at ourselves in a mirror. But if we ask the stranger to lend us a dollar, he puts his hand in his pocket—if he does it at all—only after a good deal of hesitation, of delay. This does not seem to deserve the name of reflex action. Modern physiology, however, has gradually put less and less emphasis on the quickness of the response in the case of so-called reflexes, and more on its definiteness. For example, in such reflexes as coughing, or vomiting, or intestinal action there is not necessarily any great
( 48) quickness, but a great definiteness of response. Now, we may most naturally regard the definiteness as being determined by the fact that of the innumerable motor outlets of the excitation ;just one is superior to all others in being reached over the shortest path, offering the least resistance. Such a path may be represented by one of the arches of our diagram, since each arch assigns to each sensory point just one motor outlet of specially low resistance. We may, then, call these arches reflex arches, but we must remember that by giving them this name nothing is proved or disproved, nothing is made clearer than it would be otherwise,—we have merely referred to a traditional physiological term when occasion seemed to offer.
Each reflex arch contains four points. We may give them different names by calling them peripheral points (Sa and Ma, for example) and central points (S1a and M1a for example). These terms "peripheral" and "central" must not, however, be understood literally. The peripheral points are not always located on the anatomical periphery of the body, the skin;— some sensory points are located in the skin and some in the inner parts of the body, whereas the motor points, in muscle fibers, are necessarily, without exception beneath the skin. Neither are the central points always located in a central part of the body. The words peripheral and central are used in a loose sense, as a telephone station in a city is called central, although it may be located far from the centre of the city. We may then speak, not only of a peripheral sensory point and a peripheral motor point, but also of a central sensory point (S1a) and a central motor point (M1a). This does not signify that these points are in a strict sense sensory and motor, but merely that one of them (S1a) is nearer to a sensory than to a motor point of the body, and the other (M1a) is nearer to a motor than to a sensory
( 49) point of the body. It is easily seen, then, that in the diagrams of our figures several central sensory points are collected, so to speak, by neurons (central sensory neurons) into a central sensory point of a higher order. For example, S1a and S1b are collected into S2ab. This is done in a manner not different from that in which the peripheral sensory points Sb and S’b are collected into S1b. The central motor points are also collected (by central motor neurons) into central motor points of a higher order, for example, M1a and M1b into M2ab. The same principle of design is then applied again. From the central sensory point to the central motor point of the second order there are several paths — a direct one (S2ab M2ab) and more indirect ones, for example S2ab S3abc M3abc M2ab. We find there the same manner of connection which we found for S1a and M1a, which too had a direct and a less direct path of communication. The whole design, complicated as it looks at first, is really built upon a very simple principle, applied consecutively in regular order, —the principle, that from a sensory to its corresponding motor point there are always direct and less direct paths of communication, and that the less direct paths serve the purpose of connecting each sensory-motor system of conductors with other systems of a similar kind by the aid of a common path, always represented graphically by a horizontal line.
The letters and digits added below or above each S or M very simply indicate the peripheral points with which the point in question is most directly connected (for example, S3abc with Sb, S’b, Sc, Sd, S’d, and S’‘d) and the total number of unit conductors making up any such connection (for example, three neurons for S3bcd, two for S2ab). Where the number of units differs for different peripheral points, the digit is added, not to S or M
( 50) but to a, b, c, etc., as the case demands. For example, in the diagram of Figure 17, Sa2b3c3 indicates that the distance from this point to Sa, is one of only two unit lengths, but to Sb of three lengths, and to Sc also of three lengths.
We spoke of the physiological term reflex. We may use this opportunity to refer now to another traditional terns, that of instinct. When one observes that a bird, without having been taught by experience or by other birds, builds a nest before laying any eggs, one usually says that the bird has done this instinctively. Here again there is a certain promptness and definiteness of action as in the case of a reflex action, but promptness and. definiteness of a different kind. Let us try to make clear of what kind. The bird does not many times lay eggs which are doomed to perish because there is no place to receive them, until some time it happens to prepare a nest before the act of laying. But before the first egg is laid, the nest is prepared in quite a definite way, well suited for the hatching of the young. Of course, there are certain physiological processes in the bird's body which, previous to laying, cause excitations in definite sensory neurons, and these excitations are conducted to the motor neurons controlling the act of building. Why are they conducted just here and not to any other motor neurons? Obviously, because of short connections, of relatively small resistance, between these motor and these sensory neurons. But the figure of speech of a "mirror-like reflection" is not applicable. We cannot comprehend an instinctive activity by simply referring to a reflex arch, it is too complex and variable for that. It is definite only in the sense that it occurs at a time when innumerable stimuli acting on innumerable reflex arches make us regard almost any other kind of muscular activity as just as probable as the one which actually occurs, some of them stimuli
( 51) without the presence of which the act of building would not have occurred, some, however, quite irrelevant to the act of building. For example, a, suitable site for the nest and suitable building material must impress themselves upon the eyes. But even this site and this material might have called out innumerable other responses just as well. The specific excitation of the instinct, caused by the internal physiological stimulus aforementioned, therefore cannot be the all-sufficient cause of the action, but must be rather of the nature of a selecting and collecting agent, weakening and rejecting those excitations whose .presence is unnecessary, or whose reflex responses would interfere with the instinctive action, strengthening and uniting all those excitations whose presence is necessary for the performance of the instinctive action. Let us now try to comprehend the effect of the specific excitation of the instinct by the application of the diagram of Figure 17.
Let Sd S1d represent all those sensory neurons any excitation of which is likely to interfere, if the corresponding reflex responses are allowed, with the performance of the instinctive activity. To illustrate this by a concrete example, think of flying. It is clear that any long continued flight would make nest building an impossibility, even though short flights are with many birds an essential part of the building activity. Here a selection is needed, excluding, on the whole, such motor responses as flight. Let, further, Sb S1b and Sc S1c represent all those sensory neurons whose excitation is necessary for the instinctive activity. For example, the bird's eyes must see the site and the material. Here a collection is needed, including in the activity the motor responses upon such things as a building site and building material. Let Sa S1a represent the sensory neuron whose excitation is what we
( 52) called the specific excitation of the instinct, which selects and collects among the nervous processes coming from the other sensory points. How, then, can we comprehend the influence of the excitation coming from Sa upon the motor activities at the points Mb, Mc, and Md? Obviously, the excitation coming from Sd must be prevented from reaching the corresponding motor point Md. The excitations coming from Sb and from Sc, however, must not be prevented from reaching their corresponding motor points Mb and Mc, but their effectiveness must be enhanced as much as possible:—the bird must not do any other things, but pick up building material and drop it at the site of the nest in the proper manner leading to the formation of a nest.
It seems clear that this acting as a selecting and collecting agent as we said above, can be best understood by assuming that the excitation coming from Sa, being so strong that the direct discharge over S1a and M1a into Ma overflows the channel of its reflex arch, partly travels upwards from the point S1a. Because of the connections naturally existing it can reach equally well both M1b and M1c. over shorter paths than it could reach any other points (e. g., M1d) leading towards motor organs. Thus it enhances the muscular contractions at both the motor points Mb and Mc. The connections of the generalized design of Figure 16, however, do not fulfil this condition. They would favor Mb over Mc. The connections representing the instinct must be like those of Figure 17, which is derived from Figure 16, in the main by simply omitting certain conductors, as a comparison of the two figures immediately reveals. If we travel in Figure 17 from Sa to any motor point other than Ma over the shortest possible route, we can travel only over S1a, Sa2 b3 c3 and Ma2 b3 c3 to either Mb or M’b or Mc In this case the entire
( 53) path from the sensory to the motor points has a total length of six units. Had we traveled from Sa2 b3 c3 up to Sa3 b4 c4 d4 and Ma3 b4 c4 d4 and thence to any other motor point, for example, to M’d, the path would have had a total length of at least eight units. The control by the strong and overflowing excitation from Sa of the function of the motor points Mb and Mc, the collecting agency of the instinct, is thus graphically represented by the relative shortness of the connecting conductors. On the other hand, the inhibiting influence of the excitation coming from Sa upon the function of the reflex arch SdMd is graphically represented by the bare existence of a connection by means of the connecting neuron Sa3 b4 c4 d4 Ma3 b4 c4 d4. This connection must make it possible for the nervous process coming from Sa to capture, as it were, the nervous process coming from Sd, thus preventing it from reaching Md. But we are still far from understanding this inhibiting, selecting influence. The diagram of connection by conductors in Figure 17 is only one of the assumptions necessary for the explanation of instinctive activity. We shall return to this problem in the following lecture. It is necessary that we investigate first what other assumptions we have to make to understand theoretically what nervous activity means.
We have already compared the nervous system with a signal system, a telephone system by means of which messages may be sent. We must understand more clearly what kind of a signal system it is. It is important, however, that we keep clearly before our mind that it is only a system for signaling, not a system for the transmission of power. It can not be compared, for example, with the electric light and power circuit of a city, furnishing to many houses in many streets power which is generated in a central station. An animal's muscular power does not take
( 54) its source in the nervous conductors attached to the muscles. The power is derived from the digested food through mediation of the blood circulation. The power is thus stored in the muscles, ready to be expended at the proper signal. The nervous excitation is the signal, the message that the power should be expended. To be sure, the amount of power expended is not altogether independent of the force of the signal, of the intensity of the message received. But that the power is not derived from the messenger is clear from the fact that when a muscle is exhausted, no nervous excitation can make it contract.
What, then, is the excitation which is propagated through a nervous conductor in order to serve as a signal to a muscle fiber. This, however, is not really a good question. What it is, in other words, what name is to be applied to it, is directly of little significance. What it can be likened to, is the question which we should rather ask. What thing with which we are familiar in the ordinary functions of life, acts like a nervous conductor through which an excitation is taking its path?
Fifty years ago, when a master desired to call his servant, he rang the bell in the servant's room by means of a rigid wire connection extending over pivoted angles from his own to the other room. Now, let nobody think that this comparison is absurd from the start. Everybody knows, of course, that there are no wires and pivots in any nervous system. But we have already emphasized that it is of minor importance what there is, that we are concerned rather with what goes on there. Does the signaling going on in the nervous system permit comparison with the signaling done by pulling a bell cord? Now, it does not. If for no other reason, for this, that the bell attached to a perfectly rigid wire or rod begins to ring the very moment the other end is pulled. But when an excitation is
( 55) caused at one end of a nervous conductor, it is not at the same moment also at the other end, but measurably later, the later, the greater the distance between the two points. We must look, therefore, for a different comparison.
Everybody knows the grand musical instrument in which the player, pressing down the keys, opens at considerable distances from the keyboard the valves which make the various sources of sound speak. In the pipe organ too the messages were sent until comparatively recent times from the keys to the valves by means of rigid connections, as from the master's room to the bell in the servant's room. No delay is permissible in the response of the pipes to the touch of the fingers on the keys. The rigid connection, therefore, seemed to be the only possible one. Nevertheless, the modern organ has been freed of all rigid connections. A narrow tube runs from the key to the pipe valve. The motion of the key opens a tiny auxiliary valve which admits compressed air from the main reservoir to the tube just mentioned. At the other end of the tube is a tiny bellows which is raised by the compressed air and thus operates the pipe valve. Now, what use can we make of our knowledge of this familiar mechanism? We shall see at once that we can thus elucidate certain fundamental facts of the function of the nervous system. The pneumatic mechanism does not operate the pipe valve at the very moment when the key is moved by the finger, but at a measurably later time,—the later,' the longer the connecting tube. However, the time interval is short enough to be negligible in musical practise, provided the connecting tube is not extraordinarily long. The time interval is practically independent of the density of the compressed air in the reservoir. It is simply proportional to the length of the
( 56) tube, provided the tube is plain and does not contain in its course any additional mechanisms. The corresponding facts are found in the function of the nervous system. The muscle fiber does not contract at the very moment when the sensory point is excited, but some time later,— the later, the longer the nervous path connecting the sensory and the motor point. Yet, the time interval is, in the case of a reflex, quite negligible according to our standards of time in ordinary life,—so much so, that for centuries, until modern methods of measuring exceedingly short time intervals were invented, the time was regarded as absolutely zero, that is, the response was indeed believed to occur at the very moment when the sensory point was excited. It has further been shown by experiment, that the contracting of the muscle fiber does not occur any sooner if the excitation of the sensory point is made stronger.
With all this, however, we do not want to suggest that a neuron is a narrow tube through which a fluid is pressed. We have emphasized before that we are merely searching for familiar functions with which we may compare the nervous functions in order to assist our power of imagination and reflection. Let us use this opportunity to tell briefly what physical processes have actually been found by the neurologists to go on in the nervous conductors. Whenever anything of the nature of an excitation occurs in a neuron, an electrical phenomenon is observed. But it is generally admitted that this electrical phenomenon is not the excitation itself. There is no such thing as an electrical insulation surrounding a neuron, which would enable an electrical current to pass along a neuron. And further, the velocity with which the excitation is conducted is almost infinitely small when compared with the velocity of electricity in its conductor.
( 57) During the time a nervous excitation is conducted one way and back through an elephant or other large animal, electricity can circle the globe. The electrical phenomenon must be, therefore, a purely accidental accompaniment of the conduction of an excitation. It is highly probable that the conduction of the excitation is a process of a chemical nature. The substance of a neuron, consisting of highly unstable organic compounds, must be well adapted to the conduction of chemical changes. It is also well known that the conduction of chemical changes frequently involves, as by-products, so to speak, electrical phenomena. Indeed these electrical phenomena accompanying the conduction of chemical changes have been used technically and have become of the greatest industrial importance in the so-called accumulators or electrical storage batteries. An accumulator is essentially a conducting fluid on the sides of which there are two related, yet different chemical substances, most commonly lead compounds. One of these substances has a tendency to take up a certain more elementary substance; the other has a tendency to give off this same elementary substance. The same elementary substance is one of the components of the conducting fluid. What happens is this : A stream of elementary substance flows—or, whatever it may actually do, is imagined to flow — from one end of the conductor to the other, and this flow, the wandering of molecules or ions, as it is usually called, is accompanied by an electrical phenomenon. We are, then, probably justified in regarding the conduction of an excitation through a neuron as, not identical with, but at least analogous to the wandering of ions through the conducting fluid—the electrolyte, to use the technical term—of a storage battery.
Concerning the chemical and physical properties of the neurons hardly anything further is known which
( 58) could make the function of these wonderful structures clearer. It is not especially remarkable that the chemistry of the neuron, although it has attracted in recent years the attention of investigators, has made little progress. We need only remember that the neuron is a microscopic structure, and that for a chemical analysis a more than microscopic quantity of the substance to be analyzed is required, and we understand why no one yet knows how the excitation actually wanders from one end of a neuron to the other end and thence to the other neurons. For the very reason that the chemistry of the neurons is a thing of the future, we may picture to our own imagination the processes going on in the neurons in terms not necessarily chemical, in any kind of terms with which we are familiar and which enable us to understand the function of the nervous system as being a complex of a few —as few as possible —simple functions. We have pictured it as a process like the one going on in a connecting tube of a pneumatic organ. Let us now draw further conclusions from this assumption.
