Physiological Foundations of Behavior

? Herrick (’24, Chap. II) points out. that automaticity, or spontaneity in the restricted sense of internally excited activity, seems to be an important factor in progressive evolution, especially in higher animals, leading up to trial-anderror reactions, active search for satisfying stimuli and the impulse toward selfsatisfaction and self-realization in the psychological field. resembles the decrement which we find in various forms of physical transmission, water waves, sound waves, etc. In general, the decrement in physiological transmission is more conspicuous in the more primitive forms of excitation, but we sometimes find that in the same protoplasm a marked decrement occurs in transmission of a weak excitation while in transmission of a strong excitation the decrement may be much less or is apparently absent.

In all cases where such a decrement is present an excitation-transmission gradient results, that is, the degree, intensity or energy of excitation is greatest at the point of origin and decreases from that to some more or less distant point where it becomes inappreciable or ineffective. That such gradients are of very general occurrence in the less highly specialized forms of excitation and transmission is well known. Among the lower invertebrates, for example, we frequently see that the muscular contraction or other effect of a local excitation gradually decreases with increasing distance from the point of original excitation and at a ccrtain distance ceases. As yet we have no exact knowledge concerning the slope or steepness of such excitation-transmission gradients and their relations to different physiological and other conditions; while we have no certain direct measure of the strength, degree or intensity of excitation, there can be no doubt that in the less highly specialized forms the length of the gradient, 7. e., the effective range of transmission, very generally varies with the degree of the original excitation, as indicated by certain of its effects, and with the conductivity of the protoplasm, as determined first by its specific constitution and second by its physiological condition at the time (see for example Verworn 13, Chap. VI). On the other hand, it is commonly believed by physiologists that the specialized conduction process in the nerves of higher animals occurs normally without a decrement, 7. e., that its range is indefinite. But under certain conditions, e. g., low temperature, partial anesthesia, a very distinct decrement is present, even in such nervous conduction (Verworn, 713, Chap. VI; Lucas and Adrian, ’17).

Moreover, even in rather primitive nervous systems such as the nerve net of certain ccelenterates slight excitations are transmitted with a decrement and are limited in range, but, as the experiments of Mayer (’06, ’08, 16) and Harvey (712) on the medusa Cassiopea show, a sufficiently intense excitation may be conducted in the nerve net for an indefinite distance. Briefly stated, the experiment demonstrating this fact consists in starting a wave of excitation in one direction around a ring of tissue cut from the

umbrella of the medusa. Conduction of the excitation wave may continue for days or weeks until the tissue dies. In such a case the impulse passes through many cells and since the cells are essentially alike and excitable the impulse continues indefinitely. From such facts as these it appears that the presence or absence of a decrement in certain nerve paths may depend on degree or intensity of excitation. Conduction without decrement in the nerve fiber is interpreted as resulting from the all-or-none type of reaction (Verworn, 713, Chap. VI; Lucas and Adrian, ’17). According to the current conception, the all-or-none reaction is a reaction which is maximal for all strengths of stimulus above the threshold. In other words, the weakest stimulus which is capable of bringing about excitation at all brings about just as great or intense excitation as any stimulus of greater strength. In this sort of reaction the degree or intensity

Fig. 145.—Diagram illustrating transmission with decrement. Explanation in text. of excitation is entirely independent of strength of stimulus, except in so far as a certain minimum strength is necessary to bring about excitation. In protoplasms which show this all-or-none form of reaction the external exciting factor merely initiates the excitatory changes, as the electric spark initiates the explosion, and the further changes are determined by the potential energy of the protoplasmic or the explosive system.

At present only Suggestions or surmises are possible by way of physiological interpretation of the differences between all-or-none and other types of reaction, but consideration of certain points is of some interest. In the first place, excitation of any point A (Fig. 145) of a living protoplasm supposedly always gives rise to an electrical gradient. Since resistance increases with distance, the strength of the electric current must decrease with increase in distance of the points between which it flows. The strength of current resulting from excitation of A decreases progressively through B, C, D, and it is evident that at a certain distance, X, from A, it will be so weak that it cannot bring about excitation of more distant points. The distance AX corresponds to the distances AB, CD, etc., in Figure 132,

i. e., it represents the range of physiological effectiveness of excitation of a given point A. If the excitatory change brought about at B, C, D and other points is proportional to, or varies with, the strength of the current between A and each such point, the result of excitation at A will be merely an excitation gradient extending from A to X. In Figure 145 this gradient is indicated in a purely diagrammatic way by the shaded portion of the figure, the slope being purely hypothetical. In such a case the range of transmission is nothing more than the range, AX, of physiological effectiveness of

_ the electric current orginating from the excitation at A. This may perhaps be regarded as the simplest and most primitive form of excitation gradient. Its length may vary widely according to the electric change at A and the resistance interposed between A and other points. In protoplasms which are but little specialized, e. 9., the very simple organisms and early stages of development, and perhaps in some cases with weak excitation, physiological transmission may perhaps be little or nothing more than such an excitation gradient determined directly by the electrical gradient resulting from the original excitation.

Between such primitive transmission and the all-or-none type of conduction without decrement intermediate transmission forms are at least theoretically possible and probably occur. If, for example, excitation of the point B (Fig. 145) by the current originating at A gives rise to a sufficiently strong electric current to be effective over ~ the range BY, the total range of transmission will be extended to Y. And if this current arising at B increases the excitation at C the current arising at C may extend the range of transmission to a still farther point Z. As the range of transmission extends farther, the decrement of course becomes less steep, until finally the all-or-none type of transmission may be attained. From this viewpoint the differences between the primitive excitation gradient and all-or-none conduction depend upon the differences in what happens at the various points B, C, ete., which are excited by the current. If the current brings about changes more or less proportional to its strength a decrement results, but if any strength of current above a minimum determines maximum excitation, the all-or-none type of conduction results. In this latter case, assuming that the electric current resulting from excitation at A, is effective at all points between A and X, all these points will develop a maximal excitation and all will give rise to electric currents of the same strength as those arising at A. That is, excitation of A will be followed by maximal excitation of

the region AX and this in turn by maximal excitation of a region of equal length from X onward and so on indefinitely (Fig. 144). Nevertheless, as noted elsewhere (Child, ’21 a), transmission may show a decrement even though it is of the all-or-none type. If the path of such transmission, e. g., the nerve fiber, is already a physiological gradient of such character that the maximal excitation decreases in strength in a certain direction, transmission in that direction will show a decrement and, if the path were long enough, might conceivably attain a limit. In short, the all-or-none type of reaction does not necessarily mean absence of decrement in transmission.

Certain chemical aspects of the specialization of excitation. — As regards the differences in the nature of the excitatory changes which underlie the differences in transmission, we have no positive knowledge. Protoplasms in general undoubtedly contain molecules of very different degrees of lability and from the facts at hand it appears probable that in the more primitive processes of excitation different molecules or atomic groups may be concerned in the same protoplasm, according to intensity of energy of stimulus and condition of the protoplasm. A weak excitation may perhaps involve oxidation of only certain molecules or groups and the velocity of this reaction and the electric changes will determine a characteristic rate of transmission and decrement. A stronger stimulus may determine the decomposition of other molecules in addition to those susceptible to the weaker stimulus and the rate of transmission and the decrement may be different in this case. In the maximal excitation all the molecules capable of reacting at all under the conditions are concerned and it is apparently demonstrated that, at least in some protoplasms, such excitation is transmitted without decrement, while submaximal excitations, or those below a certain level, are transmitted with decrement (see pp. 187-188). Frequently also we observe differences in rate of transmission with different degrees of excitation in the protoplasm. In the ctenophore plate row, for example, as in various other conducting paths, the stronger excitation is transmitted more rapidly than the weaker (Child, ’21a, p. 219). In terms of an electrochemical theory such differences in rate of transmission mean at least in part differences in the velocity of the chemical reactions concerned.

The specialization of excitable tissues, reaching its highest degrees in certain organs of special sense, jn nerve and in.muscle, appears in general to involve as an important factor a Specialization of the physico-chemical constitution of the irritable substratum of the tissue concerned. In the different sense organs such specialization is in different directions according to the form of energy to which reaction occurs most readily. In nerve and muscle specialization toward the all-or-none form of reaction occurs and in so far as chemical factors are concerned in such specialization, they must include an increase in uniformity of chemical constitution of the excitable substratum and increase in lability, such that any energy transfer above a minimum sets the whole mechanism in action as the spark does the powder. But of course we do not know whether this lability and apparent uniformity of constitution is actually chemical in nature.

It is an interesting fact that in low temperature or in partial anesthesia the nerve fiber, which normally shows the all-or-none reaction transmits with a rather steep decrement. Whether or not chemical factors are concerned in this change, it appears to consist in a return to the primitive type of transmission with a decrement, perhaps to the primitive excitation gradient determined directly by the strength of electric current. But as already noted even the normal nerve fiber may conceivably represent a gradient in lability or in other features of constitution determined in its development and it may react according to the all-or-none law and nevertheless show a decrement.

The possibility must also be noted that in the course of evolution different factors of the excitation process may play the chief parts in different protoplasms or tissues. In the more primitive protoplasms, for example, excitation appears to be rather closely associated with the fundamental metabolic reactions and to involve a relatively large amount of chemical reaction, at least in many cases. In striated muscle, on the other hand, the oxidative reactions appear to be in large part, if not wholly, an incident or result of the excitation process in the stricter sense. Forms of physiological excitation without chemical reaction are conceivable, though whether excitation actually occurs in living protoplasm without any chemical reaction may be doubted. The very close relations existing in protoplasm between physical conditions and chemical changes leave little room for doubt that the two sorts of change are very generally, if not always, associated.

We have seen that excitation appears to be fundamentally an acceleration in the rate of the changes which are concerned in living protoplasm with the liberation of energy. According to current theories it is a change ocurring in relation to protoplasmic membranes or limiting surfaces, but probably we shall not know exactly what constitutes excitation in protoplasm until we know what constitutes life. But whatever the nature of excitation, living protoplasm is able to transmit it to a greater or less degree. Transmission consists essentially of excitation of one point or region by the excitatory changes occurring in another and when such transmission follows definite morphological paths such as nerves, it is generally known as conduction. This possibility of excitation of one part or region of protoplasm by another is of fundamental significance in relation to the role which excitation plays in the life of organisms. Transmission makes possible the extension to the whole cell, cell mass or organism of the excitatory change initiated by the external factor at scme particular point. In Chapters IV and V it was pointed out that a relation of dominance and subordination is a characteristic feature of organismic pattern and that excitation and its transmission constitute the physiological basis of this relation and the primary factor in the integration of the individual organism out of a specific protoplasm, a cell or a cell mass. According to this conception, excitation and its transmission constitute the physiological basis, the starting point, of the organism as a whole. Excitation determines physiological differences in different regions or parts, 7. e., the more excited and the less excited, and transmission determines an actual physiological relation between them of such character that the more excited part brings about excitatory changes in other parts and so dominates them.

The presence or absence of decrement in transmission and the consequent limited or unlimited range of dominance are merely features of different protoplasmic constitutions, or perhaps in some cases, of different intensities or degrees of excitation and as such have _ been fully considered in another place (Child, ’21 a, Chaps. V, XII, XIII). In the more primitive protoplasms these factors determine how large a mass of protoplasm or cells of a particular constitution can be integrated into an organism, a whole. In the higher animals, however, the size of the individual is limited rather by the cessation of growth with the progress of differentiation, while the range of dominance may be indefinite, at least in certain nervous structures (p. 170).

Excitation and its transmission, then, make possible a change in the life of a mass of protoplasm, a cell or a cell mass as a whole in consequence of the local action of an external factor upon some part of it. Such a change is what we call physiological reaction or response and this in turn is the physiological basis of what we commonly call behavior, 7. e., excito-motor behavior in living organisms. Behavior in this sense is fundamentally an ordered and integrated change, reaction or response occurring in relation to local action of an external energy factor. No one except the vitalist doubts that behavior is physiologically a consequence of the irritability of protoplasm and that excitation of the irritable protoplasm and the transmission of such excitation are the primary factors in each such reaction or response. The ability to react or behave as a whole is a fundamental - physiological characteristic of the living organism and it is obvious that the material relations between the individual and the external world are affected by, and in large measure dependent upon, its behavior. In “The Origin and Development of the Nervous System” I have considered the various lines of evidence which indicate that the nervous system, the organ of behavior par excellence in all animals except the simplest, develops in the individual on the physiological basis of an excitation-transmission gradient or gradients. Moreover, its intimate relation to the primary factors in the integration of the organism is indicated by its extremely early differentiation preceding that of other definitive organs. The whole development of the nervous system is a development of mechanisms of excitation and transmission and of relations between them.

Of course the particular type of these mechanisms and of their relations in any particular organism is primarily a matter of the hereditary constitution and potentialities of the protoplasm constituting the organism, but in the development of the individual certain physiological conditions are necessary for the realization of the hereditary potentialities. As Lhave endeavored to show, these physiological conditions originate in the physiological gradients which are not independent of external factors, but represent the result of the differential action of such factors on the protoplasm. The facts force us to the conclusion that such physiological gradients are primarily excitation-transmission gradients. If this is true the physiological basis of the behavior of the individual organism and of the organism itself as a pattern and integration is the same. From this viewpoint the individual organism the “organism as a whole” is not as Morgan (’19, p. 241) puts it, “the collective action of the genes,” but rather the collective reaction of the genes to the differential action of environment. We may say then that pattern and development, as well as the function of the individual organism, repre-

sent the behavior of the germ plasm in a given environment. Or more briefly, the individual organism represents a behavior pattern in a specific protoplasm. It is doubtless evident to the reader that the whole course of the argument of preceding chapters has been concerned with the development of this purely physiological conception of the individual, but the following chapter is an attempt to make the viewpoint still more clear by a summing up, a synthesis of the data which underlie the conception.

In the preceding chapters attention has been called to various aspects of the relation between the individual organism and environment. That the individual as a pattern in a specific protoplasm has no significance except in relation to environment was pointed out in Chapter II. Chapter III was concerned chiefly with showing on the one hand the impossibility of accounting for the individual in terms of purely predeterministic conceptions and on the other the necessity for a clear distinction between the potentialities of the hereditary mechanism and the realization of certain of those potentialities in an individual. The following chapters (IV-IX) constitute essentially a consideration of the general physiology of organismic pattern, 7. e., of the individual or “organism as a whole,”’ as distinguished from the hereditary substratum out of which it develops. This consideration has led us to the conception of the physiological gradient as the basis, the starting point of the order or pattern and of the integrating mechanism of the individual organism. As I have endeavored to show, the conception of the physiological gradient has developed from, and is supported by, many different lines of experimental and observational evidence. Moreover, it is not in conflict with any data of investigation, but only with certain speculative predeterministic and vitalistic hypotheses.

It now remains to consider the relation between the more or less permanent physiological gradients and the phenomena of excitation and transmission and to determine whether the facts justify us in regarding the individual organism as a behavior pattern in a specific protoplasm. As we have seen, excitation in its more primitive forms is very generally, if not always, transmitted with a decrement in energy or intensity, or, since we do not know its nature, but only its effects, we may call it a decrement in physiological effectiveness. Consequently the more primitive phenomena of transmission in living protoplasms usually appear as excitation-transmission gradients with a certain

range and limit of effectiveness. This range of effectiveness may vary widely with the degree or intensity of the primary excitatory change and with the physiological state, that is, the irritability and conductivity of the protoplasm concerned. According to the electrochemical theories of excitation, such a decrement results primarily from the fact that electrical resistance increases with distance, and in the simplest forms of transmission the decrement may be little or nothing more than the electric decrement due to resistance (pp. 188-189). Only in the all-or-none type of reaction is transmission without a decrement possible, but even here the decrement is not necessarily absent (pp. 186-191). The electric gradient is of course present in the wave-like conduction of the nerve impulse, even though no decrement in height or steepness of the wave occurs. At each level of the electric gradient above the threshold maximal excitation occurs. But the excitation does not reach the maximum instantaneously. The facts at hand indicate that after a brief latent period the excitatory change begins as a minimal change and requires a certain length of time to attain the maximum. The advancing front of the excitation wave is then always an excitation gradient from maximum excitation at the summit to minimum excitation at the point where excitation is just beginning. This gradient of the wave front is then a gradient in degree of excitation and therefore analogous in a way to the gradient in degree of excitation characteristic of the more primitive forms of transmission, but its relation to the electrical gradient is not necessarily the same as that of the latter.

Moreover, the gradient of the wave front of the nerve impulse is not a stationary gradient but advances, and any given point occupies successively every level from the low to the high end and from the high to the low end of the descending gradient which follows the maximum. Even in the transmission of the nerve impulse, then a quantitative gradient appears in the wave-like form of the - impulse, or at least of the electrical changes associated with it. This gradient, however, is apparently not simply an expression of resistance, but may include other factors, such as velocity of reaction at any point of the nerve. In short, the nerve impulse is an advancing gradient followed by reversal while the primitive gradient is stationary and may or may not be reversible.

The chief point in this discussion is that a quantitative gradient in physiological condition, unquestionably involving both chemical and physical factors, is a very general, if not universal characteristic of _ the protoplasmic transmission of excitation, Even in the highly specialized nerve fiber of higher animals the electrical] gradient is present as in the primitive form of transmission and is undoubtedly associated with an excitation gradient. In the most primitive forms of trans- ‘mission the gradient is stationary and may persist and become a basis for differentiation. In the nerve fiber it is the advancing front of the wave-like impulse and undergoes rapid reversal. According toan electrochemical theory of excitation the existence of such gradients depends first upon the increase of electrical resistance with distance, and second, on the fact that the changes concerned in excitation at any given point are not instantaneous, but proceed with a certain velocity.

Moreover, even if the nerve fibers of higher animals conduct impulses without any decrement so that the changes involved are the same in velocity and amount at all levels of the fiber there is good reason to believe, as Lucas (’17) has pointed out, that certain regions of the reflex arc, such as the neuro-muscular junctions and probably the synapses between individual neurons, are regions in which a decrement occurs. If we accept the views which I have advanced elsewhere (Child ’21 a) concerning the relations between the origin and development of the nervous system and the general axial gradients of the body we must expect. to find quantitative differences as regards excitation in the neurons of different levels of an axis. In general, the neurons developing at the higher levels of an axial gradient may be expected to show a greater irritability, 7. e., a lower threshold, and probably also a higher excitation maximum than those of lower levels. According to this view the reflex arc as a whole originates physiologically in an excitation-transmission gradient, which in turn is an expression of a more or less permanent physiological gradient of the body (Child, ’21 a, Chaps. XII, XIII).

The significance of excitation-transmission gradients in the behavior of the original organism can scarcely be overemphasized. In the simpler organisms, both animals and plants, such gradients determine the degree and extent of integration following a particular excitation. In such forms the reaction following a slight excitation is more or less local and disappears at a certain distance from the point of primary excitation, while the reaction to a stronger excitation extends to greater distances and may involve the whole body. In hydra or the sea anemone a slight excitation of a single tentacle tip brings about contraction of only the distal region of the tentacle or of the whole tentacle, without affecting other tentacles or other regions of the body, while a stronger excitation is followed by contraction of a number or all of the tentacles and perhaps of the whole body. The behavior of

the simpler animals shows everywhere such differences in extent and complexity of reaction correlated with differences in degree of excitation. It is probable that excitation-transmission gradients play a part in determining the different behavior patterns exhibited in the motor reactions of such forms as the starfish (see pp. 65-66). Certainly the differences in behavior of the different arms must be correlated with differences in degree of excitation.

Even the behavior of higher animals indicates that in the nervous system as a whole somewhat similar relations must exist. For example, the summation in the reflex arc of impulses individually too weak to be effective suggests the existence somewhere in the arc of a region of decrement (pp. 195-197). Again a local excitation of a sense organ may at first give rise to a simple reflex action involving only certain nerve paths and certain muscles or other effectors. But with continuation of the excitation other nerve paths are affected, other effectors are involved and sooner or later the whole organism may be involved in the reaction. The familiar experiment of local excitation of the hind leg of a decapitated frog by mechanical, chemical or other means affords a good illustration of this point. The first reaction is the relatively simple reflex directed toward removal of the source of excitation, but if this is not effective and the excitation continues, other reflex ares become involved until finally a large part of the skeletal muscular system may take part in the reaction (Herrick, ’24, Chaps. XVII to XIX).

These few illustrations will perhaps suffice to indicate that excitation-transmission gradients are certainly of fundamental importance in neuromotor behavior. As regards the higher animals the situation is of course very complex, but even here it is difficult to go very far in a physiological interpretation of behavior without postulating the existence of such gradients in some form or in certain — parts of the nervous system. But we may go further than this and assert, I believe, without exaggeration that the excitation-transmission gradient is a factor of fundamental importance in the reactions of living protoplasms to their environment. Apparently it is the most primitive and most generalized form of reaction of protoplasm

and therefore the physiological basis of the integrative behavior of living things. The question whether or not excitation is completely reversible has been touched upon elsewhere (Child, ’21 a, pp. 82-85), but its importance for the present purpose makes necessary some further discussion at this time. We are accustomed to regard excitation as a temporary change followed by recovery, 7. e., by return to the condition preceding excitation. It is of course true that the organism, the muscle or the nerve persists apparently unchanged as regards its more conspicuous aspects after a period of excitation. There are, however, many facts which indicate clearly enough that even in the so-called irritable tissues, muscle and nerve, some of the changes concerned in or associated with excitation are not, under ordinary conditions of individual life, completely reversible. In muscle, for example, frequent excitation, if not carried to the point of exhaustion, brings about growth, 7. e., functional hypertrophy, and other changes in the muscle, while lack of excitation after a certain stage of development results in atrophy and disappearance of the muscular tissue. In the earlier developmental stages the intrinsic rate of metabolism is apparently high enough to maintain the muscular structure without external excitation, but as the tissue grows older and its intrinsic rate of metabolism decreases, the increase in rate associated with excitation is necessary for its continued existence and may bring about hypertrophy in it. The change in form, the contraction of the muscle which is an expression of the excitation, is of course completely reversible, but under ordinary conditions the process of excitation as a whole is certainly not rapidly completely reversible. But changes in condition may bring about more or less complete reversal of these effects of excitation in muscle which are commonly more or less persistent. The hypertrophy resulting from a period of activity may disappear during a following period of inactivity and after injury muscle cells may undergo more or less dedifferentation. In spite of such changes as these, it is true that under ordinary conditions certain of the changes associated with excitation are more or less persistent.