Irritability: A Physiological Analysis of the General Effect of Stimuli in Living Substance

by Max Verworn

Fig. 7.

Orbitolites complanatus. A—Before stimulation. B—Under influence of a constant current.

It is sometimes the case that under the influence of a stimulus a new equilibrium is developed, which may remain as long as the stimulus persists. This most frequently occurs as a result of weak stimuli. That which is usually termed “individual adaptation” belongs in this category. Likewise some of the natural and artificial immunizations may also be included. The continued stimulation in such cases of adaptation as we learned before in the example of Amœba limax and radiosa or Branchipus stagnalis and Artemia salina becomes a vital condition for the living substance in its new state.

The other result, namely, that of death ensuing sooner or later, is most frequently produced by stronger stimulation. Through the effect of the prolonged stimulation, the change in the living system is so great that all harmonious interaction of the various processes of life become after a time impossible. The disturbance of this equilibrium after a longer or shorter time becomes so great that life ceases. By far the greater number of all diseases furnish examples of this kind. Disease is nothing else but reaction to stimulation. Should a constant stimulus persist and if the development of a new equilibrium of this system is not established, the result is premature death.

In most cases, as, for instance, the nerve impulses which move toward an organ, or better still the electrical stimuli as used for experimental purposes, it is not a question of a permanent but of a temporary alteration in the external vital conditions. The stimulus starts, then ceases after a longer or shorter period. In this way there is added to the deviation at the start also the alteration at its termination. The latter takes place with different degrees of rapidity, in a manner analogous to that of the initial alteration, and can bring about response. With this the curve of the duration of the course of the stimulus becomes somewhat more complicated and in consequence a like effect is observed in the response. The “making,” duration and “breaking” of the constant current furnishes the example of this type. The “making” of the current being a quick alteration calls forth a strong and sudden excitation (in the muscle contraction); the continuation of the current maintains weak excitation of equal intensity (in the muscle a continued contraction) and the “breaking,” being a sudden alteration, is followed again by a stronger excitation (in the muscle a contraction). The duration of the change can, however, be so short that its intensity does not remain at two periods of time at the same height, but instead the ascent of the intensity is immediately followed by its descent to zero. Induction shocks of short duration, the duration of which have been observed more in detail especially by Grützner,[39] offer typical examples. Here a single effect of the stimulus results from the rise and fall of the intensity curve. Hence the induction shocks as momentary stimuli are universally used for experimental purposes.

In contrast to the single stimuli, which find their ideal in induction shocks, another form of stimulation should receive our attention, namely, the series of stimuli which produce a rhythmical alteration of vital conditions. These show among their complex combination of simultaneous and successive actions of their single stimuli relatively the simplest and most easily understood regularity in their effects. They are of particular interest, because they develop in the normal physiological happenings of the animal body in the form of rhythmical intermittent impulses of the nervous system.

Here again it is self-evident that with regard to the course of response, we must first consider the character of the single stimulus of the series, and this must be done from all those standpoints already here discussed. However, a new factor is met with here, that is, the frequency of the single stimuli of the series, or that which has the same meaning, the duration of the intervals between them. This is a feature upon which the result of stimulation depends in a very high degree. But here, too, however, it is not a case of the absolute frequency of the single stimulus, but simply of the relative frequency in regard to the rapidity of reaction of the particular living system. I should like to remark here that it is of greatest importance whether the interval between the two single stimuli of the series is sufficiently long or not to allow the living system time to completely recover from the effect of the preceding stimulus. In the cases, for instance, where we have recovery, we have the same rhythm of stimulation as that of response. When recovery does not occur, interferences of the response are developed, which are of great physiological importance, with the analysis of which we shall later on find occasion to occupy ourselves in detail. The physiological example for these stimuli is the rhythmical discharge of impulses of the nerve centers; the physical method, which is most widely used for experiments, is the faradic current.

It is apparent that the question of frequency must again be combined with all those factors previously discussed in connection with the single stimulus. In consequence another complication arises and with this another point must be taken into consideration, namely, the fact that the duration of the single stimulus in a series undergoes alteration by increasing frequency beyond a certain limit. Beyond this limit the duration of the single stimulus must become less and less. As the result of the fact that stimulation is, as we have seen, dependent on the duration of stimulus, it is evident that, depending upon the rapidity of response of the living system, sooner or later the rhythmical stimulation must become ineffectual. Nevertheless, this effect of shortening the duration of the single stimulus can be compensated by a corresponding increase of its intensity. In this connection Nernst[40] showed a very simple relation for induction currents of higher frequency of interruption, which furnishes a law according to which such a compensation takes place. In conjunction with Barratt he found, namely, that the intensity must increase proportionately to the square root of the number of single stimuli if the threshold value of the stimulus is to be maintained, that is, I : √m = const., in which I is the intensity of the current and m the frequency of interruptions. The limits of the validity of this law cannot at present be conclusively established.

This exhausts the small number of elementary factors concerned in the course of the stimulation, and which are of importance in considering its effect. The combination of the different varieties of these single factors, that is, the nature, the direction, the intensity, the rapidity, the duration and number of alterations in the external vital conditions of the organism produce the enormous variety of effects of stimulation which we observe in the living world.