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

by Max Verworn

Such are the instances to which one has up to the present applied the “all or none law.” The question if, as a matter of fact, such a condition has ever been realized in any living substance has until now found no final answer. Most authors, who accept the validity of the “all or none law” for certain living substances, do so with a certain reserve and speak only of the possibility or probability of such behavior. The subject has, however, as will be shown later, a great and even vital interest in another direction. For this reason I should prefer to postpone the treatment of the same to a later occasion. Here I wish simply to say, that if the “all or none law” is valid in a strict sense for certain structures, then there exists no general constancy of the relations of the intensity of the stimulation and the amount of response, applicable to all living organisms.

We will now return from this digression concerning the relations between the intensity of the stimulus and the response, to the further characterization of the properties of the stimulus. Besides the quality, the direction and the intensity of every alteration in vital conditions, an equally important factor is the duration of the alteration. The time relations, under which a deviation of the external vital conditions takes place, present immense and manifold variations in nature. In many cases the change is very complicated, as for instance, the alteration of the static pressure or the temperature under the influence of air or water currents, the osmotic pressure or chemical factors in diffusion currents, and the light intensity produced by the movement of clouds. These very irregular alterations have practically little interest for us. Here we are concerned rather with the differentiation of the time alterations of the processes of the simplest fundamental types, which are of importance in studying the course of the reaction. For it is of such simple elements that the complicated and irregular alterations of the above-mentioned kinds are composed.

The simplest form of an individual change in the external vital conditions would be a regular and constant alteration of intensity which can be graphically represented as a straight line, wherein the intensities are the ordinates and the time the abscissa. (Figure [3], A.) A regularly rising pressure would, for instance, represent a stimulus in its simplest form. But such forms of stimuli are only very rare in nature and are also experimentally very difficult to produce. It is, for example, not easy to give the electrical stimulus, so much used for experimental purposes, this form. Fleichl and v. Kries have only accomplished this by means of complicated apparatus. The usual form of the individual stimulus is not a straight line, but a logarithmic curve. (Figure [3], B.) The alteration hardly ever progresses with equal rapidity from its beginning until it reaches its highest point, but as a rule, with decreasing rapidity. This is the usual course of alterations of concentration, also of chemical and osmotic stimuli, of changes of temperature and of electric stimulation.

Fig. 3.

The rapidity of alterations in vital conditions has quite an important influence on the development of the response to stimulation. It is well known that if a constant current, which reaches its highest intensity rapidly, is permitted to act upon a muscle, the effect differs from that following the application of a current of the same intensity but in which this is reached very slowly. In the first case there is a sudden strong twitch, in the second none at all. In spite of this there can be no doubt whatever of the current in the last case being effective. That the muscle is also excited when the current is slowly increased is shown by the contracture, which grows more and more plainly perceptible with the increasing intensity of the current and in higher intensities by the so-called Porret’s phenomenon, which consists in a curious wave-like movement of the muscle-substance. In reference to the rapidity of the alterations in the factors which act as stimuli, the behavior varies greatly. Many stimuli because of their nature never have a steep ascent or descent of intensity, as, for instance, alterations in the concentrations of soluble substances, that is, chemical or osmotic stimuli; likewise temperature variations may be mentioned. They always act relatively slowly. On the contrary there are forms of stimuli which have now a rapid, now a slow, ascent or descent of their intensity, such as the photic and mechanical stimuli. Finally, there are other stimuli that nearly always show a very abrupt change of intensity, such as the electrical form.

The most important factor to be considered in producing the response to variations of intensity, is not the absolute rapidity, but rather the relative rapidity; that is, the rapidity in relation to the characteristic rapidity of reaction of the particular living substance concerned. The rapidity of the reaction to stimuli is very different in various forms of living substance. On the one hand, we have forms reacting very quickly, as the nerve and the striated muscle; on the other, those which respond very slowly, such as a great number of unicellular organisms. Between these are a great number of living substances which, as far as the rapidity of the reaction is concerned, occupy intermediate positions of every varying degree. It is clear that the adequate stimuli for slowly reacting substances must be those having also a slow change of intensity; for quickly reacting, those having a rapid change of intensity.[33] If a nerve muscle preparation is simulated with the single induction shock, the “break” as well as the “make” shock has effect. But even here a difference is noticeable. The “make” shock has a weaker effect than the “break” shock. This difference is due to the difference of abruptness in its course, which when the current is made is less than that of opening, for, when the current is made, the ascent of the primary current is retarded by the extra current flowing in the opposite direction, whereas, when broken, with the fall of the intensity of the primary current, the extra current in the primary coil flows in the same direction. In consequence of this there is a perceptible difference in the rapidity of the alteration of the “make” and “break” shocks. (Figure [4.])

Fig. 4.

Course of induction shocks. 1 and 2 make and break of the primary current. 1₁ and 2₁ make and break induction shocks. (After Hermann.)