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

by Max Verworn

Now slowly reacting forms of living substance, such as certain foraminifera, in which the extended pseudopods are stimulated with single induction shocks, the break as well as the make shocks are wholly without effect, as both take place far too quickly for the slow responsivity of these organisms. I have made such observations on various forms of foraminifera of the Red Sea, on Orbitolites, Amphistegina and others. The movement of granules in the pseudopods is not influenced by the induction shocks in the least. It also continues without interruption when the pseudopods are extended. Even with the strongest induction shocks at my disposal I could not induce them to contract; the faradic current, also, the intensity of which I found quite unbearable, remained utterly without effect.[34] These two extreme cases, the nerve and the foraminifera, show plainly that the effect of a stimulus is not produced by the absolute rapidity of the increase of intensity, but is solely influenced by the relative rapidity of the same.

Fig. 5.

A further point for consideration in the duration of an alteration in a vital condition in producing a stimulant action is the length of time the stimulus remains after reaching its highest point. In the forms of stimuli occurring in nature the duration of the alteration after reaching its highest level can vary considerably. The stimulus may remain indefinitely at a certain level, when this is once reached. (Figure [5], A.) The alteration likewise persists. This would be the case, for instance, with the changes of concentration in the transfer of an organism from fresh into sea water. The alteration can also, however, immediately after attaining its highest level, return, so that the original state is at once reestablished. (Figure [5], B and C.) Here it is a case of a quick deviation in the external vital conditions. A sudden jar would be a case in point. Between these two extremes we have all variations in the duration of all natural and experimental forms of single stimuli.

Now we arrive at the question: Has a prolonged stimulation really a prolonged effect? This question might seem superfluous, as from a conditional standpoint it is self-evident that every alteration in any one of the conditions of a system is followed by an alteration in the system. But this very question played an important rôle in older physiology and led to prolonged discussions for the reason that a special case was taken into consideration in this connection, which at that time was not clearly understood. Du Bois-Reymond,[35] as a result of his investigations on the nerve muscle preparation of the frog, formulated a law of nerve excitation, according to which it is not the absolute value of the intensity of the constant current which produces an excitation of the nerve and contraction of its muscle, but an alteration of the intensity from one moment to another. The more rapidly these changes are produced, the greater is the excitation. His arguments were based upon the fact that a contraction can only take place on the “making” or “breaking,” or by rapidly strengthening or weakening the constant current; it is possible to subject a nerve muscle preparation to a current of considerable strength without a muscle contraction resulting, provided it is slowly increased. One might be disposed to conclude from this that the constant current, when showing no fluctuations, has no stimulating effect whatsoever. Should this observation be carried even further and the attempt made to extend it into a general law of excitation by assuming that the effects of stimulation are only produced by variations in the intensity, not by its continued duration, one would commit the error of judging the occurrence of a stimulus only by the unsatisfactory criterion of an abrupt muscle contraction. Today we know with positiveness that a continued effect also exists during the uninterrupted flowing of a constant current in nerve or muscle, though much weaker, however, than in the case of the excitations produced by sudden fluctuations of the intensity. This is shown in the nerve by an altered excitability, which continues at the poles during the whole duration of the current. In the region of the anode the excitability is diminished, in that of the cathode it is increased. An excitation can also be demonstrated which extends from the cathode through the nerve, which can easily be detected by sufficiently delicate methods. Among other effects of prolonged stimulation is that of cathodal contracture, which remains localized in the region of the cathode and which excitation persists as long as the current continues. This permanent excitation can be particularly well observed in the single cells of the rhizopods. If a constant current is allowed to flow through an Actinosphærium,[36] the straight, smooth, ray-shaped pseudopods of the cell body at the moment of “making,” show evidence of contraction by being drawn in, particularly those directed towards the anodic and in less degree also those towards the cathodic pole. This excitation, greatest at the time of “making” of the current, though diminishing rapidly in intensity during its continuance, remains, however, to a less degree, and leads to a progressive disintegration of the protoplasm on the side towards the anode, which lasts until the current is again broken. (Figure [6].) Thus even though there can be no doubt, on the one hand, that the effect of stimulation, which appears at the moment of the entrance, is to produce alterations, which develop very rapidly, and that by a continuation of this state there is a more or less rapid fall to a low level; on the other hand, it is just as certain that the alterations in the living system persist throughout the duration of the changed external conditions, or to put it more concisely: the effect of the stimulus never wholly disappears unless the changes in the external vital conditions return to their original state.

Fig. 6.

Actinosphaerium eichhornii. Four stages showing the progressive influence of a constant current. Protoplasmic disintegration at the side toward the anode.

But more, an effect of the stimulus cannot indeed take place without a certain duration of stimulation, which is related in its turn to the rapidity of reaction of particular living system. This can be much more readily observed in more slowly reacting substances. Fick[37] first proved this fact on the muscle of the Anodonta. I have also been able to demonstrate the same fact in the slowly reacting sea rhizopods[38] by the use of the constant current. When Orbitolites is stimulated with a constant current lasting approximately the tenth of a second, no response is seen in its extended pseudopods, which are directed towards the poles. The same is the case if the induction current is employed. Only when the constant current of the uniform strength lasts approximately .05 seconds, a barely perceptible response occurs, manifested by the sudden stoppage of the centrifugal flowing of granules in the anodic pseudopods, which, however, after the lapse of one to three seconds continues again unaltered. Should the duration of the constant current be still further prolonged, typical symptoms of contraction are seen being manifested by a heaping up of the protoplasm in the pseudopods in the form of spindles and balls, whilst the protoplasm flows in a centripetal direction towards the central cell body. (Figure [7].)

Two effects can be realized by the alteration in the living system as the result of prolonged stimulation. Either a new state of equilibrium is established by the prolonged action, or sooner or later death develops. In considering both results, however, we will ignore for the present the fact that every living system in the absence of such prolonged stimulation is always in a state of change, i.e., development. Only with this restriction can an equilibrium of the living system be spoken of.