Curve of irritability as demonstrated by action current of two nerves in nitrogen, which are alternatively stimulated (plain line) and at rest (dotted line). Recovery in nitrogen is always merely partial and relative. It only increases on introduction of oxygen. (After Thörner.)
To briefly summarize in conclusion, I will repeat that just as all living systems show a refractory period after an excitation, in which irritability is reduced, all living systems are likewise capable of fatigue. Both are most intimately connected and are based fundamentally on the facts of metabolism.
An excitating stimulus disturbs the metabolic equilibrium of rest by suddenly bringing about increased decomposition of certain substances. During and directly after the breaking down, irritability is reduced in the same degree as the amount of substances required for disintegration in response to a succeeding stimulus is decreased and the quantity of the decomposition products is increased. This is the refractory period. By the metabolic self-regulation in accordance with the principle of chemical equilibrium, the original metabolic equilibrium is restored after every excitation. Irritability, therefore, increases in the same measure as this occurs, that is, in the form of a logarithmic curve, until it again reaches the specific degree of irritability of the particular system. The refractory period diminishes. If the processes of disintegration and self-regulation are delayed, either by want of substance necessary for breaking down or the accumulation of decomposition substances, the refractory period is prolonged and the response to every further stimulation decreased, that is, the system is fatigued. In all aërobic organisms the retardation of the course of excitation and self-regulation under a continuous influence of stimuli is the result of the relative want of oxygen. The processes of oxydative disintegration are prolonged and restricted by relative deficiency of oxygen and merge more and more into anoxydative decomposition. The products of incomplete oxydative and anoxydative decomposition accumulate. Both factors decrease the strength of the response after every stimulation. Thus the want of oxygen leads to reduced activity. In the anaërobic organisms the refractory period and symptoms of fatigue are, of course, produced by the relative deficiency of other substances. Fatigue in the anaërobic systems has, however, so far not been investigated. We advance very slowly, step by step, in physiology, and, as in every science, an acquirement of a new knowledge means a new problem. In this lies the inexhaustible charm of our scientific research.
CHAPTER VIII
INTERFERENCE OF EXCITATIONS
Contents: Examples of effects of interference of stimuli in unicellular organisms. Interference of galvanic and thermic stimuli in Paramecia. Interference of galvanic and thermic stimuli and narcotics. Interference of galvanic and mechanical stimuli. Interference of galvanotaxis and thigmotaxis in Paramecia and hypotrii infusoria. Real or homotop interference, apparent or heterotop interference. The two effects of homotop interference of excitations: Summation and inhibition of excitations. Theory of the processes of inhibition. Hering-Gaskell theory. Inhibition as an expression of the refractory period. Individual possibilities of interference of two stimuli. Interference of an excitating and a depressing stimulus. Interference of two depressing stimuli. Interference of two excitating stimuli. Analysis of the interference of two excitations. Interference of two single stimuli. Conditions upon which the result of interference is dependent. Heterobole and isobole living systems. Intensity of the two stimuli. Interval between the stimuli. Specific irritability and rapidity of reaction of the living system. Latent period. Interference of single stimuli in a series. General scheme of the development of the effect of interference. Summation and inhibition. Apparent increase of irritability. Conditions of summation. Tonic excitations. Conditions of inhibition. Various types of inhibition. Interference of two series of stimuli. Relations in the nervous system. Peculiarities of the nerve fibers. Conversion of the nerve by relative fatigue from an isobolic into a heterobolic system.
Until now the mechanism of the single excitation has received the major portion of our attention. It was not until we reached the subject of the origin of fatigue that we became acquainted with the effects of repeated stimulation. Here we found a case of interference of individual excitations. But fatigue is simply a special instance of such interference, for the subject of interference action occupies a much greater field.
Every cell of the larger organisms, and more especially the single celled organisms, is subjected to manifold stimuli. It is indeed, quite common that two stimuli interfere with each other and manifold effects follow, depending upon the specific reaction of the cell and the quality, intensity and duration of the interfering stimuli. Sometimes the interference effect is readily understandable from a knowledge of the specific effect of the individual stimuli concerned. At other times, however, the specific reaction seems entirely different in nature than would be expected from a study of the effects of the individual stimuli.
Fig. 39.
Galvanotaxis of Paramaecium aurelia.