I have described this experiment somewhat in detail as it contains facts which are the key for the comprehension of a general physiological process of paramount importance. I refer to fatigue. The refractory period and fatigue are inseparably connected, for fatigue is founded on the existence of the refractory period and is an expression of prolongation of the former, brought about by want of oxygen. This is shown at once by closer analysis. It is here necessary to differentiate somewhat more in detail the factors which bring about the prolongation of the refractory period in deficiency of oxygen.
If we first turn our attention to the normal refractory period which occurs in a system in metabolic equilibrium of rest in direct connection with dissimilatory excitation, following a momentary stimulus, we find that reduction of irritability or, more exactly expressed, the lessening of the response is, as we have seen, determined by the time involved in the metabolic decomposition and recovery. Both these processes require time and until their completion the quantity of substance demanded for the oxydative disintegration is decreased in a given space, and every stimulus must consequently be followed by a weaker response. Our conceptions of the physical details of these processes depend essentially upon the question, if the oxydative disintegration itself in the given living system occurs in one single phase, in that the oxygen is the activator for the oxydative splitting up of the carbon chain, or if this takes place in two periods, in which the carbon chain is first anoxydatively split up into larger fragments by the stimulus, which are then seized upon by the oxygen to be split up into carbon dioxide and water. As we have seen, this question must remain for the present undecided as far as the metabolism of rest as well as the excitation produced by a single momentary stimulus is concerned. It is highly probable that a uniformity of the process for all living systems does not exist. We are, therefore, not justified in assuming that these special chemical processes resulting from single stimuli are uniform throughout the refractory period.
On the contrary it is different in the case of oxygen deficiency. Here we see with increasing want of oxygen a constantly increasing duration of the refractory period, a prolongation which may be attributed to the retardation of the oxydative disintegration. It is necessary, however, that we now study more clearly these alterations brought about by the deficiency of oxygen.
If we follow the course of the changes from that of the normal state of equilibrium of metabolism, wherein oxygen is sufficient to bring about complete disintegration of the molecules to the formation of carbon dioxide and water, we must assume in spite of the great explosive rapidity of this process on the basis of our chemical knowledge, that first a series of intermediate products are produced before finally the end products are formed. In this way the oxydative disintegration produced by a stimulus becomes more and more prolonged by an increasing want of oxygen. If, as I have previously suggested, the amount of energy which is liberated in a given space and time by an excitating stimulus is taken as a standard of irritability, it is apparent that the more the oxydative disintegration following a stimulus is retarded, the greater must be the decrease in irritability. The less oxygen there is at disposal and the more incomplete the oxydative breaking down, the smaller is the degree of irritability, the weaker the response and the slower the return of irritability after every stimulus. In other words, with the increasing deficiency of oxygen, the response is not merely reduced for every stimulus, but the duration of the refractory period is likewise progressively prolonged until finally with an absolute want of oxygen, constant and complete depression takes place. In the genesis of this process another factor, however, has the same effect.
While with a sufficient supply of oxygen disintegration leads to the formation of carbon dioxide and water, therefore to end products, which can quickly and easily be removed by diffusion, the want of oxygen produces complex products of incomplete combustion and finally of anoxydative decomposition, such as lactic acid, fatty acids and even more complex substances in constantly increasing quantities. These products permeate the protoplasmic surfaces with great difficulty, if at all, and as they cannot subsequently be oxydatively split up, constantly accumulate. These asphyxiation substances, as they may be briefly termed, produce a depressing effect on further disintegration. This can be experimentally demonstrated.
For this purpose I have modified the experiment previously described in the way that after every introduction into the blood of oxygen-free saline solution and after the injection of strychnine, the artificial circulation was stopped so that stagnation of the oxygen-free saline solution took place in the vascular system. The processes then occurred in exactly the same manner with the exception that the state of non-irritability appeared somewhat earlier. If after the beginning of complete depression artificial circulation with oxygen-free saline solution was again started, a certain degree of recovery took place within one or more minutes. The stimuli were once more effective and produced a number of contractions. At times, several single contractions, following each other in more or less quick succession, could be brought about. But complete recovery or the appearance of even incomplete tetanic convulsions was never again obtained, whereas by the introduction of oxygen complete recovery could at once be brought about. If, however, the circulation with oxygen-free saline solution was continued, irritability gradually decreased. The refractory periods after the individual stimuli became longer, and in spite of continuous artificial circulation irritability again disappeared. The experiment shows that by the circulation of oxygen-free solution irritability can simply be reduced up to a certain degree. This partial restitution is produced by washing out the depressing metabolic products. Being desirous to verify the results of this investigation with greater exactitude I have requested Dr. Lipschütz[143] to repeat the experiments, taking the utmost possible precaution in respect to the absolute exclusion of oxygen. Lipschütz has tested the normal saline solution made oxygen free with the sensitive Winkler method, in which the slightest trace of oxygen is shown by the oxydation of manganous chloride to manganic chloride in which the latter in a saline solution sets free an amount of iodide from iodide of potassium corresponding to that of the consumed oxygen. These experiments of Lipschütz have shown that even with the absolute exclusion of the slightest trace of oxygen a partial recovery can be brought about by artificial circulation. There can be, therefore, no doubt that recovery is actually founded on the removal of the depressing asphyxiation substances by artificial circulation. Moreover Fillié[144] has previously succeeded in the laboratory at Göttingen in obtaining by the same methods a corresponding result for the nerve. In both cases the experiments are extremely complicated and must be carried out with the most painstaking care. The depressing influence of the asphyxiation products need not be regarded as a specific effect of poisoning. It can be solely an expression of mass relations, if we assume that the anoxydative decomposition is controlled by a chemical equilibrium between masses capable of disintegrating and products of the disintegration. It is not possible to give any detailed account as to the part taken by accumulating asphyxiation substances in the prolongation of the refractory period. Indeed, we must for the present relinquish the attempt to delimitate quantitatively the part taken by the individual constituent processes in the symptoms of depression resulting from the deficiency of oxygen. We can merely say, the individual alterations produced by the want of oxygen, that is, the restriction and retardation of the oxydative disintegration, the corresponding increase of the anoxydative decomposition and the accumulation of the products of incomplete oxydation and anoxydative breaking down have the same influence in that they decrease the strength of the response and retard the rapidity of the decomposition process. These are the general effects perceptible in the refractory period by the deficiency of oxygen.
The establishment of these facts of the dependence of the refractory period upon oxygen are of the utmost importance for the genesis of fatigue, for the state of fatigue in all aërobic organisms is invariably brought about by deficiency of oxygen. In other words: fatigue is invariably asphyxiation. A deficiency of organic reserve substances never occurs in fatigue before the effect of oxygen deficiency leads to complete depression, for the quantity of organic reserve substances at the disposal of the cells is greater comparatively than that of oxygen. This is shown by transfusion experiments in which the time involved before complete paralysis was brought about in the frog by the introduction of an oxygen-free saline solution was ascertained and compared with the period which elapsed before complete paralysis took place, when the same solution saturated with oxygen was used.
Although the previously described experiments on the strychninized frog show clearly the relations of fatigue to the refractory period, I should, nevertheless, like to illustrate them somewhat further.
The state of fatigue as it is developed in a living system by a continuous functional activity is characterized by a series of symptoms which can be best studied in the fatigue of the muscle, the nervous centers, and the peripheral nerves.
If the muscle of the frog is isolated and rhythmically stimulated with single induction shocks and the muscle contractions graphically recorded, it will be found that the first perceptible alteration during the course of stimulation is the increasing height in the curve, which appears directly after the first contraction and becomes more and more noticeable after every succeeding one. With the isolated apex preparation of the frog’s heart an effect is produced which Bowditch[145] has termed the “Treppe” and Tiegel,[146] Minot[147] and others have obtained the same result for the skeletal muscle. The Treppe has been often regarded as an expression of increasing of capability of the muscle following each succeeding stimulus in spite of the fact that it is physiologically incomprehensible that an isolated muscle can become more capable by increased demands. Fröhlich[148] first threw light on this seeming contradiction by showing that the increase in height of the muscle contraction in the Treppe is in reality the first indication of the beginning of fatigue, and Fr. Lee[149] arrived at the same result. The increase in height of the contraction curve depends upon the retardation of the course of contraction. As the contraction extends over the muscle substance in the form of a wave, a longer stretch of the muscle will be in a state of contraction when the wave is more extended than when it is shorter, that is, the shortening of the muscle will be greater, the contraction curve higher, when the wave is more extended. With increasing fatigue the retardation in the course of contraction, as Rollet[150] already has shown, becomes continuously greater. (Figure [34].) The consequence of this retardation in the course of contraction is, therefore, perceptible in the rhythmically activated muscle in the form of contracture. As fatigue increases, the muscle requires an increasing length of time to relax to its full extent and in consequence the period between the two stimuli is very soon insufficient for this to occur. There remains a certain amount of shortening, when the next contraction begins. This characteristic extension of the individual contraction curve of the fatigued muscle is an expression of the retardation of the oxydative disintegrating processes and of the Treppe. It shows us that fatigue is perceptible to a slight degree even after the first excitation. After every succeeding stimulus the oxydative decomposition in the fatigued muscle is increasingly prolonged. It is, therefore, self-evident that the capability of action of the muscle likewise becomes less with increasing fatigue. Every state of fatigue is, in fact, distinguished by the decrease of response. This is perceptible in the later stages by the decline of the height of contraction. Hence all symptoms of fatigue which we observe form the expression of one single process; it is the constantly increasing slowness of oxydative disintegration with increasing fatigue.