Curve of action current of the musculus sartorius excitated by two successive stimuli (St. 1 and St. 2). The effect of the second stimulus is the less and the latent period is the longer the more quickly the first stimulus is followed by the second. (Keith Lucas.)
We will now examine the alterations of irritability which are perceptible during the refractory period to complete restitution of the specific irritability of the particular system, and endeavor by the analysis of their special conditions to render them comprehensible from a physical standpoint of view.
The first fact to take into consideration is, that, as is shown in the heart, the refractory period begins at the moment of the appearance of the systolic excitation. The irritability of the heart is absent and remains so until the excitation has reached its highest point, that is, shortly before the beginning of the diastole. From this point the restitution of irritability begins, which does not reach the maximum until the end of the diastole. In other words: irritability undergoes the greatest reduction by disintegration produced by the stimulus and is restored by the metabolic self-regulation following the decomposition.
This point of view enables us to interpret this state from a physical standpoint. In this discussion on the relations between irritability and the extension of excitation, I have taken the amount of energy which is produced during the time unit and space unit in a living system as the general standard for the degree of irritability, at the same time duly regarding the individual components involved. This amount of energy is determined in a given system by the quantity of substance broken down by a stimulus of a given intensity. It is, therefore, clear that during the time in which an increased disintegration produced by a stimulus takes place, the irritability in response to a second stimulus must be reduced, as during this period the second stimulus has less of necessary decomposable substances at its disposal, and at the same time there are more products of disintegration in a given space. If a living organism is the subject of consideration, to which the “all or none law” is applicable, as, for instance, the heart at the moment of the beginning of excitation, irritability is completely obliterated, as shown by the fact that the second stimulus of any strength remains without response, for during the excitation there is a complete breaking down of all the substances capable of decomposition. If, on the contrary, a system is the subject of observation, for which the “all or none law” is not valid, then irritability is merely reduced but not wholly obliterated during an excitation, and whether or not a response is obtained to the stimulus depends upon its strength. To impress the relations between the degree of irritability and the intensity of the stimulus, I have, therefore, employed the term “relative refractory period” in contrast to the “absolute refractory period,” in which irritability is obliterated even for the strongest stimuli. It is self-evident that irritability must again increase in the same degree as the restitution of the living system by metabolic self-regulation takes place, for the more molecules capable of disintegrating are restored and the more products of disintegration removed, the more molecules necessary for decomposition in the unit of space are attacked and broken down by the stimulus. All these are self-evident facts which are in accordance with the conception we have here developed of the course of the process of excitation and its physical nature. But another important point is evolved from the observations we have made of the nature of the process of self-regulation. The process of self-regulation is founded on the same principle as that which governs the taking place of all chemical equilibrium, for metabolic equilibrium is merely a special kind of a chemical equilibrium. The development of a chemical equilibrium between reacting substances and reaction products has, as known, a characteristic course in regard to its duration. If the rapidity with which the equilibrium is reached is expressed by a curve in which the abscissa represents the time, while the ordinates signify the number of contacts of the interacting molecules, the rapidity of reaction is altered with the approach to the equilibrium in the form of a logarithmic curve; that is, the approach to the state of equilibrium, which is represented by ordinate value zero, takes place at first very rapidly, then with more and more decreasing speed, for with the decrease of the number of reacting molecules and the increase of the amount of products of reaction, the contact of the interacting molecules and with this the opportunity for the reaction occurs less and less frequently. Although the self-regulation of metabolic equilibrium is by no means such a simple process as, for instance, that of the well-known example of the forming of ethylester from acetic acid and æthyl alcohol, we have still in every case to deal with the taking place of a chemical mass equilibrium. Hence the progress to the metabolic equilibrium must likewise correspond with a logarithmic curve, i.e., restitution after a disturbance of the equilibrium must take place at first rapidly, then at a constantly decreasing rate. For reasons readily to be understood the special form of this restitution curve has so far not been accurately ascertained for any kind of living substance. Even in those cases where the restitution occurs very slowly we meet with the difficulty that, when the tests are applied which are necessary to determine the restitution at different intervals, with each testing stimulus irritability is each time reduced. Hence the construction of the restitution curve can only be achieved by indirect means, and we must content ourselves with the ascertainment of a smaller number of its points from which by interpolation its form can be constructed. Indeed in this connection a certain number of results have already been gained quite sufficient to experimentally confirm the correctness of these types of curves, primarily obtained by purely theoretical deductions. That irritability very gradually reaches its maximal height has been already shown, as previously mentioned by Bowditch[133] in his investigations on the influence of rhythmical induction shocks on the apex of the heart of the frog. He found that in order to produce response, the weaker the stimuli the longer must be the intervals between them. It follows from this, that after a discharge the irritability in response to strong stimuli reappears more rapidly than for weak, i.e., that they only gradually regain their maximum. The exact periods of time for the course of the return of irritability for the heart have unfortunately not been so far ascertained. On the other hand, the investigations of Ishikawa[134] furnish the material for the construction of the restitution curve for the centers of the spinal cord of the frog. Ishikawa did not employ the threshold of stimulation as an indicator for the course of restitution, but used instead the duration of the reflex time following on a stimulus of a certain strength. The reflex time is greatly prolonged after an excitation of extended duration and only regains its normal value in the same degree as restitution takes place. By a great number of painstaking experiments Ishikawa ascertained the duration of the reflex time at intervals of thirty seconds to one minute, and obtained figures which show that restitution does actually take place, at first rapidly and then with constantly decreasing speed. The detailed study of the course of self-regulation of the individual forms of living substance will doubtless be more exactly determined in the near future. But even at the present we are fully justified in describing the form of restitution curve as a logarithmic in type. Therefore, a relative refractory period must be present in every metabolic self-regulation after an excitation, during which stronger stimuli produce response, while weaker are still without result. This is a fact which, as we shall see later, is of fundamental importance for the comprehension of the various kinds of interference responses to stimuli.
From the information here gained on the nature and origin of the refractory period the conclusion must inevitably be drawn that in all living substance there must exist, directly following an excitation, a period of time in which its irritability is reduced, that is, under proper conditions a refractory period can be demonstrated for every living organism. Every living system possessing irritability undergoes a period of reduced irritability at the time of and subsequent to every excitation, for every excitation momentarily decreases the amount of products capable of disintegration and increases the disintegration products in the unit of space. As restitution involves time, a stimulus occurring in the phase preceding complete restitution cannot break down the same quantity of molecules as would be the case after the establishment of complete restitution, that is, the response is weaker, the irritability is decreased. The refractory period during and subsequent to excitation is as much a general property of the living substance as irritability and metabolic self-regulation.
This conclusion appears so self-evident that it would seem hardly to call for emphasis were it not that even at the present time the view is still widely held that the refractory period is a special characteristic of certain forms of living substance. This assumption is explained on the one hand by the fact that our information concerning the refractory period is still of comparatively recent date and that few physiologists are in the habit of connecting special observations with general physiological conceptions, but also for the reason that some investigators have vainly tried to find a refractory period in certain forms of living substance. Langendorff and Winterstein,[135] for instance, have not succeeded in proving a refractory period for the spinal cord of the frog. Langendorff stimulated the central sciatic stump with two stimuli in quick succession and used the contractions of the triceps as indicator of the response. He found that when the stimuli, if consisting in either single induction shocks or faradic shocks, followed each other even at intervals of .004 seconds the second stimulus was still operative, this being perceptible in an increase of the contraction or with greater intervals of time in a summation of two contractions. Winterstein concludes from this that the development of a refractory period after a stimulation is not a general property of all nerve centers. If the experiments of Langendorff failed to show the presence of a refractory period it is not for the reason that this does not take place in the centers of the spinal cord but rather results from the fact that the conditions for the investigation were not suited for its demonstration. In fact, Fröhlich[136] and especially Vészi[137] have incontestably proved the existence of relative refractory periods in the normal spinal cord.
If the existence of the refractory period is based on the fact that during the time of and subsequent to an excitation the quantity of substances necessary for disintegration is decreased and that of the breaking down products increased, and if it is limited by the restitution of the substances required for decomposition and the elimination of the disintegration products, its duration must be dependent upon the length of these processes. All factors which lessen the decomposition and hasten the metabolic self-regulation must, therefore, shorten its duration. This is completely confirmed by experimental investigations. As can be understood, the factors of special interest for us are those which influence the duration of the refractory period in the physiological occurrences of the organism.
One of these factors is temperature. As we know, the rapidity of chemical reactions increases with ascending and decreases with falling temperature. As in the disintegration as well as in the restitution, processes are chemical in nature, it is to be expected that the duration of the refractory period is influenced in like manner by temperature. Indeed, Kronecker[138] found some time ago that in the isolated frog’s heart a much more frequent rhythm of stimulation is effective at a higher than at a lower temperature. When the heart is stimulated at a temperature of 11–12° C. with twelve rhythmical induction shocks in the second, every stimulus is operative and produces a systole. If a stimulus of the same frequency is used at a temperature of 5° C., the heart responds merely to every second stimulus. This shows that the refractory period is of longer duration at a lower than at a higher temperature.
A factor of particular interest is the supply of oxygen, for we know its fundamental importance in all aërobic organisms in the breaking down of the living substance. The life of these organisms is primarily dependent upon the supply of oxygen from without. Organic reserve substances for restitution after disintegration are contained in ample quantity in the reserve stores in the living cell substance, whereas oxygen is present in very small quantities in relation to the former. It is, therefore, self-evident that the rapidity of the breaking down processes is very closely dependent upon the amount of available oxygen at hand. Nevertheless it is not the absolute quantity but the relative amount of oxygen in relation to the momentary requirement which is of importance. For instance, the quantity of oxygen present may completely suffice for the oxydative disintegration in the metabolism of rest or at lower temperature, whereas the same amount would be much too small to meet the demand increased by excitation or at higher temperature. In the latter case “a relative deficiency of oxygen” occurs. I have introduced the term “relative deficiency of oxygen”[139] for I have found that a number of authors by neglecting the relations of the available oxygen to that which is required at the moment have been led to false conclusions. There is no living object so preëminently fitted to demonstrate in such a striking manner the dependence of the duration of the refractory period upon the supply of oxygen as the spinal cord centers of the frog, when their irritability has been increased to the maximum by strychnine.[140] Various observers, such as Loven, Buchanan, H. von Baeyer and others, investigated the action current by the capillary electrometer. As a means of studying the number of impulses in the strychnine tetanus, we can upon the basis of their figures roughly assume the number of impulses to equal ten per second at room temperature. In short, in the freshly strychninized frog the duration of the refractory period is about .1 second. By means of the method of artificial circulation already mentioned a deficiency of oxygen can readily be brought about. It has been demonstrated that the rhythmic in contrast to the continuous method of introduction of circulatory fluid is superior in that the former reproduces more closely the natural conditions of the circulation of the blood and renders the smallest capillaries more permeable. In consequence I have recently constructed a small appliance for artificial circulation, which accomplishes this in a manner as simple as it is complete. (Figure [31].)
