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

by Max Verworn

A further very interesting example of depression produced by oxygen deficiency is furnished by heat depression. It has long been known that with increasing temperature the vital manifestations of all poikilothermic organisms at first undergo a heightening of their intensity. If, however, after a maximum is reached, the temperature is still further increased a sudden depression sets in. The increase in the rapidity of the vital process as a result of increased temperature is readily understood when based on the well-known law discovered by van’t Hoff. Numerous investigations on the rapidity of the course of special vital manifestations, as, for instance the growth of the eggs of the frog and sea urchin, the assimilation of carbon dioxide in green plant cells, the number of vacuole pulsations in the infusoria cells, the frequency of the heart rate of the frog and of the mammal, etc., have shown that their increase does in fact follow the van’t Hoff law, being doubled or tripled in amount with every increase of ten degrees of temperature. The genesis of depression produced by heat, developed in different organisms at various heights of temperature, requires a closer analysis. This depression takes place at temperatures below that in which coagulation of proteins occurs. Therefore, under certain conditions, with which we shall presently become acquainted, it is capable of being recovered from, whereas in higher temperatures, in which albumen coagulates, vital activity is permanently obliterated. Depression produced by heat is, therefore, in itself not a necrobiotic process, which, as such, must necessarily lead to death. But rather like fatigue it must be looked upon as an asphyxiation process. Its relations to oxygen exchange have been chiefly demonstrated by Winterstein[202] by his investigations on the central nervous system of frogs and on medusæ. He found that when placed in a heated chamber in a temperature of 32–40° the activity and reflex excitability of the frog are at first augmented. Within the lapse of a short time this increase has become so great that the slightest touch produces tetanic contractions, similar to those characteristic of strychnine poisoning. Very soon, however, this state of high excitation is followed by one of depression, in which no response to stimuli can be obtained. The animal remains entirely motionless in any position in which it is placed, in the same manner as a frog whose nerve centers have been completely exhausted by strenuous activity. On the basis of our knowledge of the rôle played by the deficiency of oxygen in the bringing about of exhaustion the thought arose, if in this heat depression exhaustion might not likewise be the result of oxygen deficiency. This assumption has been most strikingly confirmed by the investigations of Winterstein. It has been demonstrated that recovery of the animal in a state of heat depression cannot be obtained by mere cooling, but is only brought about when at the same time a renewed oxygen supply is provided. For instance, a frog is depressed in the warm chamber and even when a strychnine injection has been introduced, does not show the slightest reaction to stimuli. In the warm water bath artificial circulation is now applied in the previously described manner with an oxygen-free saline solution at 30° C., so that the blood is displaced and thus the renewed oxygen supply to the nervous centers prevented. The animal can now be cooled and the warm saline solution be replaced by a cooled one without the least recovery taking place. If, however, blood of the ox with contained oxygen is substituted for the oxygen-free saline solution, the frog shows signs of recovery within a few minutes and after ten or fifteen minutes responds as a result of the strychnine to the merest touch with tetanic contractions of the whole body. By modifying these methods of investigation to a certain extent Bondy[203] has confirmed these results to the fullest extent. Later Winterstein by quantitative determinations of oxygen consumption on medusæ showed that at 30–35° C., at which temperature heat depression sets in, the consumption of oxygen shows an increase of about three and a half times compared to that in a temperature of 11–12° C. These facts show that we have in heat depression a process which, as far as its genesis is concerned, is completely analogous to that of fatigue. In fatigue, a relative want of oxygen is produced by the increased consumption following functional activity, in heat depression by the increase of the entire metabolism producing a corresponding increase of oxygen requirement. In both instances we have an excitation produced by external stimuli which result in an increase in the amount of oxygen required, and in both instances the oxygen at disposal is not sufficient to permanently meet the augmented demand. In both types, therefore, decomposition must become more and more anoxydative and the well-known series of processes is developed, which find their expression in depression.

In another direction likewise heat depression is of special interest, that is, in regard to the theory of nature of the processes in the living substance. According to the van’t Hoff law we may assume that every individual constituent metabolic process, if we imagine it as isolated and taking place in a test tube, undergoes in more or less the same degree as all others an increased rapidity of reaction as a result of increased temperature. At the same time, in living substance we find on the contrary that the van’t Hoff law is only within certain narrow limits more or less applicable to the sum total of all metabolic processes. Beyond certain degrees of temperature no further increase of the vital process takes place, instead a retardation occurs. The analysis of depression produced by heat shows us in the clearest and simplest manner the reason for this apparent deviation from the general law of van’t Hoff. This reasoning is based on the fact that the rapidity of reaction of a chemical process is not merely dependent upon the temperature, but likewise upon the mass relations of the reacting substances. In spite of the effect of the temperature in increasing the rapidity of reactions, the process undergoes retardation which extends to a complete cessation if the supply of material necessary to its existence does not keep pace with the increase produced by temperature. In the present instance the amount of reserve supplies for the building up of the disintegrating molecules exists in abundance, and it is merely the available oxygen which is in relatively a very small quantity. As soon, however, as metabolism in its entirety, or even merely in those parts in which oxygen is directly required, is increased by whatever means, the oxydative processes would be the first to fail and it must be from this point that the disturbance of the harmony in the interacting of the individual metabolic processes proceeds. This principle which we here see manifested in its simplest form in the effect of temperature on oxygen exchange in the form of a disturbance in the correlations of the individual constituent processes based on an alteration of the mass relation and the rapidity of reactions of individual members is, however, not merely restricted to effects of temperature and the results quickly following on a relative oxygen deficiency. It has, indeed, a much more general significance for all manner of constituent metabolic processes, for it is applicable to all nutrition and to all growth, and forms one of the most important factors which influence the process of development, that is, the gradual “metachronic” alterations in metabolism to which all living systems are subjected as long as life endures.

A very extensive group of depression processes is produced by the action of chemical stimuli. Among these the processes to which we apply the collective term of “narcosis” must claim our special interest. As is well known, an enormous number of substances of very different chemical nature, such as carbon dioxide, alcohol, ether, chloroform, chloral hydrate, etc., exist, which, possessing the property of producing cessation of the vital activities in all living systems, after withdrawal of their application, if it has not been too prolonged or intense, permit a complete restoration to normal vitality. These are the general narcotics. Besides these there are a series of substances which have a depressing effect only upon certain forms of living substance, and which we may, therefore, term special narcotics. As, however, the particular nature of depression following the application of chemical substances has hitherto been closely studied only in a very few instances, we are not, at present, in a position to sharply define the limitations of the conception of narcosis, a conception which originally had hardly any further meaning than the production of unconsciousness by chemical means. In the following discussion, therefore, we shall deal merely with narcosis produced by the well-known general narcotics, such as carbon dioxide, alcohol, ether, chloroform, etc. From the time of the introduction of ether narcosis into medical practice by Jackson and Morton in the year 1848 up to the present day, the theory of this process has awakened the liveliest interest. Many attempts have since been made to explain the physical nature of this interesting process without, however, any generally acknowledged theory of narcosis being established. I will refrain from entering into these former theories in detail as they have been exhaustively treated by Overton[204] in his studies on narcosis.

In connection with our present observations, however, I will more closely analyze the process itself, following the results of investigations extending over more than ten years carried out by my coworkers and myself. In these investigations it has been found that narcosis belongs to this group of depressing processes. A satisfactory theory of narcosis, however, and this I must explain from the first, can even today not be arrived at. Such a theory would require the ascertainment of all primary and secondary alterations produced by the narcotic in the course of normal vital activity. For this, however, a number of minute details are still lacking. Nevertheless, the careful and detailed investigations during the last ten years have acquainted us with a large number of alterations, which, acting as conditioning factors for the process of narcosis, must be taken into consideration, and which to a certain extent give us an idea of the mechanism of this process. They are equally interesting from a theoretical as well as from a practical point of view. The presentation will become more detailed as more of such conditioning factors are established by the deeper penetrating of future analysis. I will deal here with the facts found up to the present and then proceed to the deductions which these furnish for the theory of narcosis.

In the first place narcosis is stamped as a typical process of depression, being characterized by a decrease of irritability with a corresponding decrement of the extent of excitation. The chief feature of all narcotized systems is, that in slight narcosis excitating stimuli produce a greatly weakened excitation, and that in deep narcosis no perceptible response is obtained. This can readily be ascertained in the various forms of living substance. According to the previous observations on the inseparable relations between conduction of excitation and irritability, it is self-evident that with decrease of irritability there must be a corresponding decrease in the capability of the conduction of excitation from the point of stimulation. This decrease in conductivity must, therefore, be the greater the more irritability is reduced; that is, the deeper the narcosis, the greater must be the decrement undergone by the wave of excitation in its extension from the point of stimulation. These facts can be observed in the highest perfection in the nerve, and have, as we have seen, been demonstrated by the investigations of Werigo, Dendrinos, Noll, Boruttau and Fröhlich.[205] Upon deeper analysis of this process of depression, the next task for the investigator must be the ascertainment of the special components of the metabolic activity, which are depressed as a result of the narcotic.

As a consequence of the result of my investigations on fatigue, the idea occurred to me to test if possibly oxygen exchange likewise undergoes depression during narcosis. The spinal cord centers of the frog, which had served me in ascertaining the rôle played by oxygen in the bringing about of the depression of activity, appeared likewise a favorable object for this investigation. Indeed, the question if consumption of oxygen takes place during narcosis, could be experimentally determined in direct connection with the investigations on fatigue. This was based on the following consideration. If an oxygen-free saline solution is introduced into the aorta of a frog and in order to increase the activity of the spinal cord centers to the maximum the animal is poisoned with strychnine, after a very short time complete exhaustion takes place as a result of oxygen deficiency. This exhaustion can only be removed by the introduction of oxygen. In this condition the oxygen requirement of the centers is enormously increased. If the centers are narcotized by adding a narcotic to the oxygen-free circulating fluid in amounts which, as experience has found, would produce complete loss of reaction in the normal animal, for example, about 5 per cent. of alcohol, it can then be tested if, in this state of narcosis, the centers are capable of oxygen consumption. It is merely necessary to replace the oxygen-free saline solution containing alcohol by blood rich in oxygen, containing alcohol in an amount sufficient to continue the narcosis, but supplying an abundance of oxygen. If, after this artificial circulation has lasted for a sufficient period, the blood is then displaced by an oxygen-free saline solution containing alcohol, and then this, in turn, is replaced by an oxygen- and alcohol-free saline solution, so that cessation of the narcosis is now produced, it can be ascertained by the responses of the animal if consumption of the oxygen, when at the disposal of the centers during narcosis, has taken place or not. If the former is the case, then on the cessation of narcosis reflex contraction must occur in the same manner as in every strychninized frog totally exhausted by oxygen deficiency and into which a saline solution containing oxygen is reintroduced. If during narcosis, on the other hand, oxygen has not been consumed by the centers, depression must continue to be present after cessation of narcosis. Testing the recovery of the animal on the introduction of blood, rich in oxygen, serves as an indicator for the vital activity and capability of recovery of the centers. A great number of experiments based on this scheme of investigation were undertaken at my request by Winterstein.[206] These were carried out with alcohol, ether, chloroform and also carbon dioxide. His experiments have shown in the most uniform manner that, in spite of the requirement of oxygen by the centers being increased to its highest extent, and notwithstanding the most ample oxygen supply during narcosis, after cessation of the same and the introduction of an oxygen-free saline solution no trace of recovery occurred, whereas after a supply of oxygen was introduced tetanic contractions reappeared at once. During narcosis, therefore, the centers, in spite of their great requirement of oxygen, lose their capability of oxydative splitting up and consumption of oxygen.

After the methods for asphyxiation of the nerve had been worked out and perfected the wish arose likewise to carry out for these structures an analogous series of experiments to that employed for the centers and based on the same chain of reasoning. These investigations have the advantage of essentially simpler conditions. After having convinced myself by experiments, that the results on the nerve were in complete conformity with those on the spinal cord, at my suggestion Fröhlich[207] repeated and continued these experiments on a more extended scale. A nerve was asphyxiated by the previously described method. This is accomplished in the simplest manner by the opening or closing of stop cocks in the apparatus I have employed which permit of pure nitrogen, or nitrogen with ether, and finally also oxygen with ether or pure oxygen being conducted at will through the glass chamber. If the nerve was so far depressed in pure nitrogen that conductivity became obliterated for about two cm. of the asphyxiated stretch, it was then narcotized in nitrogen. Following this oxygen with ether was supplied for a time. Then the oxygen-ether mixture was displaced by one of nitrogen and ether and finally by pure nitrogen. Even after a prolonged period, a recovery in pure nitrogen never took place. On the other hand, the nerve recovered at once, as soon as oxygen without ether was introduced. The results of these investigations are, therefore, completely in harmony with those undertaken by Winterstein on the nervous centers. They were later likewise entirely confirmed by similar experiments of Heaton.[208] All these investigations furnished the proof that in narcosis, living substance, notwithstanding even the greatest oxygen deficiency, is not capable of producing oxydation, neither can consumption of oxygen take place, with which, after cessation of the narcosis, oxydative splitting up can be carried out.

Recently Warburg[209] has likewise found an oxydative depression during narcosis in the eggs of the sea urchin and in the red corpuscles of geese, and the same fact has lately been also demonstrated by Joannovics und Pick[210] for the oxydative activity of the liver cells of the dog.

This fundamental establishment of the fact that narcosis prevents oxydations in living substance is at once followed by the further problem, in what manner do the disintegration processes undergo alterations during narcosis? That they must be altered, and this in the form of a reduced energy production, is clearly shown by the decrease of irritability and the increase of the decrement of the conduction of excitation. Both become the greater the deeper the narcosis. The observations just discussed render these facts at once self-evident. They follow as a simple and necessary result of the elimination of the oxydative processes. If these are suppressed further breaking down, if not influenced by addition of other factors, proceeds anoxydatively. The previously observed series of processes is developed, which invariably take place when oxygen deficiency occurs and which produce in the clearest form the results of asphyxiation on the withdrawal of oxygen supply. If, therefore, the disintegration processes are not influenced in some other manner during narcosis, they must then take place in the same way as in the withdrawal of the oxygen supply. The question, if this is actually the case, can be experimentally decided by comparing, on the one hand, the development of the course of asphyxiation during narcosis, and on the other, the withdrawal of the oxygen supply. We have carried out this comparison for the spinal cord centers as well as for the medullated nerve. A prolonged series of experiments have been made by Bondy[211] with the apparatus constructed for this purpose by Baglioni.[212] Two frogs under uniform conditions of temperature were submitted to artificial circulation, the one merely with an oxygen-free fluid, the other with the same, but with the addition of 5 per cent. of alcohol. In order to render the least trace of irritability perceptible, responsivity was increased in both animals by the employment of strychnine. It then appeared that, on the average, irritability was obliterated in the narcotized frog in about the same time as in the animal simply asphyxiated. These experiments were controlled by introducing at their conclusion a saline solution containing oxygen into both frogs and by ascertaining the degree of recovery. In like manner Fröhlich[213] has established the same fact for the nerve. The period of asphyxiation for the nerve in a nitrogen-ether mixture is approximately the same as in pure nitrogen. Analogous experiments have been carried out in amœbæ by Ishikawa.[214] Here also it has been shown that living substance becomes asphyxiated in narcosis and can finally recover only when oxygen is supplied. In more than a hundred experiments Ishikawa has, however, obtained the uniform result that amœbæ asphyxiate rather sooner in narcosis than in pure nitrogen. The most striking experiments are those which Heaton[215] has carried out on the nerve. Using both sciatic nerves of the same frog, he passed each one through a separate glass chamber, as previously described, and laid the central stumps projecting from the chamber over a pair of platinum electrodes, while the stretch within was likewise placed on platinum electrodes. The muscles served as indicator of the capability of conduction and irritability. The alterations thereof were tested by the ascertainment of the threshold of stimulation. The nerve in the one chamber was then subjected to a pure nitrogen current, that in the other merely to one of pure air with ether. In order to test the degree of asphyxiation the air-ether current in the latter chamber was replaced from time to time by an ether-nitrogen current, and then by one of pure nitrogen, so that the narcosis was interrupted without the entrance of oxygen being possible in the mean time. During this suspension of the narcosis, the nerve recovered each time in nitrogen, its irritability again increasing and its capability of conduction returning with every test. However, recovery showed itself as less and less complete. Finally irritability had sunk so low that the capability of conduction disappeared entirely. At the end of the experiment as control, nitrogen was displaced by air in the two chambers and in both nerves recovery took place.

In both cases recovery could only be brought about by an introduction of oxygen. From the sum of all these experiments it results that during narcosis in air the nerve, even when a sufficiency of oxygen is present, gradually asphyxiates and loses its capability of conduction, and this in about the same length of time as the other nerve in pure nitrogen. These investigations furnish two important facts for the theory of narcosis. First, that in narcosis living substance becomes asphyxiated notwithstanding the presence of an ample oxygen supply, and secondly, that asphyxiation occurs in the same time, or somewhat more rapidly, in pure nitrogen under otherwise similar conditions than without narcosis. In other words, it is shown that the breaking down processes of metabolism continue in narcosis as anoxydative disintegration. In narcosis, therefore, asphyxiation takes place with approximately the same or a somewhat greater rapidity than that in an oxygen-free medium.