Muscular Activity

Fic. 6. Recovery HEAtT-PRODUCTION IN OxyGEN: Four DURATIONS OF Trranus at 20°C.; Two Durations or Tetanus at 0°C. (REsutts Corncipz.) (Hartree and Hill, 1922) by any possible lack of oxygen. In such small contractions there is sufficient oxygen already dissolved inside the muscle to account for considerably more heat than the total amount liberated in contraction and recovery. It is possible therefore to follow the chemical dynamics of the recovery process uninterfered with by considerations of

recovery heat production has required more elaborate and careful experiments than could be made in 1913, and since the War I have had the cooperation in this and other matters of my friend W. Hartree, whose skill in experimental work and calculation has made it possible to reach a degree of certainty in the analysis which could never have been attained otherwise. By photographic recording and accurate numerical analysis of the deflection for ten minutes after stimulation, it has been possible to describe the whole

Fig. 7. Errect or CO, on THE TIME-COURSE OF THE Recovery Huat- Inset, the effect of varying percentages of CO, on the maximum rate of heat-production during recovery. (Hartree and Hill, 1924.) of the time-course of the delayed heat production. (See fig. 6.) Its absolute magnitude in oxygen is 1.5 times the total © initial heat; in nitrogen only 0.25 times; the delayed oxidative heat therefore is almost exactly equal to the total anaerobic heat: its rate is influenced by temperature in the way usual in biochemical reactions, and is decreased by a rise in the hydrogen ion concentration (fig. 7). It

starts from a low rate, rises to a maximum, and declines to zero again. It seems to occur with a velocity approximately proportional to the square of the concentration of the bodies whose removal constitutes recovery. Taking Meyerhof’s latest value for the heat liberated per gramme of lactic acid produced, we find that one-fifth only of the lactic acid is oxidised in recovery: the rest must be restored to its precursor. It is clear that the recovery process is a fundamental part of the whole mechanism of muscle. As we shall see later, oxygen is used only in the recovery process, a fact which has led to a new conception of the nature of the muscular machine.

The analysis of the recovery heat-production is comparatively simple because its evolution is slow. It is desirable, however, also to analyse the production of heat in the earlier phases of contraction, a more difficult matter. After a single shock the muscle shows two phases of contraction, namely, the development and the disappearance of the mechanical response,—contraction and relaxation. Tetanus shows three phases, contraction, relaxation and maintenance. It was desirable to produce an outline of the chemical breakdown processes associated with each of these phases.

A muscle is stimulated, the galvanometer deflects, a curve is recorded on a moving photographic paper. A control curve is made later, after the muscle is dead, for which a known small amount of heat is liberated rapidly, e.g., in 0.1 second, and another deflection is recorded. It is found even in the earliest phases, long before the development of the recovery process, that the two curves differ from one another. The curve for the live muscle rises less steeply and falls less steeply than the control curve, _a fact which can be due only to a spreading out of the heatproduction. The curves do not differ largely from one another because the whole duration of the initial phase is

so short compared with the time relations of the galvanometer and thermopile themselves: though Hartree is ference, however, is sufficient to make the analysis practicable, and by keeping the muscle at a low temperature it is possible to slow down the events and to attain a greater degree of accuracy in the analysis.. That the analysis is really valid has been shown by certain tests recently ap- Fia. 8. Tor ANALYSIS OF THE DeFLECTION CAUSED BY A Known Huat-

plied to it by Hartree (fig. 8), in which a known artificial heat production has been analysed with reasonable exactitude from its records. The result that appears is that 1 Considerable improvement has recently resulted also from (1) increasing the figure of merit of the galvanometer system: A.C. Downing has constructed a galvanometer with a figure of merit of 18,000, which moves about 4 times as rapidly, at the same sensitivity, as the instru-

ment previously employed; and (2) using slower muscle, the biceps cruris of the tortoise. outburst of heat, during the maintenance of the contraction there is a continued production of heat, reaching a constant rate as the contraction is prolonged, and a comparatively large and sudden evolution of heat during relaxation (fig. 9). An excited muscle behaves like an excited electro-magnet: energy is required to magnetise the iron and to make it develop its pull: energy is required to cause it to maintain

Fie. 9. Heat Liseratep By Sartorius Muscies at 0°C. In Oxyarn The height of each rectangle represents the heat given out in the interval corresponding to the base on which it stands. Skeleton rectangles represent the heat liberated during contraction, black rectangles the heat liberated during or immediately after relaxation. (After Hartree and Hill, 1920.) its magnetic condition and to continue its pull; heat is liberated when the magnetic condition, and the energy associated with it, disappear as the current is cut off. In the muscle energy is required to set up the contraction, and, as we shall see later, more energy is required if work be done. Energy is needed to maintain a contraction, as,

weight for a long time. During relaxation the potential energy of strain possessed by the muscle during contraction has to disappear, and we find it as heat. Perhaps the most important point brought out by this analysis of the initial heat-production is that relating to the influence, or rather to the absence of influence, of oxygen. The essential conclusion can be drawn without any analysis, merely by comparing the curves of deflection with and without oxygen. The proof that oxygen has no effect whatever on the time-course of the initial heat-production complements an observation by Weizsicker, working at Cambridge in 1914, that the presence or absence of oxygen has equally no effect on the magnitude of the initial heatproduction. No difference whatever can be detected between the curves obtained (a) from a muscle in pure oxygen and (b) from one which has been deprived of oxygen in the most rigorous manner for several hours. The conclusion is important and supplements the observations previously described on the recovery heat-production. Oxygen is not used at all in the primary breakdown but simply in the recovery process. A muscle behaves like anaccumulator, which can be discharged without any kind of combustion or any kind of provision of energy from without: it requires external energy only when it has to be re-charged. It had long been known, of course, that muscles can go on working for some time in the absence of oxygen, but it was open to anybody to suppose that the processes in the presence and in the absence of oxygen were different. So they are, to the extent that recovery follows one but not the other; the complete absence, however, of any effect of oxygen on the initial processes of contraction shows that — those initial processes are identical in both cases. The analogy of the accumulator is exact.

If we take the case of the prolonged isometric contrastion, heat is liberated in each of the three initial phases of contraction, maintenance and relaxation. If the contraction be maintained for a long time, there is a steady heat-production during the whole of that time, propor- Fig. 10. Renation Betwenn H/T1 anp Duration or STIMULUS, FOR DirFrerEnt TEMPERATURES The curves all start from the same point, corresponding to H = T1/5.5, and rapidly become linear. (After Hartree and Hill, 1921.)

tional to the tension developed (fig. 10). We may describe the phenomenon mathematically in the formula: m7 A+ Bt where H is total heat-production, T is force developed, / is length of muscle, ¢ is time during which the contraction is maintained, and A and B are constants. It is found that A is independent of temperature, having, in the case of the sartorius muscle, always a value of about 1/5.5; B, which we may regard as the inverse of the “efficiency of maintaining a contraction,’ depends on the type of muscle, on temperature, on fatigue, and on many other factors.’ If we apply this result to the important practical case of human movements, so much energy is required to set them up, so much to maintain them for a given time. The amount required to set up a given contraction is constant, the amount required to maintain it is variable, depending on the nature of the muscle and its condition at the time. Anything which slows the single twitch of a muscle makes summation more easy and B smaller: the muscle becomes more efficient for maintaining a contraction. The quickest muscles of all are the least efficient when it comes to exerting a force for a long time. It has often been discussed whether the mechanical response and the internal metabolic changes which precede it may be separated from one another. If it were possible to have a thermal change, signalising chemical reactions inside the muscle, without any external mechanical effect, it would provide a most valuable method of research and allow us to draw conclusions of considerable importance. For example, if, as has been asserted, a muscle may still produce considerable quantities of heat at a moment when it can exert no tension at all, then it would be of first-class

accompanies the chemical reactions, or only the mechanical — no means of separating the internal metabolic changes and - the external mechanical changes has ever been found. — 2A has approximately the same value, in the biceps cruris of the tortoise, as in the sartorius of the frog. B in the frog may be many times as great as in the tortoise. found to be incorrect. In a most careful series of experiments Gasser and Hartree recently proved that when a muscle is caused to diminish its response reversibly by alcohol or hypotonic solutions, the heat and tension disappear together. Even the highest sensitivity of the methods available only showed that the line relating heat to tension passes through the origin.

The same effect obtains in connection with the action of drugs. If a contracture is produced, one may always safely look for an accompanying heat production. Veratrin has been said to delay relaxation and so to cause its characteristic prolonged contraction. This is definitely untrue. It is possible to analyse the course of the production of heat during veratrin contracture, and it is found that the rate of heat production runs parallel to the tension maintained throughout and that the ratio of the two is not only constant but has identically the same value as obtains during the contracture of a tetanus. The precise details of the working of veratrin remain to be investigated, but in a general way it is certain that it merely liberates continuously the chain of processes normally released discontinuously by a shock or a series of shocks. In the normal muscle a shock causes a momentary opening, which is followed by the rapid closing, of the door through which energy can pour in the form of lactic acid and heat. In the veratrin contraction the door is opened but the closure is only slow. The energy may escape continuously and produce tension continuously, and the relation between the two is the same as in an ordinary tetanus. Caffein shows an analogous effect. Soaking a muscle for a few minutes in 0.05 or 0.06 per cent caffein leads to the slow development of a typical contracture. Matsuoka has supposed that the course of this contracture is determined by - the admission of caffein into the muscle. This is definitely ‘not the case. The contracture develops in an hour or two even when the muscle was previously soaked only for

five minutes in a caffein solution and then removed to air. Apparently the caffein gradually changes the permeability of the surface which restrains the free passage of lactic acid, gradually opens the door which normally controls the outflow of energy, so that there is a continual escape, slower indeed but more persistent than that of the veratrin contraction, and leading in the end to the lactic acid maximum, to final rigor. The whole process is accompanied by an evolution of heat and a production of lactic acid; the maximum of contracture occurs at the same moment as the maximum of heat rate. Caffein appears merely to release slowly and continuously the chemical events which normally occur only rapidly and discontinuously on stimulation. The whole curve of heat liberation after caffein may be reproduced by applying a shock every few seconds and so maintaining a continual release of energy.

A prolonged contraction may be produced in still another way, namely, by soaking the muscle, not in normal Ringer’s solution, but in one of NaCl. The muscle in pure salt solution shows a more prolonged contraction, often a contracture analogous to that produced by veratrin. Again response is accompanied by an increase in the amount of heat liberated and (Hartree) by a prolongation of the heat production. It is difficult not to come to the conclusion that whenever we find prolonged contraction—at any rate in a voluntary muscle—there we shall find also a prolonged evolution of heat and a prolonged liberation of lactic acid.

We have dealt so far, for several reasons, with the simple case of the isometric contraction: first, because it involves us in one less variable, namely, the length of the muscle, — and second, because myothermic experiments in the isometric contraction are so much simpler and freer from the possibility of error. Recent developments, however, of production in a muscle which is allowed to shorten as much as one desires, and provided that stringent precautions be taken, reliable results are obtained. The temperature equilibrium in the muscle chamber must be so good that shortening over the junctions of the thermopile does not bring cooler or warmer parts of the muscle into contact with them. It must be remembered that this constancy of temperature must be to the nearest 1/100,000 of a degree because our scale of temperature is so small: one must beware of trusting to one’s ‘‘common sense” in the matter. During the last two years Fenn, followed by Azuma, has studied the effects of shortening and doing work upon the total liberation of energy by a muscle. Fenn found, in a very important series of experiments, that when work is done, there is an ‘‘excess” liberation of energy over and above that necessary in an isometric contraction of the same duration (fig. 11). The muscle is excited and allowed to shorten from one length to another length, to lift various loads from one to the other. The work done is proportional to the load. If the load be held up at the end of the contraction so that the muscle relaxes unloaded, the excess energy is just about equal to the work done. [If the muscle contracts freely and then lowers the load, excess energy is liberated of the same order of size as the work done by the load in falling: the muscle requires energy both to lower and to raise a weight. This is one only of the curious and important results which Fenn found. If a muscle be caused to lengthen during the development of its contraction, it gives out less energy than in an isometric twitch. If it be held fast initially and allowed to shorten during relaxation only, then again it will give out less heat. Shortening during contraction, lengthening during relaxation, appear to require excess liberation of energy.

Lengthening during contraction, shortening during relaxation, appear to cause an excess “absorption” of energy, ie., to lead to a total energy liberation less than that of We are studying here the curious power which a muscle possesses of adapting its liberation of energy to the work it has to do. We are dealing obviously with a very fundamental property of the machine. For those who like mechanical and electrical analogies the case of the electric motor may be suggestive. If the motor be run unloaded,

Shortening limited to 2.66 mm. After-loaded isotonic contraction: work varied by varying load. The work curve falls off at the point where the load becomes too heavy to be lifted the full distance. (Fenn, 1923.) a certain amount of energy is used: increase the load and > the motor automatically takes more current: try to drive causing less current to be taken. It is clear, in any case, — that the ‘‘all or none’ principle is not completely applicable to the muscle fibre, if we state the principle in the form that the response depends only on the stimulus and on the initial

circumstances. The amount of energy which the muscle gives out obviously depends on factors which come in only after the muscle has begun to shorten, on the inertia opposing its contraction, on the load it has to lift. State it in the form that the response of a single muscle fibre cannot be varied by varying the stimulus only and it still holds. It is clear also that the hypothesis that a stimulated muscle is simply a new elastic body is more complex than we thought. A muscle may possess elastic properties, but it requires more energy to do more work, a fact which is very difficult to reconcile with the simple elastic body theory. Perhaps the experiments of Azuma, amplifying those of Fenn, may afford the first clue in a very mysterious problem.

I referred in my first lecture to the curious thermoelastic phenomena which occur when a muscle is released or stretched. Stretch a muscle and its temperature rises: release it and its temperature falls, both events being re- -versible ones in the thermodynamic sense, both, however, being followed by an irreversible process in which mechanical energy is degraded into heat owing to the viscosity of the muscle substance. These reversible productions or absorptions of heat are not small and may very well play a part in the phenomena described by Fenn. They depend upon the coefficient of thermal expansion of the elastic substance which is stretched. If the elastic body expand on heating, then on stretching it, it willcool. If it shorten on heating, then on stretching it, it will warm, in either case by an amount directly proportional to the coefficient of thermal expansion. Persisting with our view that the muscle is an elastic body, the stimulated muscle has apparently different elastic properties from the unstimulated, and the coefficient of thermal expansion also is presumably different. The coefficient of thermal expansion of resting muscle is opposite in sign to that of ‘most known substances. It may well be the case that in

the transformation from rest to activity the more normal type of coefficient will appear, so that the muscle may cool when stretched and warm when allowed to shorten, and that during relaxation there may be a gradual return from the one type of coefficient to the other. That such an explanation is required is suggested very strongly by the St eek g6 aH S2nM- 31MM 2 £ = Imm sTeeTeu. Wi "7. erty TSS PERS WT PS apr eee = eae ISOMETRIC (L) & (S) qG + & Beene) yfedpecl ee eee eaten Ree eee engage a5-0¢ Nw ereecte oF RELEASE ; F miscna U AN Ne eee 4-0 2 B SARTORWUS Done 3Enn — 33M} -

Fia, 12. Errect on tom Best PropuctTion or STRETCHING OR RELEASING 4 Muscty at Vartous Moments Durina an OTHERWISE Isommtric TwitcH experiments of Azuma (fig. 12). The thermal effects of . the quick release at various moments during a twitch are complex, but they are the exact converse of those of a rapid stretch. It is difficult to see any explanation of Azuma’s results except a thermodynamic one. To assert merely that the muscle adapts its energy output to the

work it does would be an expression of ignorance. Such may be a fact, but even so the adaptation must have some mechanism behind it, and thermodynamics and _ these thermal properties of elastic systems may be expected to provide us with an explanation of the mechanism. The above reviews contain references to all work discussed here, up to the date of the last one; the following papers are more recent: W. Harrren: The Measurement of Small Rates of Heat-production. J. of Scientific Instruments, 1, p. 265, 1924.

W. Harrres anp A. V. Hr: (1) The Effect of Hydrogen Ion Concentration on the Recovery Processin Muscle. (2) The Heat-production of Muscles Treated with Caffein. J. Physiol., 58, pp. 441 and 470, 1924. R. Azuma, Thermodynamic Phenomena Exhibited in a Shortening or a Lengthening Muscle. Proc. Roy. Soc. B. 96, p. 338, 1924. E. Sereni: The Effects of Different Salts on the Heat-production of Muscle. J. Physiol., 60, p. 1, 1925. The closing years of the nineteenth century marked what seemed to be the final establishment of a particular conception of respiration in muscle. It appeared to be established that muscular energy sprang from a more or less explosive splitting of some molecular complex, which had been built up into some highly unstable form, by the inclusion within it of oxygen taken in by the cell during rest. The breakdown of this hypothetical molecule was supposed to yield both lactic acid and carbon dioxide, these being the two obvious and recognisable products of activity.

This conception, in its earlier stages, connected the idea of “irritability’’ with that of combustibility. Spallanzani, however, showed that living tissues can long survive and continue to yield carbon dioxide without any contemporary supply of oxygen. Various workers found the same to be true of muscle, and it was on this that Hermann chiefly based his theory of inogen. He showed in 1867 that free oxygen is not present in the air pumped from isolated frog’s muscle, and yet that without any supply of oxygen from outside carbon dioxide is yielded by the muscle when it contracts or stiffens after death. At the same time lactic acid is produced, while no other recognisable chemical changes occur. Inogen therefore was the unstable precursor both of lactic acid and of carbon dioxide, a precursor in which oxygen was ready to combine explosively with carbon and hydrogen in the combustion which was to yield the energy of contraction. After the explosive

_ breakdown of this precusor, it was supposed that fresh carbon bodies, and perhaps also the lactic acid, were combined again in a newly oxygenated unstable molecule of inogen. This inogen hypothesis of Hermann was taken up again later by Pfliger but without significant change, or fresh experimental support. Pfliiger’s ‘‘giant’’ molecule, as he described it, made unstable by the inclusion within it of what he called ‘‘intra-molecular oxygen,” was the same in all essentials as the inogen molecule of Hermann.

These conceptions of Hermann and Pfliiger have had a historical importance reaching far beyond the particular enquiry into muscular energy. They summarised the only aspects of cell metabolism which had received any experimental analysis at all, and up to the end of the nineteenth century they not only represented all that was known of cell respiration and of its relations to cell energy, but they dominated also our ideas of cell metabolism in general. Yet, in the course of the last quarter of a century they have been proved completely false.'

It is not necessary to go specifically into the accumulated evidence against these theories. The work of Fletcher, of Fletcher and Hopkins, of Meyerhof and of others, has shown that all the carbon dioxide which can be obtained from a muscle in the complete absence of oxygen was there pre-formed in chemical combination: that the lactic acid which appears arises, not from any mysterious complex but from glycogen: that the energy liberated in the anaerobic contraction can be explained completely by the formation of sodium lactate from its known precursors, without leaving a balance for any other reactions, whether oxidative or not: and that the oxygen taken in by the surviving isolated muscle is used completely in the oxidation of — carbohydrate, without any remainder for storage in an intra-molecular form. The change of outlook is com-_ plete.