Muscular Activity

the greater length to that corresponding to the smaller. It did not. (See fig. 9.) It fell to, or towards, zero and then slowly rose again as the stimulus was continued. The redevelopment of tension was similar under all conditions to the initial development of tension, which occurred when the muscle was first stimulated. (See fig. 10.) If a resting muscle was stretched, and then suddenly released from one length to another, the tension at the second length was attained almost instantly, unlike the case of the stimulated muscle. (See bottom record, fig. 9.) Was the sudden — disappearance of the tension in the stimulated muscle,

when released, due to some kind of inhibition of the contractile process by the release, so to speak to letting the steam out of a boiler, so that the pressure had to be redeveloped before the engine could be set working again? The matter was simply tested. If the contractile process Fig. 11. Errect on Forcr Exertep By AN Excitep Musciz or ALLOWING Ir TO SHORTEN AT VARIOUS SPEEDS Top curve = time (¢ second); middle curve and base line = record of

shortening; bottom curve and base line = record of tension. A = most rapid shortening, D = slowest, B and C = intermediate. Note that in A and B the tension falls to zero and then redevelops after the end of contraction, while in D the tension practically does not fall at all below its equilibrium value. (After Gasser and Hill, 1924.) had really been dissipated by the sudden release, then it would not return when the muscle was immediately stretched again. As a matter of fact it returned precisely to its original value, showing that the active state was still

present but not manifest externally; the released muscle was still “contracting” though it could exert no external force. If a stimulated muscle be released not instantly but slowly, the force which it exerts during its release may be recorded by the tension lever at its other end. (See fig. 11.) _ If the release be very slow the tension falls, so to speak “reversibly,” to that corresponding to its shorter length. If the shortening is more rapid the tension falls below its final value and then rises again: the more rapid the shortening, the greater the fall of tension below its final equilibrium value.

Fig. 12. Diagram or Errect or StreTcHine aT Vartous SPEEDS IN A MaxIMaL TETANUS A, very slow (“‘reversible”’); D, rapid and very rapid; B, fairly rapid; C, intermediate between Band D. Frog’s sartorius. (After Gasser and Hill, 1924.) . The converse experiment may be made. If a muscle be stretched fairly rapidly (B, fig. 12) its tension rises, not to the final equilibrium tension, but above it, and then settles down to that final value along a characteristic curve similar to that of redevelopment of tension after a quick _ Telease. If it be stretched too rapidly, other phenomena ~ occur which we were unable to explain, but under such | conditions the muscle rapidly degenerates and is often dead after two or three observations only. A very sudden stretch of a stimulated muscle appears to shatter its mechanism. Apart, however, from this shattering by too rapid a stretch, the phenomena of stretching are the converse of those of shortening or release.

It was natural to conclude that the phenomena in question were due in some way to the viscosity of the substance of the active muscle. The much slower return to its final tension, of the stimulated muscle as compared with the stretched unstimulated one, was a sign that the viscosity of the stimulated fibre was much greater than that of the unstimulated. The matter was decided by a direct experiment. A spring was arranged to vibrate and to record

UE nea | Car nr, Vea, Sete) aly ESL, Ey UD are VAS Kay, a Oy Ney, eae Wee Fria. 18. Damping or OscILLATIONS IN A SPRING CONNECTED TO A MuscLE (a) Unmxcitmep, (0) Excirep anp (c) UNEXcITED AGAIN Note that the damping becomes enormously greater when the muscle is excited. (After Gasser and Hill, 1924.) its oscillations on a drum. A muscle was connected to it and was alternately stretched and released during the vibrations of the spring. Its viscosity damped those vibrations, and an ordinary damped oscillation was recorded on the revolving drum. (See fig. 13.) The muscle was then tetanised and the damping was found to be increased about 16 times, confirming the deduction from the experiments discussed before. Not only is the active muscle much less extensible than the inactive, but it is very

much more viscous. The increase is so large, indeed so startling, that it leads one immediately to ask, what can be the mechanism of this change in viscosity? We shall return to that later. We attempted then to construct a viscous-elastic model which would reproduce the phenomena shown by a stimulated muscle. A thin india-rubber tube, with walls about the thickness of a child’s rubber balloon, was filled with For comparison with figure 9. Upper line, ‘‘long’’ tension; middle line, ‘‘short’’ tension; lower line, zero tension. In (1) and (2) redevelopment of tension commences immediately after release, in (3) only after a certain latent period. (After Gasser and Hill, 1924.)

very viscous material, Chatterton’s compound boiled with vaseline, and the same experiments of quick release and slow release, quick stretch and slow stretch, were carried — out as we had made on muscle. All the phenomena found in the latter could be reproduced in the model (see, e.g., figs. 14 and 15) and we were led to conclude that the phenomena are indeed of viscous-elastic origin. If so, we were forced to one very striking conclusion. When a muscle is stimulated its tension develops along a

characteristic curve: when it is released its tension falls to zero and then redevelops along a curve which is identical in shape with that of its original development. (See figs. 9 and 10 above.) If the redevelopment of tension be Upper line, ‘‘long’’ tension; middle line, “‘short’’ tension; lower line, zero tension. Note rise above ‘“‘long’’ tension on stretching, and subsequent gradual disappearance of extra tension. (After Gasser and Hill, 1924.)

determined by purely physical effects in a viscous-elastic medium, we must conclude that the original development of tension is also so determined: the form of the mechanical response in muscle must be mainly conditioned by the elastic state of the stimulated fibres, and by the viscosity of the substance inside them. The change of state produced by the stimulus must occur much before the mechanical response as ordinarily shown, and the latter must follow the internal change, lagging behind it owing to the viscosity of the medium in which events occur. We were led to look for a sudden change of the mechanical condition of the muscle immediately on stimulation, and re-

Fra.16. Recorps or tHe Errect, oN THE TENSION DEVELOPED, OF Stretcuine a Froq’s Sartorrus Muscin at Various Momsnts Dunine a Twitcu. (After Gasser and¥Hill, 1924) quired a means of studying that without the lag introduced by waiting for the ordinary mechanical response. This means was found in the quick stretch of the muscle. If a muscle be given a shock its tension rises, and we should expect the effect on its tension of stretching it to be greatest at the moment when it is most inextensible, probably at the moment when its tension is at a maximum. A muscle was stimulated and then stretched at various

moments after the stimulus: also it was stretched and then stimulated. A sudden stretch followed immediately by a stimulus has no effect whatever on the response: the muscle might equally have been stretched minutes before. A stimulus followed immediately by a stretch, however, produces a very large increase in the force developed. If the stretch be delayed, the effect is less. The effect of the stretch is greatest, the increase in tension produced by it is largest, if the stretch come very early after the stimulus. The muscle is most inextensible a few thousandths of a second after it has been stimulated:

Fie. 17. Diagram or THE ErrsecTt, ON THE TENSION DEVELOPED, OF Srrercuine a Muscie at Various Moments Durine a Twitcu - it is much more extensible later on. This inextensibility _ disappears far more rapidly than the mechanical response. We are led therefore by these experiments to the conmore rapidly than the ordinary external change which is recorded on an isometric lever. We have not proposed any final explanation of these phenomena: they obviously require far more investigation. They do, however, suggest that the fundamental nature of the mechanical response is a sudden and reversible gelation

of some constituents of the fibre: due possibly (as Professor Meyerhof has suggested to me privately) to an electrical discharge of charged colloidal surfaces, followed by a dehydration. Imagine that the effect of a shock is to produce instantly, or almost instantly, possibly by the agency of the lactic acid resulting from it, a coagulation of some element of the muscle fibre analogous to the clotting of blood plasma. To consider a definite model, imagine a thin elastic tube filled with blood plasma, and allow the plasma to clot. In the first place, owing to the clot, the viscosity will be greater after than before: in the second place one knows that a clot tends to shrink when formed, its fibrils contract and tend to express the serum. Our model would tend to shorten if its fibrils ran lengthwise in the tube, but the rate at which it could shorten would depend on the viscous resistance of the fluid which had to pass between the ultramicroscopic meshes of its clot. The internal condition might be very suddenly changed, but the external force could not be manifest until the internal stress had had time to overcome the viscous resistance to the necessary change of form. For remember that no contraction is really isometric; the active portions have to shorten, and the inactive ones to lengthen, before the tension can be fully developed externally. If, however, the muscle as a whole were suddenly stretched out, its new rigidity might be made obvious at once, without having to wait for the slow spontaneous shortening of its active parts. We might be able to detect the rapid onset of the clotting by attempting to stretch the clot. We have, if we are to complete the analogy to muscle, to suppose that the — clotting is reversible and passes off rapidly as soon as the exciting cause is removed. | We did not propose such a model as an actual theory of

contraction: it merely represented the type of mechanism which seemed necessary to explain the observed results. It is possible that we are not correct in ascribing all these curious effects to viscosity. At present, however, we can only say that if it be not viscosity it is something which behaves in a remarkably similar way, a conclusion not unlike the answer to the question of whether it was Homer who wrote the Iliad or whether it was somebody else of the same name. It is obvious in any case that the study of the mechanical properties of muscle is not a sterile subject, and that much remains to be done.

One more remark, this time a piece of almost impertinent speculation. When our viscous-elastic model was stretched if the substance inside it was too viscous the model proceeded Fie. 18. Diagram or SupPosep INTERNAL (“‘FUNDAMENTAL’’) MECHANICAL CHANGE In MuscLEe 4 Broken line, observed external response, lagging, owing to viscouselastic forces, behind the internal change. (After Gasser and Hill, 1924.) to break up into a series of segments, not unlike what happens in a working intestine. It was impossible, without warming up the model, to make it revert to its previous uniform condition. This tendency clearly could have been counteracted by making the model in “water-tight compartments,”’ providing it with cross-partitions to prevent the viscous material from segmenting. May we dare to suppose that the cross-striations of a muscle, the only clue to whose function is that they are most obvious in the most rapidly moving fibres, are there to divide the fibre into “water-tight compartments,” to prevent the irreversible segmentation of its contents?

-* Lupton: The Effects of Speed on the Mechanical Efficiency of Hi ar = Muscular Movement. J. Physiol., 57, p. 337, 1928. = ce! a Mad . Pn et F ats Pel) AAS ¢ ‘ie o°2 CS en a ot at ak Ee ke peed rss a = The chief advances during recent years in our knowledge of the intimate nature of muscular activity have come from a study of its thermal and chemical changes. One of the fundamental characteristics of striated muscle and the one involving the greatest difficulty in investigation, is the great. rapidity with which events take place in it. There is no doubt that ultimately the muscle is a chemical mechanism, in the same way for example as a Daniell’s cell or an accumulator is a chemical mechanism. If we were aware of all the chemical events, we should know all that was necessary about the machine which we are studying. Unfortunately, the investigation of chemical events is a slow and laborious process. Undoubtedly lactic acid, as Fletcher and Hopkins showed some fifteen years ago, is an essential part of the machinery, but it is _ quite impossible to measure the production and removal of lactic acid instantly and contemporaneously, during and after a single unit of muscular response. It is necessary to evoke a long series of responses and finally to study the gross changes of accumulation or removal of the acid. Attempts have been made to follow the chemical processes involved in muscular activity by studying the changes of hydrogen ion concentration by physical instruments. This would appear to be more hopeful than the possibility of quickening up the study of the reactions by ordinary chemical means, but unfortunately—so far—it has been completely unsuccessful. The instantaneous and contemporary study of the events occurring inside the muscle fibre appears to be possible only in two ways, the mechanical and thermal. The mechanical changes, however, are only the end-products of activity, and if we wish to get inside the mechanism, it is necessary to study some intermediate

process, something occurring between the stimulus and the response itself, something associated with the chemical events which evoke contraction. This is provided by the investigation of the heat-production. It is obvious of course that the picture provided by thermodynamics is only a partial one; the certainty, however, with which the principle of the conservation of energy may be applied, gives us firm ground on which to start our investigations, and there would seem to be no doubt that the outline provided by thermodynamics, once established, must remain,

and must finally have the complete chemical picture painted. into it. The advantage of the study of the thermodynamics of muscle is that heat may be measured in absolute units, rapidly and at once, and the time course of its evolution analysed by suitable means. In the study of the thermal changes the most consistent and valuable results have been obtained by utilising the isometric contraction of the sartorius muscle of the frog. That muscle is a very suitable medium for this investigation, in so far as it is practically of uniform cross-section and consists of straight fibres running along its length. The isometric contraction has the advantages, first, that energy is not liberated in it in any other form than heat, so that no complications arise by having to sum the thermal changes with the mechanical work, and second, that in it movements of the muscle over the instruments are prohibited, which (on the small scale of temperature with which we are dealing) is of value in avoiding errors due to temperature differences along the muscle.

The fundamental difficulties in myothermic observation are the smallness of the changes involved and their rapidity. In the muscle twitch of a frog’s sartorius at 20°C. the rise of temperature is not more than 0.003°C., and the time occupied in the earlier phases (as distinguished from the recovery process) is only a few hundredths of a second. The first requisite therefore is a very sensitive thermometric apparatus and great freedom from temperature changes, the second is extreme rapidity and lightness in the recording instruments. Neglecting for obvious reasons the use of ordinary mercury thermometers, there are two possible methods available, those of the resistance thermometer and the thermopile respectively. The resistance thermometer has not been employed successfully in myothermic observations. Calculation with the requisite physical constants shows the existence of a certain fundamental difficulty, which is confirmed by actual experiment. Were it possible to use the resistance thermometer, it would be exceedingly advantageous, as such an instrument may be made very small and light, so that it will respond with great quickness to the temperature changes in its neighbourhood. Moreover, unlimited sensitivity may be obtained by increasing the current in the resistance wire. The fundamental difficulty is that heat is thereby produced in that wire, which is conducted into the muscle and warms it up, causing serious disturbances of the zero, and enormous negative deflections whenever the smallest movement of the muscle occurs. I have attempted recently to use a resistance thermometer for myothermic observations and found it completely impossible. There remains, therefore, only one method, that of the thermopile, which we will now discuss.

It is possible to make a small light thermopile (fig. 1), suitable for myothermic observations, containing one hundred couples of constantan-iron, each providing 52 microvolts for a difference of temperature of 1°C. With a suitable sensitive galvanometer this may give us a scale of temperature in which 1°C. is about 1 kilometre in length. Ample sensitivity, therefore, is available. Such thermopiles are laborious to make by the ordinary method of soldering the wires together, though much of the work hitherto recorded has been done with instruments thus constructed. Recently, however, an ingenious device has

tinuous piece of constantan wire is coated with silver in successive sections by electroplating, and each pair of junctions between a silver-plated and an unplated portion acts as a silver-constantan couple. By this device very small, light thermopiles may be constructed with great ease (fig. 2), and recently several of my colleagues have used such thermopiles in their work. The possibilities now available of myothermic investigation are mainly due to improvements in galvanometers and thermopiles.

Courtesy of the “Journal of Scientific Instruments” Fria. 1. SoupERED THERMOPILE ‘‘Warm’’ junctions down the middle, ‘‘cold’’ junctions on the two sides. HEH, platinum stimulating electrodes; GG, galvanometer leads. (After Hartree, 1924.) It is not practicable to register the changes of tempera- - ture in a muscle as they occur, without lag or loss: the recording instruments are too slow, they possess too great a heat capacity, the flow of heat is not rapid enough and the galvanometer—which must be very sensitive—has too long a period. It is necessary to carry out an analysis of the results obtained, preferably after photographic re-

sibility of analysing the time-course of the evolution of heat in a muscle contraction depends upon certain fortunate physical properties of the system employed. The cit Da u TvBE FoR GAS VULCANITE BLOCK _ y & bes J & a <e Fr ‘ <——— GLASS tKE ———>/h hi ° 4 fe v 8 5 ee) 2 t i z to ‘ 3 . a , THAMOPILE PLATE as i ; é 4 zauv y és a ryt "he : GLASS’ TUBES | = Pi Zz>a FLED 1 eof a NH WAR a & a | $ Zerit Ure > sy esi l= i : Yad 2 “Muscre CLarP 7 pan ae | z= 5 % mer VULCAN IT. pa A.B the | a A eee: Blocks Hw tar 7 & 2 3 h = ic 2 its f - j 3

Fig. 2. Latest Typr or THERMOPILE AND Musciur CHAMBER Modified from Fenn conduction of heat, the relation between E.M.F. and temperature, the movements of the galvanometer, its dynamics, the control on it for small movements, and its ELD FUIELSIIE OF MSE v7 het, UM + W, the terion Wi te Y = 9 % JD +f. Tike property A the sytiees mene Ww be 4 WY Cath nt, wish 6 wacker pda on wbyhe A te ation A hte, Whisls wohl herwan here tnmm thanks amount of heat. A muscle must first be killed by chloroform, and then an alternating current of known strength

may be liberated, and the resulting deflection of the gal- — in absolute units (calories or ergs) of one scale division of | the galvanometer. There are various technical difficulties — in this calibration which need not be discussed now, but in one modification or another it serves as the basis of all will refer later. I wish to speak first rather of the analysis of the timecourse of the evolution of heat in muscle when stimulated. leads to a certain deflection of the galvanometer, y = f(é), which rises to a maximum and falls slowly to zero again owing to conduction away of the heat, ete. A live muscle would give the same shape of deflection, if the heat produced in it were liberated at, or immediately after, the moment of stimulus. In point of fact the “live” deflection differs largely from the “‘control’’ one (fig. 4), a fact which can be explained only by supposing that the heat-production in the live muscle is not instantaneous at the beginning, but distributed in time, there being production or absorption of heat later on. It may be said at once that there is no evidence of an absorption of heat at any stage in the process, so that nothing but production of heat need be taken account of. It is easy enough to produce an apparent absorption of heat, by allowing temperature differences to arise

in one’s recording apparatus (see fig. 5): such observations, the galvanometer deflection persists much longer in a live muscle than in a control experiment: it remains away from its zero for periods up to ten minutes, whereas, if the production of heat were instantaneous at the beginning, it would be back to its zero in two or three minutes. This heat, and I found that this “recovery” heat is appreciable only in oxygen, being abolished by keeping the muscle in

up the oxygen dissolved in the muscle. My original analysis was rough and revealed little but the facts themselves and their order of quantities. It certainly, however, did establish the existence of a “recovery”’ heat-production, which it was natural to associate—rightly, as it has proved since—with the oxidative removal of lactic acid discovered by Fletcher and Hopkins. A rough estimate of the recovery heat production made it approximately equal to the total initial heat. This estimate appeared to answer unequivocally a question long debated, on the fate of lactic acid in recovery. Fletcher and Hopkins had found that lactic acid is removed in the presence of oxygen, though the same muscle at the end of the recovery process can liberate, during exercise or rigor, the same amount of lactic acid as before. Was lactic acid removed simply by oxidation, or by restoration to the precursor from which it came? Previous experiments had shown that the production of one gramme of lactic acid in rigor leads to the liberation of about 500 calories. Experiments by Peters had proved that the production of 1 gramme of lactic acid in exercise, or in exercise followed by rigor, leads to the liberation of about the same quantity of heat. Hence, if the recovery heat were equal to the initial heat, the

conclusion, which, however, was not universally accepted at first, seemed inevitable—that the lactic acid is not removed simply by oxidation. This conclusion has been amply confirmed later by the experiments of Meyerhof. In a large mass of muscle deprived of its circulation, the rate at which the recovery process can go on, after severe stimulation, depends on the rate at which oxygen can reach the fibres by diffusion. Such are the conditions in calorimetrical experiments. The myothermic method, however, employing small thin muscles subjected only to very short stimulation, makes it possible to follow the recovery process under conditions where its rate is quite uninfluenced