Muscular Activity

If, however, we consider how the heat-production— not its ratio to the force developed—varies with the time, we find a relation which is not strictly linear, but which becomes linear after a short interval (fig. 2). Heat is liberated continuously at a constant rate as the stimulus is prolonged. There is, so to speak, a steady stream of energy - outwards, and nothing that one can do—save raise the temperature—can increase that stream. Increase the strength of the tetanising current, increase the frequency of stimulation, and the rate of heat production during a_

prolonged contraction remains the same. There is clearly some path of limited dimensions down which must pour the stream which represents the conversion of glycogen into Fig. 2. Renation Betwenn Huat-propuction, Duration or STIMULUS The large diagram shows the curves up to 2 seconds on a small scale: the small diagram shows the initial shape of the curves up to 0.05 second, on ten times the scale. Actual observations shown by dots. (Hartree and Hill, 1921.)

sodium lactate: that path presumably is a chemical one since it is widened by a rise of temperature. Were the rate at which energy can be liberated dependent simpl; on the permeability of a membrane to certain chemical bodies, a temporary rise in permeability being effected by each stimulus, then the rate should not be much increased by a rise of temperature. It would seem rather that the reactions released by stimulation have to pass along a certain chemical channel of limited dimensions, limited that is to say by the amount of certain intermediaries or katalysts available. Each stimulus causes a momentary opening of the channel, stimuli applied with a sufficient frequency to obtain muscular summation leave it open for a sufficient proportion of the time to allow all the energy to escape which can escape. A rise of temperature widens the chemical channel, diminishes the scale of time, and allows the energy to escape more rapidly. The rate of heat liberation during a prolonged stimulus rises as an exponential function of the temperature, it has the usual type of temperature coefficient.

What the reactions may be which regulate the speed at which energy is liberated by muscle we do not know. Glycogen dissolved in water does not, of itself, change into lactic acid: presumably the reaction must proceed by virtue of some agency which—because of our ignorance—we - describe as a katalyst. In an enzyme action, under certain circumstances, the speed may depend, not so much on the concentration of the reacting bodies as on the amount of enzyme present. So, in muscle, the speed of the transformation of glycogen to lactic acid may depend upon the amount of certain agencies or bodies which assist the reaction. .

There is still another clue available in the thermodynamic relationships. During the initial outburst of heat following the commencement of the stimulus the rate of heat-production is far greater than it is finally during the steady state. The speed of the liberation of energy, and presumaply of lactic acid, is very rapid at first and tails off gradually to its final constant value. It is clear that our analogy of a channel along which the reaction can proceed is not complete: there must in addition be some quite considerable

passed—so to speak—through the channel, which is released—without further ado—as the stimulus commences. Stop the stimulus, block the output, and the store is replenished ready for the next shock. It is tempting to suppose that there is a “ready-store’’ of some intermediate body, e.g., of hexose-diphosphate, intermediate between — glycogen and lactic acid. This ‘“‘ready-store”’ is thrown into action by the stimulus, and is released at a high speed; as soon as it begins to be depleted the process of restoration gets under way, and glycogen is broken down to reform the intermediary: during prolonged anaerobic stimulation the rate of release is balanced—indeed is determined— by the speed of restoration. These processes we cannot at present define, but that some such processes occur is

When a stimulus is applied some kind of door is opened, energy is released, lactic acid is set free. In a prolonged stimulation the rate of the release is determined by the rate at which the ready store of energy can be refilled. In a single twitch, with a ready store of energy several times exceeding the total amount required, the rate of restoration does not affect the result: the energy set free during a bodies, whose escape results in the reactions, which end in the formation of lactic acid and in the mechanical response. The duration and degree of this change in permeability are determined by a variety of factors. Normally, even

at rest, there is production of lactic acid and heat, presumably the impermeability is not absolute. The degree of permeability—small though it be—determines the basal metabolism of the muscle. In a twitch the permeability rapidly rises and rapidly falls: perhaps its cycle of changes is an accompaniment of the action current. The output of energy in the twitch depends upon the duration of its alteration. It was likely that this rise of permeability would prove to be more prolonged at a lower temperature, so allowing time for more energy to escape. This expectation was confirmed by the observation that the heat-production in a single twitch is greater at a lower temperature. In a prolonged stimulation the heat-production is greater at the higher temperature, and it is difficult to explain the opposite effect for a twitch except on some such lines as these.

Again, stretching a muscle initially causes a more prolonged contraction, a greater heat production. If, as Fulton emphasises, lactic acid is freed by a wave of permeability passing along a polarised membrane, permitting in consequence the reactions of the intermediary body present in the “ready store,” the duration of the wave at any one point will determine the amount of lactic acid produced. It is very suggestive that the ‘“chronaxie” of __ rouscle is found to increase with tension, as is that of nerve.

Curare, also, according to the Lapicques, increases the chronaxie of muscle, thus rendering it unstimulable by its usual nerve impulse. Hartree and I found that curare increases the heat production of a muscle responding to a single shock, and suggested that curare may prolong the rise of permeability following the stimulus. Curare also prolongs the twitch, much as does an increase in the initial tension. All of which confirms our view that, in a twitch, the amount of energy released depends upon the duration of the primary effect—a change of permeability or whatever it may be—of a shock.

Other factors influence the release of energy. In a muscle treated with veratrine, about 0.002 per cent, there is a prolonged liberation of heat accompanying the prolonged contraction resulting from a single shock (fig. 3). The rate of liberation of heat runs exactly parallel to the force maintained—so much tension, so much heat-production per second: and not only that, but the relation of heat-production to tension is precisely the same as in normal tetanised muscle. There is no truth whatever in the conception that veratrine has its effect by delaying relaxation: it works by affecting the duration of the liberation of

energy following a shock. In the normal muscle there _ is some “‘passage’’ of escape for the energy and lactic acid, which is opened by a shock and then rapidly closes again. In a muscle subjected to a large dose of veratrine the instability of the galvanometer zero shows that the passage is so nearly open that small spontaneous outbursts occur. With a weaker dose, the “passage,” once opened by a shock, remains open for some time, allowing considerable quantities of lactic acid to pour out, with a prolonged maintenance of tension and a prolonged production of heat. The effect of veratrine may be largely antagonised by Ca salts, from which it may be argued that its action lies in a change produced in some membrane. However that may be, there is no doubt of the existence of some mechanism regulating the duration of the release of energy, and of the action of veratrine thereon.

Again, caffein applied to a muscle gradually causes a state of contracture, a liberation of heat, a production of develop does not depend upon the gradual diffusion of eaffein into the muscle substance. A few minutes subjection to a dilute solution of the drug is enough, and after that, in a period of several hours, the release of energy occurs. The same total amount of energy may be liberated, either in oxygen or in nitrogen, by discontinuous stimulation over the same period, instead of subjection to caffein (fig. 5). The action of caffein on muscle is merely to

release, slowly and continuously, the chemical processes, anaerobic or oxidative, normally induced, suddenly and - discontinuously, by stimulation. Presumably, therefore, in terms of our permeability hypothesis, the action of caffein is to induce a permanent and slight impairment of — the resting impermeability of the membranes, which — normally restrain the reactions producing activity in muscle. Still a simpler case may be considered: the complete removal of Ca and K from the fluid bathing a muscle often causes after a shock (at any rate in Rana temporaria), — a prolonged contracture not unlike a weak veratrine con- ©

Fia. 4, HEAT-PRODUCTION (GALVANOMETER DEFLECTION) IN MuscLE IN NITROGEN, AFTER TREATMENT WITH 0.050 To 0.075 Per Cent CarrriIn creased, here again an analysis of the heat-production shows The absence of Ca and K results in a prolongation of the interval of altered permeability. The argument has been advanced that muscular contraction must be due to surface action, since the force developed in a twitch diminishes with a rise of temperature —which is what surface tension does. This argument is invalid. The relation of tension to heat production in a twitch is independent of temperature, a given liberation of lactic acid produces, at all temperatures, the same rise of tension. The system, moreover, is certainly never in a

Fie. 5. Heat-propuction (GALVANOMETER DEFLECTION) IN MuscLE Excitep DisconTINUOUSLY TO CoMPLETE FatTiauE By A Lona Suc- CESSION oF SHoRT Txrranic Stimuti Every Hair Minute The scale of time is twice as great in the nitrogen experiment. Each stimulus causes a deflection of the type shown in the small diagram inset; in each experiment the upper curve (‘‘max.’’) is drawn through the tops of the successive deflections, the lower curve (‘‘min.’’) through their bottoms, the middle curve (“‘mean’’) approximately half way between the other two. (Hartree and Hill, 1924.)

state of reversible equilibrium; hence no such thermodynamic arguments can be applied. It was possible, nevertheless, in spite of the fallacy of this reasoning, that a change in surface tension was the cause of the muscular response. The following calculation shows that this is only substance known to be formed; and that substance must presumably be spread upon the surface where it interesting calculation results. In the twitch of a striated muscle the ratio H/Tl (where H is heat-production (measured in ergs), 7 is force developed in dynes, J is length in centimetres), is fairly accurately about 1/5.5. The amount of lactic acid liberated, per calorie, in the initial anaerobic process is 1/296 = 0.0034 gram, since 370 calories per gram of lactic acid are produced in the total anaerobic process, and of this 1/5 is delayed. ‘Thus,

per erg of heat liberated, the amount of lactic acid formed — Now, to produce a force of one dyne in one centimetre length of muscle, the initial anaerobic heat-production, as shown above, must be 1/5.5 erg, accompanying which must of lactic acid. Now one atom of hydrogen weighs 1.66 x 10-*4 grams, one molecule therefore of lactic acid 90 x 1.66 x 10-* = 149 x 10-* grams. Thus almost exactly 10" molecules of lactic acid are required to cause the development of a tension of 1 dyne in 1 em, length of muscle. So far we are on certain ground, direct calculation from fairly exact experimental facts. Let us advance further by the hypothesis that this lactic acid is spread out in a layer one molecule thick.

attribute the same density to the lactic acid molecule. Its volume then will be 1.49/1.25 x 10-” = 1.19 x 10 ce. Let us imagine, for simplicity of calculation, that the molecule is cubical in shape: then the area of one of its faces? is (1.19 x 10-%)2/* = 2.42 x 10-% sq. cm. Hence 10“ molecules would occupy an area of 2.42 x 10-4 sq. em. Now, by hypothesis, our muscle is 1 cm. long exerting a force of one dyne. If that force be due to surface tension the surface energy of the film causing it is 1 dyne x 1 cm. = lerg. But the energy of any surface is:

per centimetre. Hence, on the hypothesis that our lactic acid forms a monomolecular layer, the rise of surface tension produced by it must be 4100 dynes per centimetre. This is about 55 times the surface tension of water, 178 times the surface tension of alcohol, 74 times the surface tension even of mercury: clearly quite an impossible quantity. It is obviously unreasonable, therefore, to regard a monomolecular layer of lactic acid as causing a change in surface tension large enough to explain the actual force developed by a muscle. With a layer thicker than a monomolecular one the difficulty is only enhanced. It is hard to imagine a change to occur in surface tension much larger than that between water and paraffin oil, say 50 dynes per centimetre. To obtain the requisite surface energy we must then distribute our lactic acid molecules until only about one eightieth part of the whole surface ts occupied by them. It is difficult—indeed impossible—to suppose that a change in surface tension, as large as 50 dynes per centimetres, could be caused by molecules of lactic acid dotted at such wide intervals about the surface. Clearly the surface tension hypothesis involves us in grave perplexities.

2 Just the same area for the cross section of a fatty acid molecule, floating on water in a monomolecular layer, is given by the experiments of N. K. Adam, effect in question. Something must produce the effect, — however, something in a layer presumably not less than one sponsible agent it must be produced in the twitch in amounts © efficient of surface tension. No such body, however, is known: its existence would seem extremely improbable. If so, therefore, we may conclude that a change in surface tension is not, and cannot possibly be, the cause of muscular contraction. ;

If surface tension be not the cause we seek, it may still be possible that the mechanical effect is ultimately a surface one, and due for example to the neutralisation of an — electric charge. A surface endowed with o electrostatic — of specific inductive capacity K, is subject to an electro- — metre: with ¢ = 10 it amounts to 7.8. Now 10" ions carry 10" xX 4.77 X 10-19 = 47.7 electrostatic units of charge, so that if the whole of the charge were disposed in an area of 1 sq. em. it would lead to a “pressure” of 178 dynes

smaller than 1 sq. cm, the force would be proportionally greater. It is impossible to calculate the force exerted by such a charge without knowing the details of its disposiin the potential ions of lactic acid, if suitably disposed, . could yield forces quite adequate to explain the mechanical response of muscle. substances in the form of a combination with inorganic bodies: often these protein substances are sodium-protein salts. We suspect, moreover, that lactic acid, when formed during activity, is set free in high local concentration at surfaces from which it passes away rapidly by diffusion and neutralisation. During its sojourn in the immediate neighbourhood of the sodium-protein membranes, or surfaces of muscle fibres or fibrils, it might well cause—owing to its ‘‘mass’”’ effect—a de-ionisation of the protein according to the scheme

This reaction we know normally liberates a large amount of heat (Meyerhof): under other circumstances it might perhaps yield a large amount of mechanical energy. Imagine that the protein surfaces exist at rest under their own intrinsic tension, balanced by the electrostatic repulsion _ of their individual elements of charge. Then a sudden de-ionisation of their surface would lead to a disappearance of those electric charges and their consequent repulsion, and as in a capillary electrometer the tension manifested externaliy would rise: to fall again as soon as the acid had diffused away and been removed by neutralisation. In a general way this theory is not prohibited by simple numerical facts, as is that of surface tension: there may be other objections to it, and the precise nature of the reactions which result in contraction and relaxation cannot but remain, for the present, a matter of speculation.

To pry into the chemical nature of the actual processes of life is very attractive—but very hard. It is so difficult to get inside the living cell without destroying its structure and stopping its machinery. Yet we may be encouraged by the history of chemistry and physics. For long no direct evidence of any kind was forthcoming, as to the actual nature, or even the existence, of atoms, molecules and chemical structure. Their objective presence was

sometimes doubted, the theories built up around them ~ seemed to work, but it was open to anyone to believe that these were only useful hypotheses, useful but not ultimately true. Yet of recent years the work of physicists and — chemists, of Thomson, Wilson, Perrin, Rutherford of atoms, molecules and stereochemistry, are based upon real objective fact: we can see the a-particles forming a streak of cloud: we can study the molecular impacts of Brownian movement: we can weigh, as Adam does, the repulsions of fatty acid molecules swimming on the surface of water, with their —COOH groups pointing downwards. — So it will be with physiology, though our way be much

harder and longer than that of those who deal with such simple things as molecules, atoms and electrons. Fuiercuer: Survival Respiration of Muscle. J. Physiol., 23, p. 10, 1898. Fietrcuer: Influence of Oxygen upon Survival Respiration of Muscle. J. Physiol., 28, p. 354, 1902. Fietcusr: Relation of Oxygen to Survival Metabolism of Muscle. J. Physiol., 28, p. 474, 1902. Fietcuer: Osmotic Properties of Muscle and Their Modifications in Fatigue and Rigor. J. Physiol., 30, p. 414, 1904. FiercHer AND Hopkins: Lactic Acid in Amphibian Muscle. J. Physiol., 35, p. 247, 1907. FLETCHER AND Horxins: Croonian Lecture. The Respiratory Process in Muscle and the Nature of Muscular Motion. Proc. Roy. Soc., 89 B, p. 444, 1917. R. A. Peters: The Heat Production of Fatigue and its Relation to the Production of Lactic Acid in Amphibian Muscle. J. Physiol., 47, p. 243, 1913. Meryeruor: Atmung der Froschmuskulatur. Pfliigers Arch., 175, p. 20, Meryeruor: Verbrennung der Milchsiure in der Erholungsperiode des Muskels, Pyhigers Arch., 175, p. 88, 1919. Mryerunor; Energieumwandlungen im Muskel. Pfligers Arch. I. Beziehung der Milchsiure zur Warmebildung und Arbeitsleistung in der Anaerobiose. 182, p. 232, 1920. II. Schicksal der Milchsiure in der Erholungsperiode. 182, p. 284,

A.V. Hitt anp O. Muyeruor: Vorgiinge b. d. Muskelkontraktion. Hrgebn. Fostmr anp Moy.e: Interconversion of Carbohydrate and Lactic Acid in Muscle. Biochem. J., 15, p. 672, 1921. EmsppEn AND Coworkers: Untersuchungen iiber das Lactacidogen u. ii. d. Bedeutung der Phosphorsiure fiir Muskeltatigkeit und Leistungsfahigkeit. Zeitschr. f. physiol. Chem., 113, pp. 1-312, 1921. Emspren snp Lawaczecxk: Bildung anorganischer Phosphorsaure b. d. Kontraktion. Biochem. Zeiischr., 127, p. 181, 1922.

Laquer. Bildung von Milchsiure and Phosphorsdure im Frosch muskel — Zettschr. f. physiol. Chem., 93, p. 60, 1914. Furusawa: Muscular Activity and Carbohydrate Metabolism in the Normal Individual. Proc. Roy. Soc., 98 B, p. 65, 1925. Futon: Effect of Initial Tension on the Magnitude and Duration of the Mechanical Response. Proc. Roy. Soc., 96 B, p. 475, 1924. It is a common observation that after exercise breathing is deep and rapid—that recovery is necessary. The same process must occur as in the isolated muscle, complicated in this case by the presence of the respiratory and circulatory mechanism. It can be shown, though by less direct evidence, that lactic acid occurs in human muscles as it occurs in isolated amphibian muscles. It had not been realised, however, till recently, how extensive are the changes connected with the lactic acid which is produced in the human body during exercise. When an athlete is running as fast as he can, about 4 grams of lactic acid per second are being liberated in his muscles. About 130 grams may be produced before exhaustion is complete. There are several signs of this lactic acid in the body. It occurs in the urine, though only in small amounts, after prolonged hard exercise. In the blood of a resting man there is 0.01 to 0.02 per cent; during and after severe exercise there may be as much as 0.12 per cent, or under exceptional circumstances, 0.20 per cent. This may be estimated directly by chemical analysis, or it may be inferred, as Barcroft did, from the change it produces in the oxygen dissociation curve. There is a very high respiratory quotient during, and especially just after, severe exercise (see fig. 1); in our experiments we have found values as high as 2; the lactic acid raises the hydrogenion concentration of tissues and blood; this stimulates the respiratory centre, with the result that CO, is eliminated

the respiratory quotient falls to very low values as the 37 is lactic acid is removed. It may remain for considerable periods below the value corresponding to the oxidation of fat (fig. 2). The chief sign, however, of the amount of lactic acid present in the body after exercise is the magnitude of the so-called ‘‘oxygen debt.’”’ A muscle which has been active requires oxygen to carry out the combustions needed to provide the energy for the restoration process. The oxygen used in recovery is a measure of the magni-