(Cambridge and Paul Instrument Co., Ltd) was used. This instrument. increasing the frequency of excitation, up to the limit when complete

Transcription

1 THE ORIGIN OF, THE ELECTRICAL CHANGE IN MUSCLE. BY B. A. McSWINEY AND S. L. MUCKLOW (Platt Physiological Scholar). (From the Department of Physiology, Manchester.) IN 1913 Mines(l) suggested that the liberation of lactic acid is largely the cause of the action current of muscle. In 1914 Hill(2) showed that increasing the frequency of excitation, up to the limit when complete tetanus results, never decreases but almost invariably, sometimes largely, increases the heat-production of muscle. After complete tetanus is reached the heat-production per second is independent of the frequency of excitation. In 1921, Hartree and Hill(3) determined the relation between the heat-production in a tetanus and the duration of the stimulus. They found that in a prolonged contraction there is an extensive outburst of heat as the result of the first moment of the stimulus, and that successive equal periods of stimulation produce progressively smaller effects until the heat produced per second of stimulation attains a constant value. By employing ballistically a high resistance sensitive galvanometer of long period, it is possible to record, for any time of stimulation up to (say) five seconds, the total amount of electricity sent round a circuit connected to an injured and an uninjured spot on the muscle, the injury current being balanced. There is every reason to suppose that the heat-production runs parallel(4) to the liberation of lactic acid. If therefore the supposition be correct that the electrical change in muscle is due to the liberation of lactic acid, then the total electrical change measured as above ought to behave in the same way as the heat-production, if the experiments are performed in the same manner. To test this the following experiments were carried out. Method. A sensitive, moving-coil, mirror galvanometer, type L 115 (Cambridge and Paul Instrument Co., Ltd) was used. This instrument had a long period and a resistance of approximately 2099 ohms. To avoid stray currents the galvanometer was mounted on vulcanite blocks, and all the instruments (with the observer) were insulated by similar means. The muscle chamber was that used by Hill in his earlier experiments on the heat-production of muscle (5). It was fitted with a vulcanite block and a screw clamp. The femoral end of the gastrocnemius muscle was PH. LVI. 26

2 398 B. A. Mc.SWINEY AND S. L. MUCKLOW. pinned down to the block, and the screw clamp held the tendinous end of the muscle. By this means the contraction was made perfectly isometric. A glass cover made the chamber air-tight. A second cover of brown paper was used to exclude light which might otherwise have affected the silver chloride electrodes. In those experiments where readings were taken at different temperatures, a double-walled metal cover, fitted with an inlet and an outlet tube and a hole for the thermometer, was used instead of the glass cover. This metal cover provided a water jacket around the chamber, and the temperatiire of the interior was varied by running water of different temperatures through the metal cover. The air in the interior of the chamber was kept moist by lining the glass or metal cover with strips of blotting paper, soaked in Ringer's solution. Oxygen was passed into the chamber, the gas being first bubbled through a wash bottle containing water. The duration of the stimulus was controlled by a drum with two adjustable arms, driven at a uniform rate by a gramophone motor, as used by Keith Lucas and Hill(6, 7). Platinum electrodes mounted in vulcanite were used to stimulate the sciatic nerve. To lead off the action current from the muscle to the galvanometer, non-polarisable electrodes of different types were tried, and finally silver chloride electrodes were chosen as being the most suitable. Porcelain boot electrodes, as supplied by the Harvard Apparatus Co., were first used. The muscle was connected to the electrodes by a strip of natural wool soaked in Ringer's solution. The wool moving when the muscle contracted caused errors in the galvanometer readings, but this difficulty was overcome and several sets of experiments were carried out. Silver chloride electrodes were then tried and found very satisfactory if kept in the dark. Light however has a deleterious action on them. They were made by passing a strong current into a silver wire (anode) in a solution of sodium chloride, a silver plate being used for the kathode. In the experiments where the duration of the stimulus was varied, an induction coil with a Neef 's hammer was used to stimulate the nerve. In those where thefrequency of the stimulus was varied an eight-cylinder magneto (8) driven by an electric motor was employed. One shock from the secondary circuit of the magneto was obtained, with every four revolutions of the magneto, at each of the eight segments of the distributor. By connecting up leads to 1, 2, 4 or 8 segements of the distributor, 1, 2, 4 or 8 shocks could be obtained at equally spaced intervals. The speed of the magneto could be varied by altering the belt on the pulleys. The current from the magneto was passed through a potentio-

3 ELECTRICAL CHANGE IN MUSCLE. 399 meter in order to control the strength of the stimulus. From this it passed into a transformer, which insulated the stimulating circuit from direct electrical connection with the magneto, and so helped to avoid electrical disturbances. It was then transformed again, a make and break key (K2) being placed in the primary circuit of this second transformer and the nerve being stimulated by the current from the secondary. A key (K1) was placed in the secondary circuit as a short-circuit key, the keys K1 and K2 forming part of the contact breaker, so that the interval between the opening of K1 and K2 determines the duration of the stimulus. The tendinous end of the muscle of a fresh gastrocnemius-sciatic preparation of Rana temp., was immersed in boiling Ringer's solution in order to obtain a monophasic electric response. By means of the screw adjustment the muscle fibres were stretched so that when stimulated only a minimal movement should occur. The two non-polarisable electrodes of the galvanometer circuit were hooked on, one electrode to the uninjured end and one to the injured end of the muscle. After it had been covered up the preparation was allowed to stand for half-anhour, so that it might reach a steady state, and then the injury current balanced. OAVNOfETER FRO K2. 1lAGNETO TR5FORTIER. STI FIULATING ELECTRODES.?OTEtlOMlETER ELETOE 100 Fig. 1. Fig. 2. Fig. 1. Additional connection to wiring diagram given earlier (8) for experiments on frequency. Fig. 2. Diagram of galvanometer circuit. 26-2

4 400 B. A. McSWINEY AND S. L. MUCKLOW. In the experiments in which the duration of the stimulus was varied readings were taken with increasing periods, and repeated with decreasing periods of stimulation, and an attempt made to eliminate the effects of fatigue by taking the mean of the two readings of any one period. In experiments in which the frequency of the stimulus was varied, the arms of the contact breaker were set to give a stimulus of approximately one second. A connection was made with one segment of the distributor of the magneto, so that only one shock in eight was given to the nerve. The nerve was stimulated and the reading of the galvanometer taken. Readings were then taken using two, four and eight segments of the distributor. The speed of rotation of the magneto was measured by a revolution counter and stop watch. The magneto was driven at different speeds by changing the belt on the pulleys, readings at each speed being taken from the galvanometer. In every experiment, readings were taken first with increasing and then with decreasing frequencies, and the mean taken of the readings at any one frequency, to eliminate the effects of fatigue. The experiments described above were repeated at various temperatures. After a change of temperature, the muscle was allowed to remain at rest for 30 minutes so that its temperature might approximate to that of the chamber. In all experiments the results are given in arbitrary units for galvanometer deflection. The relation between the duration of the stimulus and the total electrical change. The experiments of which Fig. 3 is typical were repeated at various temperatures as shown in the upper curve of Fig. 3. In all cases the total electrical change was proportional to the duration of the stimulus. It was found that the muscle, which had necessarily to be injured at one end, died too rapidly to allow experiments to be made on one muscle at more than one temperature. Different muscles therefore were used in these experiments, so that nothing can be gained from a comparison of the slopes of the line at different temperatures. Hartree and Hill(3) investigated the relation between the total initial heat production and the duration of the stimulus under similar conditions, and a comparison of the curves they obtained with those in Fig. 3, will show that the two sets of curves bear little resemblance to one another. The heat-production is not a linear function of the duration of the stimulus though it becomes linear if the stimulus is continued; the linear portion however, if produced backwards, does not pass through

5 ELECTRICAL CHANGE IN MUSCLE. 401 the origin. As there is every reason for assuming that the heat-production is proportional to the production of lactic acid, it would appear so M30 ;2O0 I0 0 Z V I49 DURATION OFSTItlULUS IN SECONDS. Fig. 3. Temp. 15 C. Lower curve. Experiments in October and November. Upper curve with ordinates 0 to 50. Experiments in January and February. therefore, in contradiction to the hypothesis suggested by Mines (1), that the electrical changes which occur in a muscle on contraction do not bear any quantitative relation to the production of lactic acid. The relation between the frequency of stimulation and the total electrical change. Within the limits dealt with in these experiments, the total electrical change in a stimulus of fixed duration is proportional to the frequency of stimulation. Typical results are shown in Fig. 4. Hill(2) found, under similar conditions, that increased frequency of stimulation caused increased heatproduction up to the limit when complete tetanus resulted-beyond this point however, which occurs well within the range of the linear relation of the total electrical change to frequency, the heat-production was independent of the frequency of stimulus. Hence the heat-production

6 402 B. A. McSWINEY AND S. L. MUCKLOW. and the total electrical change do not bear the same relation to the frequency of stimulation, therefore the total electrical change is not,timuli [ER5[COND. Fig. 4. Temp. 15 C. Period of stimulus 1 second. November and January. quantitatively proportional to the production of lactic acid. The linearity of the relation between the total electrical change and the frequency of stimulation is not affected by temperature, and gave similar results to Fig. 3. CONCLUSIONS. By employing a sensitive galvanometer of long period, connected to an injured and an uninjured spot on a muscle suspended isometrically and stimulated through its nerve, it is possible to sum the electrical effects accompanying a prolonged contraction. If as Mines suggested the electrical change in muscle be caused by the production of lactic acid, the summed electrical effect should bear to (a) the duration and (b) the frequency of the stimulus, the same relation as does the production of lactic acid. On many grounds the production of lactic acid in

7 ELECTRICAL CHANGE IN MUSCLE. 403 muscle may be expected to run parallel to that of heat: we show experimentally that the relations between the total electrical change and (a) the duration, (b) the frequency of the stimulus are different from those established by other authors between the heat-production and the same two variables, and we conclude that the production of lactic acid is not the prime cause of the electrical change. Our best thanks are due to Professor A. V. Hill for his help and suggestions. The expenses of this research have been borne in large part by a grant from the Royal Society. REFERENCES. (1) Mines, G. R. Journ. of Physiol. 46, p (2) Hill, A. V. Ibid. 47, p (3) Hill, A. V. and Hartree, W. Ibid. 55, p (4) Hill, A. V. Physiol. Reviews, 2, p (5) Hill, A. V. Journ. of Physiol. 46, p (6) Hill, A. V. Ibid. 46, p (7) Hill, A. V. Ibid. 54, p (8) McSwiney, B. A. and Mucklow, S. L. Journ. of Physiol. 56, Proc. Physiol. Soc. p. xxvii