effected readily by switches provided. Throughout the course of the

Transcription

1 : THE ELECTROMYOGRAM OF THE STRYCHNINE TETANUS IN THE GASTROCNEMIUS OF THE FROG. By D. H. SMYTH. From the Department of Physiology, Queen's University, Belfast. (Received for publication 4th December 1935.) WHILE using the strychnine tetanus of the gastrocnemius of the frog in the investigation of the electrical manifestation of various types of muscular activity, electromyograms were obtained which are of interest for two reasons. Firstly, certain facts appear from these records which do not support recently published results, but rather agree with much older views; and secondly, the records differ from the hitherto published electromyograms of the strychnine tetanus in that they were obtained by the use of a valve amplifier and moving-iron oscillograph, while the other records referred to were made with the capillary electrometer or string galvanometer. Some of these electromyograms of the strychnine tetanus in the gastrocnemius of the frog are therefore reproduced here, with a brief description of the apparatus used and a discussion of the results obtained. APPARATUS USED AND EXPERIMENTAL PROCEDURE. The valve amplifier and galvanometer were designed and constructed by Mr. J. Wylie of the Physics Department, Queen's University, Belfast. The amplifier follows the arrangement of Adrian [1931], and has available five stages of amplification. The first four stages are provided by indirectly heated all-metal Marconi Catkin valves, Type M.H.4, while the output stage consists of four pentodes (Marconi P.T.2) in parallel. The inter-valve coupling was wired for both resistance capacity and direct coupling, the change from one to the other being effected readily by switches provided. Throughout the course of the experiments described in this paper only three stages of amplification were used, including the pentode stage, and resistance capacity intervalve coupling was used throughout. The galvanometer is of the Blondell moving-iron type with photographic recording. As this type of instrument is not in general use for biological work, a few words of description may be given. The moving part of the oscillograph is a small piece of iron held in tension in a magnetic field by means of a bifilar suspension. The suspension is maintained by a double strip of phosphor bronze, the two sides of the

2 6 Smyth strip being 3 mm. apart. The phosphor bronze used has a breaking strain of 600 g. weight, the tension actually used is 200 g. weight on each side of the strip. The moving iron is iuaintained in an equilibrium not only by the tension of the fibre but also by a fixed magnetic field provided by a permanent magnet. Movement of the iron is caused by a varying electromagnetic field produced by two coils placed one on each side of the piece of iron, with their axes at right angles to the plane of the iron, i.e. so that the magnetic field is at right angles to the permanent magnetic field. The electrical resistance of the coils is 3000 ohms. These coils are connected in series aswith the anode circuit of the pentodes, and fluctuations in the current flowing through these coils will cause an angular displacement in the position of the piece of iron. It can be shown that as long as the deflection is small the angular movement is proportional to the strength of the auxiliary magnetic field. and therefore to the current flowing through the coils. Damping of the movement of the iron is effected by medicinal paraffin. The movement of the iron is indicatel by a small mirror attached to it. Light from an arc lamp, after passing through a cooling bath, is focussed by means of a biconvex lens on to the mirror. and reflected on to the cylindrical lens of a recording camera. An isometric lever was arranged so that the mechanical change ill the muscle could be recorded simiultaneouslv with the electrical change, both appearing together on the same record. Preliminary experiments xinere miade to test the performance of the amplifier and galvanometer, and the following points were determined. It is not possible to obtain an absolutely steady base line, but the oscillatory disturbance in this has a small amplitude in relation to the voltages to be recorded afterwards in the biological work. The response of the system to known test voltages applied is a linear one within certain limits. These limits included all the work described in this paper. The system was found to be capable of recording frequencies much higher than any afterwards examined. Adequate damping is provided by the paraffin. The procedure used in the experimental work was as follows. Winter frogs were used (Rania ternporaria). Anesthesia was produced by pithing the brain, the spinal cord being left intact. Strychnine sulphate 1-5 y per g. body weight was injected into the dorsal lymph sac. The gastrocnemius was then exposed and freed, the tendo Achilles divided and attached to the isometric recording lever. Two hypodermic needles, size 12, inserted into the muscle belly about 2 cm. apart served as electrodes. Leads from these were taken to the input of the valve amplifier. When the galvanometer reading had become steady a strychnine tetanus was elicited by touching the frog with an insulated rod, and a record was made of the accompanying mechanical and electrical change in the gastrocnemius.

3 Electromyogram of Strychnine Tetanus in the Frog By this means a number of records were made of the strychnine tetanus under various conditions. First, a series of records were obtained of freshly strychninised unfatigued frogs at room temperature. Secondly, records were made of the tetanus after warming and cooling the muscle and the spinal cord respectively. The warming and cooling was done by the application of saline at a suitable temperature, the femoral artery being tied to prevent as far as possible equalisation of temperatures of the different parts of the body. Thirdly, expeiiments were made with the muscle in various stages of fatigue and recovery from fatigue. The results of these experiments are described below. RESULTS OBTAINED. In all cases the increased tension of the muscle during the tetanus is accompanied by an electrical disturbance of an oscillatory nature, which continues throughout the duration of the mechanical change. The general form of the electromyogram follows closely that: of the meclianogram. This is illustrated in fig. 1, where it is seen that after 7 i FIG. 1. Electromyogram of the Strychnine Tetanus. Upper record Mechanogram. Lower record Electromyogram. Time in i sec. a rather irregular initial disturbance there follows a series of oscillations beginning at a rate of about 10 to 12 per sec. and gradually slowing down to 3 to 4 per sec. It is seen that the frequency of these oscillations diminishes markedly toward the end of the tetanus and the amplitude of the oscillations diminishes only slightly. A more intimate knowledge of the events taking place can be obtained by recording the electromyogram with the camera driven at a faster rate, so that the record is more spread out. When this is done it is obvious that each oscillation seen in fig. 1 really consists of a group of fine oscillations which have a frequency of about 100 per sec. Fig. 2 illustrates this. It is also seen that in between these groups of oscillations there is no electrical activity. These results confirm very clearly those obtained by Buchanan [1901] with the capillary electrometer. Buchanan

4 8 Smyth recognised that the so-called tetanus produced by strychnine really consists of a series of very short tetani. The results of the experiments dealing with the effect of heat and cold on the electromyogram also confirmn Buchanan's view that the FIG. 2. Electronmyogramn of the Strychnine Tetanus showing groups of oscillations. Upper record Mechanogram. Lower record Electroinyogram. Time in 1 sec. frequency of the groups of oscillations is dependent on the condition of the spinal cord, while that of the oscillations is dependent on the local condition of the muscle. One set of results is given in the accompanying tables, which show the frequency of the groups and of the oscillations in each group at certain temperatures of the leg and the spinal cord. TABLE I. EFFECT OF ALTERING THE TEMPERATURE OF THE LEG WHILE THE TEMPERATURE OF THE CORD IS AIATNTAINED CONSTANT. Temp. of leg. Frequenc'- of Frequency of (froulps. oscillations. 34' C. 16 C. 5 0 C. 11 per sec. 1(),, 159 per see. 114, 95,,.. TABLE 1I. EFFECT OF ALTERIN(G THE TEMPERATURE OF THE SPINAL (CORD -WHILE THE TEMPERATIURE OF THE LEG IS MAINTAINED CONSTANT. Temp. of spinal cord. Frequency of groups. Irequenev of oscillations. 340 C. 18 C. 5o C. 11 per see. 8,. 6.,I i 150 per sec. 145,- 140,, 1

5 Electromyogram of Strychnine Tetanus in the Frog On these findings Buchanan based the hypothesis that in the strychnine tetanus in the frog, impulses are sent out from the cord at a frequency corresponding to that of the groups of oscillations in the electromyogram; thus the frequency of the groups was dependent on the condition of the spinal cord, so that raising the temperature of the cord caused an increase in the frequency of the groups, while lowering the temperature reduced the frequency. As each impulse arrived at the muscle from the cord, the muscle responded by going into a short tetanus. The frequency of the oscillations in this tetanus was a property of the intrinsic muscular mechanism, and therefore experimentally it was found to vary with the local temperature of the muscle. Lorenz [1927] carried out further investigations on the subject, using the string galvanometer. He agreed on general lines with the Buchanan view, but sought to expand the theory. He obtained evidence not only of a rhythm of groups of oscillations dependent on the spinal cord, and a rhythm of oscillations dependent on the local condition of the muscle, but in addition he found a third rhythm which was seen in each group of oscillations. This rhythm had a frequency of about 20 per sec.; it was manifested by large oscillations occurring at regular intervals in each group: the frequency of this rhythm was dependent on the condition of the spinal cord. Lorenz interpreted this third rhythm as being a property of each separate discharge from the cord. Each discharge was therefore held to be really a group of discharges. When each group of discharges arrived at the muscle, this responded by its own peculiar rhythm of oscillations, superimposed on that of the discharge. Lorenz also obtained evidence of a more or less continuous electrical oscillatory activity of the muscle even between what he calls the "periods," i.e. the groups of oscillations. With these points in view I obtained and examined a large number of electromyograms of the strychnine tetanus, but I have been unable to find confirmation of Lorenz's view. It is true that in each group some of the oscillations are larger than others, but the large ones do not occur at any regular intervals, nor would it be possible to fit them in with a definite rhythm. No records show any evidence of a rhythm which could be interpreted as being a rhythm occurring in the separate discharges from the cord. Also, as far as can be seen, there is almost always complete rest in between the groups of oscillations. An occasional exception is sometimes seen at the beginning of a tetanus where the first few groups may be connected by small irregular oscillations. This, however, is rare, and the usual rule is sharply marked of groups of oscillations with complete rest of activity between. It might be thought that the differences between Lorenz's records and those of my own might be due to distortion by the oscillograph. In order to have some evidence on this point, several records were made with a Matthew's oscillograph. Such a record is seen in fig. 3. This 9

6 10 Smyth agrees in its essential features with those obtained by the Blondell oscillograph. All my findings, therefore, fit in with the unmodified Buchanan theory, and no confirmation of Lorenz's views has been obtained. A number of experiments were also carried out on the effect of fatigue on the strychnine tetanus. These are not in the nature of a complete or thorough investigation, but a few observations have been made which FIG. 3.-Electromyogram of the Strychnine Tetanus recorded with Matthew's Oscillograph. may be of interest. It is well known that if a strychninised frog is repeatedly stimulated it ultimately fails to respond to further stimulation. Before the animal fails to respond the spasms are much less severe than in the fresh animal. This state where the animal no longer responds I have called "complete fatigue." If such an animal be allowed to rest it reaches a state again when spasms can be elicited. Studies were made of the electromyogram in the gastrocnemius during the tetanus elicited in various stages of fatigue and recovery from " complete fatigue." From a series of records of this kind it was noted that increasing fatigue is accompanied by (1) progressive decrease in the duration of the tetanus; (2) progressive diminution in the initial frequency of the groups of oscillations; (3) progressive diminution in the amplitude of the oscillations: while recovery from fatigue is associated with (1) progressive increase in the duration of the tetanus; (2) increase in the initial frequency of the groups of oscillations; (3) increase in the amplitude of the oscillations. From (1) it is suggested that in all cases the tetanus is brought to an end by fatigue of the animal. Where does the fatigue phenomenon occur, is it a central or a peripheral fatigue? If we examine the electromyogram of a tetanus in a fresh frog it is seen that during the course of the tetanus the frequency of the groups gradually slows down from per sec. to 3-4 per sec. When

7 Electromyogram of Strychnine Tetanus in the Frog records of fatigued frogs are examined the initial frequency of the groups is found to be much smaller, e.g. 8-6 per sec. slowing down to 3-4 per sec. This suggests that the slowing down of the groups during the course of the tetanus is a fatigue phenomenon, and since the frequency of the groups depends on the spinal cord this is evidence of a central fatigue. On the other hand during the course of a tetanus the amplitude of the oscillations decreases only very slightly, in some cases scarcely at all, but more usually to about 75 per cent. of their initial size. This would indicate that a fatigue of the muscle must play little or no part in bringing about the end of the tetanus. Evidence is shown, therefore, that the tetanus elicited in a strychninised frog is brought to an end by a fatigue of the centres in the spinal cord and not a peripheral fatigue of the muscles involved. SUMMARY. An electrical recording system for biological work is described, consisting of a valve amplifier and an oscillograph of the Blondell movingiron type. The electromyogram of the strychnine tetanus in the gastrocnemius of the frog is described. It consists of groups of oscillations, the frequency of the groups depending on the temperature of the spinal cord, and the frequency of the oscillations depending on the temperature of the leg. No evidence has been obtained of a rhythm which could be regarded as the rhythm of each discharge from the cord. The end of each tetanus appears to be brought about by central rather than by peripheral fatigue. I am indebted to Professor T. H. Milroy for his advice and criticism during this work. 11 REFERENCES. ADRIAN, E. D. (1931). J. Physiol. 72, 132. BUCHANAN, F. (1901). Ibid., 18, 117. LORENZ, G. F. (1927). Z. Biol. 85, 167.