XXVIII
THE RESONANT RECORDER
The exact determination of extremely short intervals of time is an important problem in various investigations. This is specially so in the measurement of time relations of different phases of response of living tissues.
When the sensitive pulvinus of Mimosa pudica is directly stimulated, say by an electric shock, a responsive contraction and fall of the leaf is initiated after the lapse of a short interval. After the completion of the fall of the leaf the contracted pulvinus slowly recovers as seen in the re-erection of the leaf. The lost time between the incidence of stimulus and the beginning of responsive movement is designated as the Latent Period. If instead of direct stimulation of the pulvinus, stimulus be applied on the petiole at a certain distance from the motile organ, then an excitatory impulse is transmitted through the intervening distance. The time-interval between stimulus and response will now be longer than under direct stimulation; the latent period of the pulvinus and the length of transmission afford sufficient data for the determination of the velocity of excitatory impulse in the plant, which I have shown elsewhere, is analogous to the nervous impulse in the animal.
The response of Mimosa is recorded by means of a writing lever suitably attached to the leaf (fig. 115). The friction of the writing point against the smoked glass surface, however, introduces serious error in the accurate record of the amplitude and time-relations of the response-curve. The difficulty has been overcome by the writer being thrown into resonant vibration in
Fig. 115. Diagrammatic representation of the Recorder.
consequence of which the record consists of a series of dots; the error arising from friction of continuous contact is thus completely avoided.
The Resonant apparatus consists of the writer, made of fine steel wire tuned in different cases, to vibrate 10, 20, or 200 vibrations per second. The writer is suspended at the centre of a circular electromagnet, the current in which is periodically interrupted by the reed C (fig. 116). When the writer and the reed are exactly tuned then the vibration of the reed throws the writer into a sympathetic vibration.
Fig. 116. Upper part of Resonant Recorder. (From a photograph.)
Determination of the Latent Period.
The reliability and accuracy of this method of automatic record of extremely short intervals of time is shown in the following curve (fig. 117) which gives the latent period of the pulvinus of Mimosa. The writer
Fig. 117. Record of Latent Period of Mimosa with 200 Vibration Recorder.
was tuned to vibrate 200 times in a second, and the interval between successive dots therefore represents 1/200 of a second. It is not difficult to measure one-fifth of the distance between successive dots, and calculation can therefore be carried into thousandths of a second. In the present case there are 15.2 spaces between the stimulus and the initiation of response. The latent period of the specimen is therefore 0.076 of a second. The average value of the latent period of the pulvinus of Mimosa is found to be 0.1 second.
Determination of the Velocity of Impulse
The complete apparatus is shown in figure 118. An electrical shock was in the following experiment applied on the petiole at a distance of 30 mm. from the pulvinus, at the instant marked by the vertical line (fig. 119). The frequency of vibration of the writer was 10 per second. The interval between stimulation and the beginning of response is 16.2 spaces, or 1.62 seconds. A repetition of the experiment gave the same result. Stimulus was next applied directly on the pulvinus and the latent period was found to be about 0.12 seconds. The velocity of transmission of excitation in the particular specimen was thus found to be
Fig. 118. Apparatus for determination of Latent Period and Velocity of Transmission of Excitation in Mimosa.
Fig. 119. Determination of velocity of Transmission of Excitation in petiole of Mimosa.
20 mm. per second. In thinner specimens the velocity is often found to be as high as 400 mm. per second. The velocity of impulse in Mimosa is slower than the nervous impulse in higher but quicker than that in lower animals.
The transmission of excitation is correspondingly modified by all conditions which modify the transmission of excitation in the animal nerve. The polar action of a constant electric current is identical in the two cases. In both Mimosa and animal nerve the velocity of transmission is increased within limits by a rise of temperature (fig. 120). In both, transmission
Fig. 120. Effect of Rise of Temperature in enhancing Velocity of Transmission. The three records from below upwards are for temperatures of 22° C., 28° C. and 31° C. respectively.
can be arrested temporarily or permanently by various physiological blocks. The conducting power is temporarily arrested by a block produced by the passage of an electric current in a portion of the conducting tissue through which the impulse is being transmitted; this electrotonic block is removed on the stoppage of the current. Finally, poisonous solutions abolish the conducting power of both animal and plant. These results offer conclusive proof that the conduction in the plant is a phenomenon of propagation of protoplasmic excitation as in the nerve of the animal.
The characteristics of contractility and conductivity are thus shown to be exhibited not only by animal but also by plant tissues. In the animal the cardiac tissue exhibits automatic and rhythmic pulsations. In the plant a similar activity is manifested by the leaflet of Desmodium gyrans, the well known Telegraph plant.
The Resonant Cardiograph
The record of the pulsation of the animal heart by the Cardiograph labours under the unavoidable difficulty of continuous frictional contact introducing error in the correct record of the amplitude and time-relations of the heart-beat. The drawback of continuous contact has been removed in my Resonant Cardiograph which records the pulsation with great precision by means of periodic dots, the Cardiogram being also its own chronogram. The automatic method of registration of extremely short intervals of time can be utilised with great advantage for this and other investigations.
The Characteristics of Cardiac Pulsation.
The Resonant Cardiograph inscribes the different phases of the heartbeat with unprecedented accuracy. The systolic contraction and its persistence, the diastolic expansion and the subsequent pause, and any variation of these under external agencies, can thus be quantitatively determined. The cardiograms of different animals show, moreover, certain characteristic differences in regard to time-relations, as illustrated by records of tortoise, of frog and of Ophiocephalus fish given below.
The writing lever was tuned to vibrate 20 times in a second, the magnification produced being about 8 times. The time-relations, it is to remembered, is found from the intervals of successive dots 120 second apart. The records show the auricular contraction preceding the ventricular. The period of a complete cycle is much longer in tortoise. The total period is 34 dot-intervals, or 1.7 seconds. In the particular specimen of frog the total period is represented by 14 dot-intervals or 0.7 second, while in the fish it is 16 dot-intervals or 0.8 second. Again in tortoise after the commencement
Fig. 121. Characteristic cardiograms of tortoise, frog and fish. The successive dot-intervals represent 0.05 sec.
of the less pronounced systolic contraction of the auricle, the contractile wave reached the ventricle in the course of 0.6 second, whereas in the frog and in the fish the interval is only 0.2 second, or one-third that in tortoise. The records show other characteristic differences and give very striking visual demonstration of the relative activity at different phases of the pulsation. When the activity is very great the markings are very wide apart and the dots are lengthened into dashes; with slowing down of activity the dots become thin and drawn close together (fig. 121). Under the action of depressing agents the post-diastolic pause becomes greatly prolonged.
Similarity of Rhythmic Mechanism in Animal and Plant
I have demonstrated elsewhere the remarkable similarity of rhythmic mechanism in animal and plant. In both, lowering of temperature slows down the pulsation culminating in an arrest. Rise of temperature up to an optimum, on the other hand, enhances the frequency. Diminution of internal pressure causes an arrest in both. The rhythmic tissue in animal and plant, has a long refractory period. In both, application of external stimulus has no effect during systolic phase of contraction, whereas an extra-pulsation is produced by stimulus during the diastolic phase of expansion.
The effect of drugs is often remarkably similar in the two cases. Dilute solution of potassium bromide causes a depression of cardiac pulsation; this depression is removed by the physiological antagonism produced by certain drugs.
I have recently been engaged in investigating the action of various medicinal plants of India on the activity of the animal heart. Among these I find that a heart in a state of depression is greatly stimulated by extracts pf Abroma augusta.
In the following record the normal heart-beat of fish is seen to be greatly depressed by KBr solution. Subsequent application of Abroma extiact caused a remarkable revival of activity (fig. 122). Even more remarkable
are the exactly parallel reactions of KBr and Abroma on rhythmic pulsations of Desmodium gyrans (Fig. 123).
