Collected Physical Papers/The Response of Inorganic Matter to Mechanical and Electrical Stimulus
THE RESPONSE OF INORGANIC MATTER TO MECHANICAL AND ELECTRICAL STIMULUS
Certain changes take place when a living muscle is pinched or an electric shock passed through it. A responsive twitch is produced; the muscle is changed in form, becoming shorter and broader; the particles of the living substance are strained under the stimulus. The effect of the shock then disappears, and the muscle relaxes into its usual form.
This sudden change of form then, is one, but not the only mode of response of a living substance to external stimulus. Under external stimulation the muscle is thrown into a state of strain. On cessation of the stimulus it automatically recovers. As long as it is alive, so long will it respond and recover, being ready again for a new response. This brief disturbance of the living poise, to be immediately restored to equilibrium of itself, is quite unlike the rolling of a stone downhill from a push. For the stone cannot regain its original position; but the living tissue at once reasserts its first stable poise on the cessation of stimulus. Thus a muscle, as long as it is alive, remains ever-responsive. It is in intimate relation with the forces by which it is surrounded, always responding to, and recovering from, the multitudinous stimuli of its physical environment.
The living body is thus affected by external stimuli—mechanical shock, electrical stimulus, and the stimuli of heat and light—which evoke in it corresponding responses.
In the case of the contraction of muscle by mechanical or electric shock, the effect is very quick, and the contraction and relaxation take place in too short a time for detailed observation by ordinary means. Physiologists, therefore, use a contrivance by which the whole process may be recorded automatically. This consists of a lever arrangement, by which the contracting muscle writes down the history of its change, and recovery from that change. The record may be made on a travelling band of paper, which is moving at a uniform rate (fig. 62). A single response to a single stimulus consists of a contractile up-curve followed by the down-curve of recovery, the entire process being completed within a definite period of time. This autographic record gives us the most accurate information as to the characteristic properties and condition of the muscle. It gives us, too, all its individual characteristics.
Just as one wave of sound is distinguished from another by its amplitude, period, and form, so are the curves of different muscles distinguished. For example, the period for tortoise muscle may be as long as several seconds, whereas the period for the wing of an insect is as short as 1th of a second. In the same muscle, again, the form of the curve may undergo changes from fatigue, or from the effects of various drugs. In the autographic record of the progressive death of a muscle, the graph is vigorous at first, but grows lethargic on the approach of death. In some strange way the molecules lose their mobility, rigidity supervenes, and the record of the dying muscle comes to an end. We may thus find out the effects of various external influences by studying the changes in the muscle-curve.
We may stimulate the living substance in various ways—by light, or by thermal, chemical, electrical, or mechanical stimuli. Of these, the electric means of stimulation is the most convenient, whereas the mechanical causes fewest complications. With regard to the response of living substances, the most important matters for study are the responses to single stimulus and the modification of response by fatigue and by drugs.
A single shock causes a twitch, but the muscle soon recovers its original form. The rising portion of the curve is due to contraction, whereas the falling portion exhibits recovery (see curve in fig. 62).
When the muscle is continuously excited it gets fatigued. The height of the curve becomes continuously less.
Drugs may act as stimulants, or produce depression, according to their nature. As extreme cases of such depressing agents we may instance poisons, which kill the response of living tissue. All signs of irritability then disappear.
Other Modes of Response
This mechanical method of studying the response of living substances is, however, very limited in its application. For example, when a piece of nerve is stimulated, there is no visible change. When light falls on the retina there is no change of form, but it responds by transmitting to the brain a visual impulse. What, then, is this visual impulse which is sent along the optic nerve, causing the sensation of light?
Thanks to the work of Homgren, Dewar, McKendrick and others, it is possible to answer this question. If we excise an eye, say of a frog, and substitute a galvanometer in the visual circuit in the place of the perceiving brain, it is then found that each time a flash of light falls on the eye there is produced an electric response—that is to say, there is a sudden production of a current, which ceases on the cessation of light-stimulus. Stronger light produces stronger electric response in the galvanometer, just as it produces stronger visual sensation in the brain.
The visual circuit is therefore like an electric circuit. The retina is the sensitive element. The nerve is the conductor. The brain like the responding galvanometer is a detector of the impulse. Unless these three elements are in good order, no light-message can be perceived. We must have a sensitive retina, and the conducting nerve, free from injury. Finally, just as the galvanometer will fail to detect a current if it is injured by rough usage, so also after a violent blow, the brain will no longer perceive, though the terminal organ, the retina, and the connecting optic nerve may be intact.
If we take a piece of living muscle whose surface is uninjured, then any two points A and B on such a surface being in similar molecular conditions, their electrical level or potential will be the same. They are iso-electric. No current will be exhibited by the indicating galvanometer when two non-polarisable electrodes connected with it are applied to A and B. But if one of the two points, say B, be injured by a cut, or burn, then, the conditions of A and B being different, there will be a difference of electric level or potential between them, and a current will flow from the injured to the uninjured, that is from B to A (fig. 63). This current remains approximately constant as long as the muscle is at rest, and is for this reason known as "current of rest." As it is primarily due to injury, it is also known as "current of injury." If now the muscle be thrown into an excitatory state by stimulus, there will be a greater relative change at the uninjured A, and the original difference of electric level will be disturbed. There is then a negative variation or diminution of the original current of rest. This negative variation or "action current" constitutes the "response," the intensity of which increases with the intensity of the stimulus.
If a piece of muscle be taken, and simultaneous records of its response be made by the mechanical and electrical recorders, it will be found that the one is practically a duplicate of the other.
Response in Plants
I find that the electric response seen in animal tissues is also strongly exhibited by the tissues of plants. Various parts of plants—leaves, stems, stalks, and roots—give electric response. In some there is a rapid fatigue, whereas in others there is little fatigue. A more detailed account of these responses and their modifications by anæsthetics, poisons, and other agencies will be found in a subsequent paper.
Universal Applicability of the Test of Electric Response
Nothing has yet been said of the advantage of the electrical over the mechanical method of obtaining response. As has been said before, the mechanical method is limited in its application. A nerve, for example, does not undergo any visible change of form when excited, and its response cannot therefore be detected by this method. But by the electrical method we are able to detect, not only the response of muscles, but also of all forms of living tissue.
The intensity of electrical response is also a measure of physiological activity. When this is diminished by anæsthetics, the electrical responses also become correspondingly diminished. And when the living tissue is in any way killed, the electrical response disappears altogether.
Thus, electrical response is regarded as the criterion between the living and non-living. Where it is, life is said to be; where it is not found, we are in presence of death, or else of that which has never lived: for in this respect there is a great gulf fixed between the organic living and the inorganic or non-living. The phenomena of the inorganic are supposed to be dominated merely by physical forces, while on the other side of the chasm, in the domain of the living, inscrutable vital phenomena, of which electric response is the sign-manual, suddenly come into action.
But is it true that the inorganic are irresponsive, that forces evoke in them no answering thrill? Are their particles for ever locked in the rigid grasp of immobility? As regards response, is the chasm between the living and inorganic really impassable?
Let us take a piece of thin wire from which all strains have been removed and hold it clamped in the middle at C (fig. 64).
A and B will be found iso-electric and no current will pass through the galvanometer. If we now stimulate the end A by a tap, or better still by rapid torsional vibration, a "current of action" will be found to flow in the wire in one direction. Stimulation of B on the other hand, gives rise to a current in the opposite direction. The object of the block C is to prevent the molecular disturbance produced by stimulus at one end of the wire from reaching the other.
Quantitative stimulation is applied by producing a sudden torsional vibration, say of 90°, to the end A, with the result of an up-response of the galvanometer; on application of similar stimulus to the end B, there is produced an equal and opposite response (fig. 64). If now both the ends A and B are stimulated simultaneously, the responsive electromotive variation at the two ends will continuously balance each other and the galvanometer spot will remain stationary. Fig. 65 gives the record of series of equal and opposite responses by alternate stimulation of A and B.
The similarities of response of inorganic and living substances are now sufficiently evident. We have to extend the inquiry to see whether this similarity extends to this point only, or goes still further. Are the response-curves of the inorganic modified by the influence of external agencies, as the living responses are found to be?
Effect of Superposition of Stimuli
It has been said that under rapidly succeeding stimuli, the intermittent effects of single shocks become fused, and the muscle responds by an almost unbroken tetanic curve (fig. 66). If the frequency is not sufficiently great, there is an incomplete tetanus and the response-curve becomes jagged.
The very same thing occurs in metals. I subject the wire to quickly succeeding vibrations. The curve rises to its maximum; further stimulation adds nothing to the effect, and the deflection is held, as it.were rigid,
so long as the stimulation is kept up. With lesser frequency of stimulation the tetanus is incomplete, and the curve becomes jagged (fig. 66).
Amongst living substances the nerve is practically indefatigable. Successive curves are exactly similar. Muscles, however, exhibit fatigue, which disappears after a period of rest.
Inorganic substances likewise exhibit fatigue specially after prolonged action. The fatigue curve here reproduced was obtained from tin that had been subjected to very prolonged stimulation; its remarkable similarity to the curve of fatigue in muscles will be at once apparent (fig. 67). Fatigue in metal is also removed by a period of rest.
Stimulus of Light
I have in this investigation mainly used the mechanical form of stimulation as being the simplest and giving rise to fewest complications. Time does not allow of my entering here upon the subject of the response under electric stimulation. I may, however, say a few words on the effect of the stimulus of light.
If one of the sensitised wires in the cell already described be subjected to light it gives an electric response, and under certain circumstances an oscillatory after-effect is seen to occur on the cessation of light. This latter fact probably explains certain phenomena of visual recurrence to be noticed presently.
The molecular strain produced by stimulus can not only be detected by the phenomena of electromotive variation, but also by conductivity variation. Acting on this principle, I have been able to construct an artificial retina constructed of galena. The sensitive receiver is placed inside a hollow spherical case, provided with a circular opening in front, in which a glass lens is placed, corresponding to the crystalline lens. You now see before you a complete model of an artificial eye. When this is interposed in an electric circuit, with a sensitive galvanometer as indicator, you observe the response to a flash of light by the galvanometer deflection. I throw red, yellow, green and violet lights upon it in succession, to all of which it responds. Note how strong is the action of yellow light, the response to violet being relatively feeble. Indeed, the most striking peculiarity of this eye is that it can see lights not only some way beyond the violet, but also in regions far below the infra-red, in the invisible regions of electric radiation. It is in fact a Tejometer (Sanskrit tej-radiation), or universal radiometer.
Observe how each flash of invisible light I am producing with this electric radiation apparatus, calls forth an immediate response, and how the eye automatically recovers without external aid. This will show the possibility of an automatic receiver which will record Hertzian wave-messages without the intervention of the crude tapping device.
This retina has, as will be seen with regard to spectral vision, an enormous range, extending far beyond the visual limits. We can, however, reduce its powers to a merely human level by furnishing it with a water lens, which, in its liquid constitution, approximates closer to the lens of the eye than does the glass substitute. In this case the invisible radiations are absorbed by the liquid, and do not reach the sensitive retina. Perhaps we do not sufficiently appreciate, especially in these days of space-signalling by Hertzian waves, the importance of that protective contrivance which veils our sense against insufferable radiance.
Binocular Alternation of Vision
It will thus be seen how, from observing the peculiarities of an artificial organ, we are led to discover unsuspected peculiarities in our own. We stand here on the threshold of a very extended inquiry, of which I can only say that as it has been possible to construct an artificial retina, so I believe it may not be impossible to imitate also other organs of sense.
Effects of Chemical Reagents
I now return to the consideration of mechanical stimulus and the modification of its responses, as shown by metals. We have seen the remarkable parallelism between organic and inorganic responses under various conditions. There still remains the study of the effects of chemical reagents. Drugs profoundly modify the response of living substances; the effects of which may be classified under three classes, some acting as stimulants, others as depressors, and yet others again as poisons, by which response is permanently abolished. Amongst the last may be mentioned mercuric chloride, oxalic acid and others. Again, drugs which in large doses become poisons, may, when applied in small quantities, act as stimulants.
It may be thought that to these phenomena, inorganic matter could offer no parallel. For they involve possibilities which have been regarded as exclusively physiological. Accustomed in the animal to find the responsive condition transformed into the irresponsive state at the moment of death, we look on this sequence as peculiar to the world of the living. And on this fact is based the supreme test by which physical and physiological phenomena are differentiated. That only can be called living which is capable of dying, we say, and death can be accelerated by the administration of poison. The sign of life as given by the electric pulses then wanes, till it ceases altogether. Molecular immobility—the rigor of death—supervenes, and that which was living is no longer alive.
Is it credible that we might, in like manner, kill inorganic response by the administration of poison? Could we by this means induce a condition of immobility in metals, so that, under its influence, their electric responses should wane and die out altogether?
Before we attempt the action of poisons let us study the exciting effect of stimulants. You observe the normal extent of response under successive uniform stimuli applied to one wire of the cell. I now add a few drops of diluted solution of sodium carbonate and you observe the growing exaltation of the response. (fig. 69).
I now pass on to the effect of poisons. Any of the substances already enumerated may be used as the
toxic agent. After obtaining the normal response I apply a poisonous solution of oxalic acid. The electric response is now completely killed and all our efforts, by intense stimulation to reawaken them, fail (fig. 70).
But we may, sometimes at least, by the timely application of a suitable antidote, revive the dying response, as I do now. See how the lethargy of immobility passes away; the electric throb grows stronger and stronger, and the response in the piece of metal becomes normal once more.
There remains the very curious phenomenon, known not only to students of physiological response but also in medical practice, that of the opposite effects produced by the same drug when given in large or in small doses. Here too we have the same phenomenon reproduced in an extraordinary manner in inorganic response. The same agent which becomes a poison in large quantities thus acts as a stimulant when applied in small doses.
I have shown you this evening autographic records of responses of the living and non-living. How similar are the writings! So similar indeed that you cannot tell one from the other apart. We have watched the responsive pulses wax and wane in the one as in the other. We have seen response sinking under fatigue, becoming exalted under stimulants, and being "killed" by poisons, in the non-living as in the living.
Amongst such phenomena, how can we draw a line of demarcation, and say, "here the physical process ends, and there the physiological begins"? No such barriers exist.
Do not the two sets of records tell us of some property of matter common and persistent? Do they not show us that the responsive processes, seen in life, have been foreshadowed in non-life?—that the physiological is, after all, but an expression of the physico-chemical, and that there is no abrupt break, but a continuity?
If it be so, we shall turn with renewed courage to the investigation of mysteries which have long eluded us. For every step of science has been made by the inclusion of what seemed contradictory or capricious in a new and harmonious simplicity. Her advances have been always towards a clearer perception of underlying unity in apparent diversity.
(Friday Evening Discourse, Royal Institution, May 1901)
- The excitatory reaction is, in the case of some living substances, of a more or less local character. In others, as nerves, it may be conducted to distant points.