Response in the Living and Non-Living

I shall now give a record which will exhibit an exactly similar transformation from the abnormal to normal response after continuous stimulation. Here the normal responses are represented by ‘up’ and the abnormal by ‘down’ curves. This record was eiven by a tin wire, which had been molecularly modified (fig. 76). We have at first the abnormal Abnormal ‘ down’ response in tin (fig. 76) and in platinum (fig. 77) transformed into normal ‘up’ response, after continuous stimulation, T.

responses ; successive responses are undergoing a diminution or tending towards the normal; after continuous stimulation (T), the subsequent responses are seen to have become normal. Another record, obtained with platinum, shows the same phenomenon (fig. 77). platinum—side by side, it will be seen how essentially similar they are in every respect.' This reversion to normal is seen to have appeared in a pronounced manner after rapidly continuous stimulation, in process of which the modified molecular condition must in some way have reverted to the normal. Being desirous to trace this change gradually taking

Fic. 78.—THE GRADUAL TRANSITION FROM ABNORMAL TO NORMAL RESPONSE The transition will be seen to have commenced at the thirdand ended at the seventh, counting from the left. individual stimuli continued for a long time. In this series, the points of transition from modified response to normal will be clearly seen (fig. 78). | In order to explain the phenomena of electric response, some physiologists assume that the negative response is due to a process of dissimilation, or breakdown, and the positive to a process of assimilation, or building up, of the tissue. The modified or positive response in nerve is thus held to be due to assimilation ; after continuous stimulation, this process is supposed to be transformed into one of dissimilation, with the attendant negative response.

How arbitrary and unnecessary such assumptions are will become evident, when the abnormal and norma] responses, and their transformation from one to the other, are found repeated in all details in metals, where there can be no question of the processes of assimilation or dissimilation. Increased response after continuous stimulation.— We have seen that responses to uniform stimuli sometimes show a staircase increase, apparently owing to the eradual removal of molecular slugeishness. Pos-

is the increase of re- a T b E E Fic. 79.—TuHEe Normat RESPONSE @ IN sponse Im nerve after NERVE ENHANCED TO 0 AFTER Con- TINvoUS SrimuLaTIon 'T (WALLER) The normal response in nerve is recorded ‘down.’ continuous — stimulation or tetanisation, observed by Waller (fig..79). Like the staircase effect, this Before mn After Fic. 80.—ENHANCED RESPONSE IN PLATINUM AFTER CONTINUOUS STIMULATION T contravenes the commonly accepted theory of the dissimilation of tissue by stimulus, and the consequent depression of response. It is suggested by Waller that

this increase of response after tetanisation may be due to the hypothetical evolution of CO, to which allusion has previously been made. But there is an exact correspondence between this phenomenon and that exhibited by metals under similar conditions. I give here two sets of records (figs. 80, 81), one obtained with platinum and_ the Before dh After Fic. 81.—EnNuANcED Response IN TIN AFTER Continuous StimuLaT on T other with tin, which demonstrate how the response is enhanced after continuous stimulation im a manner exactly similar to that noticed in the case of nerve. The explanation which has been suggested with regard to the staircase effect—increased molecular mobility due to removal of sluggishness by repeated stimulation—would appear to be applicable in this case

also, It would appear, then, that in all the phenomena which we have studied under the heads of ‘staircase’ effect, Increase of response after continuous stimulation, and fatigue, there is a similarity between the observations made upon the response of muscle and nerve on the one hand, and that of metals on the other. Even in their abnormalities we have seen an agreement. But amongst these phenomena themselves, though at first sight so diverse, there is some kind of continuity. Calling al/ normal response positive, for the sake of convenience, we observe its gradual modification, corresponding to changes in the molecular condition of the substance.

Beginning with that case in which molecular modification is extreme, we find a maximum variation of response from the normal, that is to say, to negative. Continued stimulation, however, brings back the molecular condition to normal, as evidenced by the proeressive lessening of the negative response, culminating im reversion to the normal positive. This is equally true of nerve and metal. In the next class of phenomena, the modification of molecular condition is not so great. It now exhibits itself merely as a relative inertness, and the responses, though positive, are feeble. Under continued stimulation, they increase in the same direction as in the last case, that is to say, from less positive to more positive, being the reverse of fatigue. This is evidenced alike by the staircase effect and by the increase of response after tetanisation, seen not only in nerve but also in platinum and tin.

The substance may next be in what we call. the normal condition. Successive uniform stimuli now evoke uniform and equal positive responses, that is to say, there is no fatigue. But after intense or longcontinued stimulation, the substance is overstrained. The responses now undergo a change from positive to less positive; fatigue, that is to say, appears. Again, under very much prolonged stimulation the response may decline to zero, or even undergo a reversal to negative, a phenomenon which we shall find instanced in the reversed response of retina under the longcontinued stimulus of light.

We must then recognise that a substance may exist in various molecular conditions, whether due to internal changes or to the action of stimulus. The responses give us indications of these conditions. A complete cycle of molecular modifications can be traced, from the abnormal negative to the normal positive, and then again to negative seen ip reversal under continuous stimulation. Relation between stimulus and response—Magnetic analogue—Increase of

response with increasing stimulus—Threshold of response—Superposition of stimuli—Hysteresis. Relation between stimulus and response.—We have seen what extremely uniform responses are given by tin, when the intensity of stimulus is maintained constant. Hence it is obvious that these phenomena are not accidental, but governed by definite laws. This fact becomes still more evident when we discover how invariably response is increased by increasing the intensity of stimulus.

Electrical response is due, as we have seen, to a molecular disturbance, the stimulus causing a distortion from a position of equilibrium. In dealing with the subject of the relation between the disturbing force and the molecular effect it produces, it may be instructive to consider certain analogous physical phenomena in which molecular deflections are also produced by a distorting force. Magnetic analogue.—Let us consider the effect that a magnetising force produces on a bar of soft iron. It is known that each molecule in such a bar is an

individual magnet. The bar as a whole, nevertheless, exhibits no external magnetisation. This is held to be due the fact that the molecular magnets are turned either in haphazard directions or in closed chains, and there is therefore no resultant polarity. But when the bar is subjected to a magnetising force by means, say, of a solenoid carrying electrical current, the individual molecules are elastically deflected, so that all the molecular magnets tend to place themselves along the lines of magnetising force. All the north poles thus pomt more or less one way, and the south poles the other. The stronger the magnetising force, the nearer do the molecules approach to a perfect alignment, and the greater is the induced magnetisation of the bar.

The intensity of this duced magnetisation may be measured by noting the deflection it produces on a freely suspended magnet in a magnetometer. The force which produces that molecular deflection, to which the magnetisation of the bar is immediately due, is the magnetising current flowing round the solenoid. The magnetisation, or the molecular effect, is measured the abscissa represents the magnetising current, and the ordinate the magnetisation produced (fig. 82).

cular deflection is slight. In the next, the curve is rapidly ascending, i.e. a small variation of impressed force produces a relatively large molecular effect. And lastly, a limit is reached, as seen in the third part, where increasing force produces very little further effect. In this cause-and-effect curve, the first part is slightly convex to the abscissa, the second straight and ascending, and the third concave. Increase of response with increasing stimulus.—We shall find in dealing with the relation between the stimulus and the molecular effect—i.e. the response— something very similar.

On gradually increasing the intensity of stimulus, which may be done, as already stated, by increasing the amplitude of vibration, it will be found that, beginning with feeble stimulation, this increase 1s at first shght, then more pronounced, and lastly shows a tendency to approach a limit. In all this we have a perfect parallel to corresponding phenomena in animal and vegetable response. We saw that the proper investigation of this subject was much complicated, in the case of animal and vegetable tissues, by the appearance of fatigue. The comparatively indefatigable nature of tin causes it to offer great advantages in the pursuit of this inquiry. I give below two series of records made with tin. The first record, fig. 83, is for to 40° by steps of 5°.

increasing amplitudes from 5 The stimuli are imparted at intervals of one minute. It will be noticed that whereas the recovery is complete in one minute when the stimulus is moderate, it is not quite complete when the stimulus is stronger. The Fic. 83.—Recorps or Responses In Trx wirH INCREASING STIMULI, AMPLI- TUDES OF VIBRATION FROM 5° To 40° base line is tilted slightly upward. This slight displacement of the zero line does not materially affect the result, provided the shifting 1s slight.

is complete. We may accomplish this within the lmited space of the recording photographic plate by making the record for one minute; during the rest of recovery, the clockwork moving the plate is stopped and the galvanometer spot of light is cut off. Thus the steps of 10°, from 20° to 160°. (The deflections are reduced by interposing a high external resistance.) A single stimulus produces the feeble effect shown in the first response. Superposition of 5, 9, 13 such stimuli produce the succeeding

tendency will be noticed for the responses to approach a limit. Threshold of response.--- There is a minimum intensity of stimulus below which there is hardly any visible response. We may regard this point as the threshold of response. record in fig. 85 shows how individually feeble stimuli become markedly effective by superposition. Superposition of stimuli.—The additive effect of succeeding stimuli will be seen from the above. The fusion of effect will be incomplete if the frequency of stimulation be not sufficiently great ; but it will tend to be more

As the frequency of stimulation is increased the fusion becomes more and more complete. Vertical line to the right represents ‘1 volt. complete with higher frequency of stimulation (fig. 86). We have here a parallel case to the complete and incomplete tetanus of muscles, under similar conditions. By the addition of these rapidly succeeding stimuh, a maximum effect is produced, and further stimulation adds nothing to this. The effect is balanced by a force

of restitution. The response-curve thus rises to its maximum, after which the deflection is held as it were rigid, so long as the vibration is kept up. It was found that increasing intensities of single stimuli produced correspondingly increased responses. The same is true also of groups of stimuli. The maximum Fic. 87.—Cycric Curve rok Maximum Errects sHowInG HysTERESIS effect produced by superposition of stimuli increases with the intensity of the constituent stimuli. Hysteresis.—Allusion has already been made to the increased responsiveness conferred by preliminary stimulation (see p. 127). Being desirous of finding out in what manner this is brought about, I took a series

of observations for an entire cycle, that is to say, a series of observations were taken for maximum effects, starting from amplitude of vibration of ‘10° and ending in 100°, and backwards from 100° to 10°. Effect of hysteresis 1s very clearly seen (see A, fig. 87); there is a considerable divergence between the forward and return curves, the return curve being higher. On repeating the cycle several times, the divergence is found very much reduced, the wire on the whole is found to assume amore constant sensitiveness. In this steady condition, generally speaking, the sensitiveness for smaller amplitude of vibration is found to be greater than at the very beginning, but the reverse is the case for stronger intensity of stimulation.

Effect of annealing.— I repeated the experiment with the same wire, after pourme hot water into the cell and allowing it to cool to the old temperature. From the cyclic curve (B, fig. 87) it will be seen (1) that the sensitiveness has become very much enhanced; (2) that there is relatively less divergence between the forward and return curves. Even this divergence practically disappeared at the third cycle, when the forward and backward curves coincided (Cc, fig. 87). The above results show in what manner the excitability of the wire is enhanced by purely physical means.

It is very curious to notice that addition of Na,CO, solution (see Chap. XV—Action of Stimulants) produces enhancement of responsive power similar to that produced by annealing ; that is to say, not only is there a ereat increase of sensitiveness, but there is also a reduction of hysteresis. Action of chemical reagents—Action of stimulants on metals—Action of Effect of ‘ poisons’ on metals—Opposite effect of We have seen that the ultimate criterion of the physiological character of electric response is held to be its abolition when the substance is subjected. to those chemical reagents which act as poisons.

Action of chemical reagents.—Of these reagents, some are universal in their action, amongst which strong solutions of acids and alkalis, and salts like mercuric Fic. 88.—Action oF Porson in ABonisHinc Response In Nerve (WALLER) chloride, may be cited. These act as powerful toxic agents, killme the living tissue, and causing electric response to disappear. (See fig. 88.) It must, however, be remembered that there are again specific poisons which may affect one kind of tissue and not others. Poisons in general may be regarded as extreme cases of depressants. As an example of those which produce moderate physiological depression, potassium bromide may be mentioned, and this also diminishes electric response. There are other chemical reagents, on the other hand, which produce the opposite effect of increasing the excitability and causing a corresponding exaltation of electric response.

We shall now proceed to inquire whether the response of inorganic bodies is affected by chemical reagents, so that their excitability is exalted by some, and depressed or abolished by others. Should it prove to be so, the last test will have been fulfilled, and that parallelism which has been already demonstrated throughout a wide range of phenomena, between the electric response of animal tissues on the one hand, and that of plants and metals on the other, will be completely established.

Action of stimulants on metals.—We shall first study the stimulating action of various chemical reagents. The method of procedure is to take a series of normal responses to uniform stimuh, the electrolyte being water. The chemical reagent whose effect is to be observed is now added in small quantity to the water in the cell, and a second series of responses taken, using the same stimulus as before. Generally speaking, the influence of the reagent is manifested in a short period, but there may be occasional instances where the eflect takes some time to develop fully. We must remember that by the introduction of the chemical reagent some change may

be produced in the internal resistance of the cell. The effect of this on the deflection is eliminated by interposing a very high external resistance (from one to five megohms) in comparison with which the internal resistance of the cell is negligible. The fact that the introduction of the reagent did not produce any variation in the total resistance of the circuit was demonstrated by takine two deflections, due to a definite fraction of a volt, before and after the introduction of the reagent. These deflections were found equal.

I first give a record of the stimulating action of sodium carbonate on tin, which will become evident by a comparison of the responses before and after the introduction of Na,CO, (fig. 89). The next record Action of depressants.—Certain other reagents, again, produce an opposite effect. That is to say, they diminish the intensity of response. The record given on the next page (fig. 91) shows the depressing action of 10 per cent. solution of KBr on tin.

Effect of ‘ poison.’—Living tissues are killed, and their electric responses are at the same time abolished by the action of poisons. It is very curious that various chemical reagents are similarly effective in killing the response of metals. I give below a record (fig. 92) to show how oxalic acid abolishes the response. The depressive effect of this reagent is so great that a strength of one part in 10,000 is often sufficient to produce complete