Relation between stimulus and response--Magnetic a.n.a.logue--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 a.n.a.logous physical phenomena in which molecular deflections are also produced by a distorting force.

#Magnetic a.n.a.logue.#--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 to 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 point 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 induced 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 by the deflection of the magnetometer. We may express the relation between cause and effect by a curve in which the abscissa represents the magnetising current, and the ordinate the magnetisation produced (fig. 82).

[Ill.u.s.tration: FIG. 82.--CURVE OF MAGNETISATION]

In such a curve we may roughly distinguish three parts. In the first, where the force is feeble, the molecular 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 is at first slight, then more p.r.o.nounced, 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 increasing amplitudes from 5 to 40 by steps of 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 recovery from the effect of stronger stimulus is more prolonged. Owing to want of complete recovery, the base line is tilted slightly upward. This slight displacement of the zero line does not materially affect the result, provided the shifting is slight.

[Ill.u.s.tration: FIG. 83.--RECORDS OF RESPONSES IN TIN WITH INCREASING STIMULI, AMPLITUDES OF VIBRATION FROM 5 TO 40 The vertical line to the right represents 1 volt.]

TABLE SHOWING THE INCREASING ELECTRIC RESPONSE DUE TO INCREASING AMPLITUDE OF VIBRATION

+---------------------+----------------+

Vibration amplitude

E.M. variation

+---------------------+----------------+

5

024 volt

10

057 "

20

111 "

25

143 "

30

170 "

35

187 "

40

204 "

+---------------------+----------------+

The next figure (fig. 84) gives record of responses through a wider range. For accurate quant.i.tative measurements it is preferable to wait till the recovery is complete. We may accomplish this within the limited s.p.a.ce 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 next record starts from a point of completed recovery, which will be noticed as a bright spot at the beginning of each curve. With stimulation of high intensity, a tendency will be noticed for the responses to approach a limit.

[Ill.u.s.tration: FIG. 84.--A SECOND SET OF RECORDS WITH A DIFFERENT SPECIMEN OF TIN The amplitudes of vibration are increased by steps of 10, from 20 to 160. (The deflections are reduced by interposing a high external resistance.)]

[Ill.u.s.tration: FIG. 85.--EFFECT OF SUPERPOSITION ON TIN A single stimulus produces the feeble effect shown in the first response. Superposition of 5, 9, 13 such stimuli produce the succeeding stronger responses.]

#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. Though apparently ineffective, the subliminal stimuli produce some latent effect, which may be demonstrated by their additive action. The record in fig. 85 shows how individually feeble stimuli become markedly effective by superposition.

[Ill.u.s.tration: FIG. 86.--INCOMPLETE AND COMPLETE FUSION OF EFFECT IN TIN As the frequency of stimulation is increased the fusion becomes more and more complete. Vertical line to the right represents 1 volt.]

#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 complete with higher frequency of stimulation (fig. 86). We have here a parallel case to the complete and incomplete teta.n.u.s of muscles, under similar conditions.

By the addition of these rapidly succeeding stimuli, a maximum effect is produced, and further stimulation adds nothing to this. The effect is balanced by a force of rest.i.tution. 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.

[Ill.u.s.tration: FIG. 87.--CYCLIC CURVE FOR MAXIMUM EFFECTS SHOWING HYSTERESIS]

It was found that increasing intensities of single stimuli produced correspondingly increased responses. The same is true also of groups of stimuli. The maximum effect produced by superposition of stimuli increases with the intensity of the const.i.tuent 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 is 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 a.s.sume a more 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 pouring 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 (C, 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_2CO_3 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 great increase of sensitiveness, but there is also a reduction of hysteresis.

CHAPTER XVI

INORGANIC RESPONSE--EFFECT OF CHEMICAL REAGENT

Action of chemical reagents--Action of stimulants on metals--Action of depressants on metals--Effect of 'poisons' on metals--Opposite effect of large and small doses.

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.

[Ill.u.s.tration: FIG. 88.--ACTION OF POISON IN ABOLISHING RESPONSE IN NERVE (WALLER)]

#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 chloride, may be cited. These act as powerful toxic agents, killing 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, pota.s.sium 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 stimuli, the electrolyte being water. The chemical reagent whose effect is to be observed is now added in small quant.i.ty 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 effect 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 taking two deflections, due to a definite fraction of a volt, before and after the introduction of the reagent. These deflections were found equal.

[Ill.u.s.tration: FIG. 89.--STIMULATING ACTION OF Na_2CO_3 ON TIN]

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_2CO_3 (fig. 89). The next record shows the effect of the same reagent on platinum (fig. 90).

[Ill.u.s.tration: FIG. 90.--STIMULATING ACTION OF Na_2CO_3 ON PLATINUM]

#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.

[Ill.u.s.tration: FIG. 91.--DEPRESSING EFFECT OF KBR (10 PER CENT.) ON THE RESPONSE OF 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 abolition. Another notable point with reference to the action of this reagent is the persistence of after-effect. This will be clearly seen from an account of the following experiment. The two wires A and B, in the cell filled with water, were found to give equal responses. The wires were now lifted off, and one wire B was touched with dilute oxalic acid. All traces of acid were next removed by rubbing the wire with cloth under a stream of water. On replacing the wire in the cell, A gave the usual response, whereas that of B was found to be abolished. The depression produced is so great and pa.s.ses in so deep that I have often failed to revive the response, even after rubbing the wire with emery paper, by which the molecular layer on the surface must have been removed.

[Ill.u.s.tration: FIG. 92.--ABOLITION OF RESPONSE BY OXALIC ACID]

We have seen in the molecular model (fig. 62, _d_, _e_) how the attainment of maximum is delayed, the response diminished, and the recovery prolonged or arrested by increase of friction or reduction of molecular mobility.

It would appear as if the reagents which act as poisons produced some kind of molecular arrest. The following records seen to lend support to this view. If the oxalic acid is applied in large quant.i.ties, the abolition of response is complete. But on carefully adding just the proper amount I find that the first stimulus evokes a responsive electric twitch, which is less than the normal, and the period of recovery is very much prolonged from the normal one minute before, to five minutes after, the application of the reagent (fig. 93, _a_). In another record the arrest is more p.r.o.nounced, i.e. there is now no recovery (fig. 93, _b_). Note also that the maximum is attained much later. Stimuli applied after the arrest produce no effect, as if the molecular mechanism became, as it were, clogged or locked up.

In connection with this it is interesting to note that the effect of veratrine poison on muscle is somewhat similar. This reagent not only diminishes the excitability, but causes a very great prolongation of the period of recovery.

In connection with the action of chemical reagents the following points are noteworthy.

[Ill.u.s.tration: FIG. 93.--'MOLECULAR ARREST' BY THE ACTION OF 'POISON'

In each, curves to the left show the normal response, curve to the right shows the effect of poison. In (_a_) the arrest is evidenced by prolongation of period of recovery. In (_b_) there is no recovery.]

(1) The effect of these reagents is not only to increase or diminish the height of the response-curve, but also to modify the time relations. By the action of some the latent period is diminished, others produce a prolongation of the period of recovery. Some curious effects produced by the change of time relations have been noticed in the account given of diphasic variation (see p. 113).