Comparative Electro-Physiology: A Physico-Physiological Study

A difficult problem in connection with electrical response is that of the discharge from the electrical organs of certain . fishes. In a large number of cases, of which Torpedo may be taken as the type, the discharge takes place in a direction from the anterior or nervous to the posterior and non- nervous surface. Pacini’s generalisation that the responsive discharge is always from the anterior to the posterior surface is negatived by the instance of Malepterurus, in which it is from the modified glandular posterior to the anterior surface. Another peculiarity of the response of electric organs in general is that the responsive current is always in the same direction—that, namely, of the organ-discharge — whether the exciting shock be homo- or hetero-dromous. No theory has yet been found which will fully explain all these peculiarities.

I have shown, however, that this phenomenon is not alone of its kind ; nor is it dependent on any specific characteristic of the animal nerve-and-muscle, or gland, of which different electric organs are modifications. The response of the electric organ simply constitutes an extreme case of differential excitability, and follows the general law of response in anisotropic organs—namely, that on diffuse stimulation the responsive current flows from the more to the less excitable. The peculiarity of the organ simply depends upon the fact that owing to the serial arrangement of its elementary aniso- tropic plates the terminal electro-motive effect becomes very large by summation. We find vegetal analogues to the two types of electrical plates of Zorpedo and Malepterurus, in the leaves of Pterospermum, and the pitcher.of Vepenthe. In the first of these, Pzerospermum, as in Torpedo, the anterior nervous surface is relatively more excitable than the mass

of indifferent tissue on the posterior surface. Hence the current of response is from the more excitable anterior to the less excitable posterior. In the second type—the pitcher of Nepenthe and the electrical plates of Malepterurus—the posterior surface being glandular and therefore exceptionally excitable, the responsive current is from posterior to anterior. In taking rheotomic observations on the response to electrical stimulation in various anisotropic leaves—virtually acting, as has been shown, like electrical plates—it was found that in sluggish specimens the maximum electro-motive value was attained ‘2 second after the exciting shock. This was also the value of the period which elapsed after the applica- _tion of moderate mechanical stimulation. With vigorous specimens, however, such as the leaves of Vymphea alba, the maximum effect was attained in a much shorter time, that is to say, in about ‘03 second. In the electrical organ of Yorpedo the corresponding period has been found to be ‘o1 second. The response of electrical organs is found to be repeated or multiple. In the rheotomic records obtained with leaves, further, the multiple apices of the curve show that the response of vegetable organs also has this multiple character Multiple response, however, is not the peculiar characteristic of the electrical organ, but has been shown to take place in various kinds of animal and vegetable tissues.

Again, that this peculiarity—of definitely uni-directioned response, whether the excitation be homodromous or hetero- dromous—is not distinctive of life, with its specific powers of assimilation and dissimilation, but of anisotropy in general, with its consequent differential excitability, was shown by the fact that similar uni-directioned responses to homo- or hetero-dromous shocks were given by an inorganic structure, consisting of prepared lead (fig. 167).

We have next to pass in review the question of the response of plant and animal tissue to stimulus of light The various motile responses, induced by light in plants, are so diverse and so apparently incapable of being explained by any single reaction of fundamental excitation, that it was thought that the effect of this stimulus was different in different cases, the specific reaction in each organ being determined by the ultimate advantage of the plant. But I have been able to show that the excitatory effect of light is normal and like that of any other form of stimulus. The various results induced by it depend, first, on the question whether stimulus has remained localised at the point of

application, or been transmitted to distant areas. The effect | is thus modified by the intensity of the stimulus and the conductivity of the tissue. These results, however, may be further modified by the differential excitability of the organ. Here, as in other cases of stimulation, the general rule holds good that response is by greater contraction and galvano- metric negativity of the more excited. As a concrete example may be mentioned the case of the pulvinus of MW/zmosa, when the upper surface alone is subjected to the stimulus of light. Here, owing to local excitatory contraction of the upper, the expelled water reaches the lower half of the pulvinus and induces there the hydro-positive effect of expansion, both of these effects conspiring, in this first stage of response, to erect: the leaf. The electrical variation at the lower half is here, then, found to be positive. But as the excitatory effect itself is gradually conducted to the lower half, it induces there an increasing contraction. The mechanical response is now therefore reversed, from one of erection to one of depression, the electrical variation of the lower half of the pulvinus undergoing at the same time a corresponding change from positivity to negativity (fig. 237). From this experiment it is clear that the electrical response under light exhibits the same stimulatory changes which are also visibly demonstrated by mechanical response. We see, moreover, from this experiment that light in general acts as a moderate stimulus. For while mechanical or thermal stimulus induces a sudden collapse of the leaf of AZzmosa, the application of light brings about only a gradual fall.

Owing to this moderateness of the stimulus of light, and to the fact that its application is strictly local, it is easy to understand the possibility of certain modifications occurring in the response. Thus, in highly excitable and conducting tissues, the responses will be by galvanometric negativity, and the state of excitation will be conducted to a certain distance. But we have seen that in tissues which are not highly excitable, stimulus, falling below the excitatory value, gives rise to positive response. Thus, under the action of light, we obtain in plants two types of response, negative and positive. Moreover, under continuous stimulation of light, these may undergo phasic alternations (— + — +) or (+ — +.—). Asan example of negative response to direct or transmitted stimulation of light may be seen the response of Bryophyllum (fig. 238), the positive response being exem- plified in the record obtained with a petiole of cauliflower (fig. 240).

It has been explained how these alternating phasic responses lead us to the phenomenon of multiple and autonomous response. | A leaflet of Bzophytum, or a Desmodium \eaflet in a state of standstill, under the continuous action of strong light, will exhibit multiple mechanical responses. The corresponding multiple electrical responses are seen in the response of the lamina of Bryophylum under the action of continuous light (fig. 242). It has also been shown that these phasic alterna- tions are brought about by the fact that the antagonistic elements in the response become effectively predominant by turns. Either of these antagonistic factors may be unmasked more effectively by the arrest of external stimulus at a particular phasic maximum, Thus, in the case where the normal alternation is (— + -— +), if the stimulus be sud- denly withdrawn at the end of the second phase, or positive maximum, the response overshoots in the positive direction (figs. 243, 244). The characteristic direct and after-effects in this Type I., then, during the application of light and its remoyal are ((— + +). In specimens whose characteristic response under continuous stimulation is (+ — + —), if stimulus be again withdrawn at the end of the second

phase—here negative maximum—the response overshoots in the negative direction (figs. 245, 247). The direct and after-effects in this Type III., therefore, may be represented by the formula (+ —...). Between these two extremes lie instances of an intermediate Type II., which has cases (a) and (6), according as the stimulus is removed at maximum of the first or negative phase, or at maximum of the second The response of the retina furnishes us with the most striking examples of the action of stimulus of light. The true character of this response has been supposed hitherto to be unlike that of other tissues, for while excited nerve and muscle were said to show response by ‘ negative variation, the response of the retina was referred to as by ‘ positive variation.’ This furnishes us with an instance of the confusion which is apt to result from making the so-called resting-current the standard of reference. On testing for the natural current, by making connections with the longitudinal surface of the optic nerve, and with the cornea, in an undetached eyeball of frog, I found that it flowed from the cornea to the nerve. But when the eye is detached, by section of the optic nerve, the after-effect of excitation on the more excitable nerve reverses this current, the nerve becoming relatively galvanometrically negative. The normal effect of transmitted excitation from the retina would now make the nerve still more galvanometrically negative, and this would appear as a positive variation of the reversed natural current. Hence, the responsive positive variation, met with in the eye under light, is in reality the same normal excitatory response, by galvanometric negativity, with which we are already familiar.

shocks, that under normal conditions the optic nerve is more. excitable than the cornea, and that the retina is more excitable than the optic nerve. The eyeball and retina have often been found by different observers to exhibit abnormal or reversed response. Now, with regard to reversed response in general I have shown it to be due to either of two different conditions which hold good for all responding tissues. These are in the first place sub-tonicity, and secondly, fatigue. The abnormal response caused by the first has been shown to be converted into normal, in the case of the retina, by the action of an agent which enhanced the excitability.

Another phenomenon which I discovered in the response of retina was that of multiple response, induced by the application of strong or of continuous stimulation. These multiple responses have visual correspondences in the multiple after-images seen in the retina, and in the visual fluctuations which occur under the constant stimulus of light. The latter of these facts was demonstrated by a specially devised stereoscope (p. 432). In this connection may be mentioned the interesting phenomenon of Binocular Alternation of Vision. 3

The various types of direct and after-effects observed in vegetable tissues under light I find to have their close correspondences in the responses of the retina. Just as in the highly excitable lamina of Bryophyllum, constituting Type I., we have the formula of (— + -i-), so also, in the highly excitable retina of Ophzocephalus, the same sequences of direct and after-effects is observed. In less highly excitable vegetable tissues, such as the petiole of cauli- flower, affording us Type III., the sequence was shown to be (+ —...). In correspondence with this may be mentioned the response of the isolated retina of fish, observed by Kiihne and Steiner. In this case, as the effect of isolation, the retina must have become sub-tonic, which supposition is borne out by the fact that its response to the immediate action of light was abnormal positive instead of negative. I found a similar sequence to occur in an isolated sub-tonic retina of Ophiocephalus (figs. 260, 261).

Finally, in somewhat fatigued specimens, an intermediate Type II. was found, in which the sequence was (— + ...) or (—...). Examples of these are afforded by the eye of the frog. These correspondences, between the effects of light in vegetable tissues and in the retina, will be clearly understood from the series of figs. 253 to 261. The next subject to be summarised is that of the electrical response of plants to gravitational stimulus. In an apogeotropic organ like the stem, when laid horizontally, the mechanical response is such as to make the shoot once more vertical. The active factor in this curvature might obviously be, either the responsive contraction of the upper side, or the responsive expansion of the lower. The question to be decided here was whether the response of the plant, to geotropic stimulus, was or was not of the same nature as its response to other effective forms of stimulation—that is to say, by excitatory contraction. An experiment has been described (p. 436) in which this question was subjected to tests. The local application of cold is known to bring about the temporary abolition of the excitatory effect, and in the present case, its application on the lower side of a horizontally laid shoot was not seen to induce any effect on the response, while, when applied on the upper, it retarded - and arrested response to gravitation. This shows that in this response it is the contraction of the upper side which is the active factor. This is independently verified by the test of electrical response, where I find that the upper side, when subjected to gravitational stimulus, exhibits the sign of true excitation—namely, by induced galvanometric negativity.

The important Theory of Statoliths offers us a suggestive explanation of the manner in which gravity exercises stimu- lation upon the responding tissue, by the weight of solid particles. When the stem is vertical, in consequence of the symmetry of distribution of the particles on all sides, there is no resultant action; and as soon as this symmetry is disturbed by laying the stem horizontally, response might be expected to be initiated. This, however, is not the case. The shoot first bends down, and it is not until after the

expiration of nearly three-quarters of an hour that the first sign of apogeotropic action appears. This anomaly is probably due to the induced mechanical curvature caused by weight which has first to be overcome. We may, however, regard ourselves as independent of the mechanical indications, when recording the effect of gravitational stimulus by geo-electric response. The excitatory electric effect, as we have seen in other cases, takes place as before, when all responsive mechanical indications are restrained. Proceeding on this principle, therefore, I found that the geo-electric response was initiated within so short a time of subjecting the specimen to gravi- tational stimulus as one minute (fig. 271). This experiment shows of what widespread application is the electrical mode of detecting the excitatory response of tissues, to many different forms of stimulus,

Transmission of excitation in plants—Vegetal nerve—Similar variations of receptivity, conductivity, and responsivity, under parallel conditions in plant and animal nerves—Conductivity balance—After-effect of section on con- ductivity and excitability—Function of vegetal nerve in plant-economy— Laminz of plant form a catchment-basin for stimulus—Motile response of nerve— Molecular cycle and characteristic changes in response of nerve— Effect of fatigue on transmitted excitation—Similarity of excitatory molecular changes in both afferent and efferent nerves—Multiple response induced by strong stimulus in nerve—Multiple excitations in nerve during drying — Individual contractile responses to constituent tetanising shocks—Negative after-effect on abrupt cessation of tetanisation—Extra-polar effects similar in plant and animal nerve —Inadequacy of Pfliiger’s Law— Under feeble E.M.F. excitability enhanced by anode and depressed by kathode—Demonstration by subjective response—Under feeble current excitation travels better against than with it—Response by variation of electrotonic current due to algebraical superposition of excitatory effect—Physico-physiological basis of sensation— Identification of positive tone of sensation with hydro-positive wave and negative tone of sensation with negative wave—Natural and artificial induc-

tion of dissociation of sensation—Physical explanation of Weber-Fechner’s — Law—Quality of sensation also a factor—Conversion from painful to pleasur- ‘able and vice versa at will by electrotonus—Memory as an after-effect of stimulus—-Persistent after-sensation— Revival of latent memory-image through differential excitation induced by diffuse stimulation—-Same effects demon- strated in the inorganic. THE next subject to be reviewed is that of the conduction of stimulus. It has been supposed that plants do not con- duct excitation by the transmission of protoplasmic changes, as certain animal tissues are known to do. Even in the well- known case of Mimosa, where stimulus is seen to induce move- ment at a distance, this was supposed to be the result of hy- dro-mechanical disturbance. This conclusion has been shown, however, to be erroneous, for pure hydrostatic disturbance

has been proved to occasion an erectile movement of the leaf with galvanometric positivity (figs. 44, 45, and 46). The transmission of true excitation, on the other hand, gives rise to a fall of the leaf and the electrical response of galvano- metric negativity. Again, the transmission of excitation in the plant is modified similarly by those varying physiological conditions- which influence it in the case of the animal. Thus, a strong stimulus is transmitted more quickly, other things being equal, than a feeble. Fatigue, on the other hand, is found to depress the velocity. The application of cold reduces or temporarily abolishes the transmission, while warmth enhances its velocity. Anzsthetics, again, are found to depress conductivity. And lastly, the polar effect of currents, in the plant as in the animal, is to induce opposite changes, according as anode or kathode is applied. I have, moreover, been able to isolate certain tissues specially fitted for the conduction of excitation. These are found in the soft parts of the fibro-vascular bundles, and are particularly easy to isolate in the case of fern. They here possess the relatively high velocity of about 50 mm. per second. It may be said, in view of their peculiar responsive charac- teristics, and the modifications of their response under given conditions, that these structures are indistinguishable from animal nerves, and may therefore be rightly designated vegetal nerves. On isolation, for example, these highly excitable vegetal nerves, like the animal nerve, when isolated, are liable to fall into a state of sub-tonicity, on account of which their conducting power is temporarily impaired. The transmitted effect of stimulus, then, as in the corresponding case of animal nerve, becomes one of abnormal galvano- metric positivity.

Continuous stimulation when in this state, however, by carrying the tissue out of the A into the B condition, converts the abnormal positive response into normal negative, through an intermediate diphasic, in the plant as in the animal nerve. When in the B stage, again, tetanisation -has the effect of enhancing response in both, The effects of ether, carbonic acid, alcohol vapour, and ammonia are the same.in the one case as in the other. The effects of various drugs on the receptivity, conductivity, and responsivity of the vegetal nerve are the same as on those of the animal, and finally, in the action of different salts, the acid and basic moieties exhibit the same characteristic effects in plant and animal nerves alike (Chapters XXXII. and

enables us to study the modifications of conductivity, excit- ability, and responsivity, induced by a given agent separately, but also to compare relative variations as between any two of these, say, for instance, conductivity against excitability, or receptive excitability against responsivity. It also enables us to compare and contrast the action of two different reagents applied simultaneously in different parts of the same nerve. In this way the factor of uncertainty introduced by the unknown individual differences between two nerves is eliminated.

_ The principle on which the Method of Conductivity Balance depends is that of applying stimulus at a point which, in the excitatory sense, is exactly midway between the electrodes E and E’; the excitatory effects at E and E’ exactly balance each other, and the galvanometric deflection is then zero. When the excitability of the right hand, E, or the conductivity of the right arm, C, of the balance is enhanced, the balance is upset and the resultant response is, say, up ; depression, on the other hand, upsets it in the opposite direction. Not only may the effects of various chemical agents be determined by this method, but it is easy also to study by its means the effects of temperature on conductivity. Cold is thus found to depress, and warmth to enhance it (figs. 307, 309). Another important investi- gation carried out by this means was on the curious phenomenon presented by the effect of section in enhancing the excitability of adjacent points. It was shown that this

was due to the molecular transformation caused by the stimulus of the mechanical or thermal section. The effect of such stimulus on neighbouring points is to induce moderate . excitation, raising them to the higher excitability of the condition B (fig. 311).. At or very near the section point itself, on account of over-stimulation, the transformation is to condition D or E, and the result should be one of loss of excitability. In accordance with this, it is found that at such points there is depression of excitability (fig. 312).

As regards the place of the vegetal nerve in the plant economy, it may be said that the normal excitability of a tissue, by which its proper functions are discharged, can only be maintained fully by a supply of energy, which must be received from the environment. Both animal and vegetal ’ nerves have been shown, when isolated, to lose their normal conductivity and excitability, their response becoming ab- normal or being abolished. It is only by the accession of fresh energy of stimulus that the normal conductivity and excitability are restored. It is known, further, that when the nerve loses its excitability, undergoing consequent degenera- tion, the attached muscle also exhibits rapid decay. It will thus be seen that the various tissues of the organism are maintained in their normal functional activities by means of energy conveyed to them through the nerves.

One of the principal forms of energy in maintaining the tonic condition of a green plant is sunlight; when deprived of this, its various normal activities come gradually to a stop, and the plant ultimately dies. But if any portion of the plant be exposed to light even its shaded parts will be found to continue in natural vigour. This is exemplified by the experiment of Sachs, in which an undetached branch of Cucurbita was kept in a dark box, and was found to grow, and produce flowers and fruits, as if under normal conditions. The fact that a plant, when totally deprived of sunlight, dies, shows how essential to its tonic condition is energy of light. The fact that so long as a portion of it is kept in light the whole flourishes, proves the transmission of energy from one

part to another, a transmission which is now made com- prehensible, being effected through the intervention of the plant-nerves, whose existence I have demonstrated. In the case of trees, again, the interior tissues whose: functions are of great'importance in various ways, are inaccessible to such external energy as that of light. But no part of them:is far removed from the vegetal nerves, whose outer endings are found in the ramified venation of the leaves. The lamine of the plant thus in their aggregation form an extensive catchment-basin for the reception of energy from ‘outside and its ultimate transmission within the plant. An experiment has been described which shows the enhancement of the excitability of the plant-nerve by energy of light

excitatory reactions in nerves. It has been supposed that in certain respects the reaction of the nerve is specifically different from that of the muscle. It has been regarded as typically non-motile, the highest power of the microscope being incapable, it was said, of detecting any effect in respon- sive change of form. I have shown, however, that this - conclusion was erroneous, there being in this respect a con- tinuity between the responses of muscle and nerve. In a particular case of frog’s nerve the responsive contraction under strong stimulation was as much as 14 per cent. of the original length, and in others, it was as much as 20 per cent. or more. With a magnification of about 200 times, which is afforded by my moderately sensitive Kunchangraph, the observer is able to study all the excitatory phenomena in nerve with as great ease, and much greater accuracy, as by the employment of a very highly sensitive galvanometer. Records of the electrical responses of nerve are obtained by the differential effects of excitation at the two contacts, when one of these has been subjected to injury. It has been shown that such injury does not always completely abolish the excitability of the second contact, for which reason there may be induced a local reaction of feeble negative or reversed

positive response. The interference of this with the normal response at the uninjured contact is thus apt to give rise to various complications. In contrast with this we have the reliability of the mechanical response of the nerve, in which the effect recorded is direct, and not differential. Again, the electrical form of stimulus, which is almost universally employed for the excitation of nerve, is liable by leakage, unless very great precautions are taken, to vitiate the results obtained by the electrical mode of response. When the response observed, however, is not electrical but mechanical, this source of error is obviously eliminated. } : By means of mechanical response, the molecular trans- formations through which the nerve passes, under the action of stimulus itself, may be observed with the greatest clearness. An isolated nerve, cut off from its natural supply of energy, generally falls into a sub-tonicity indicated in the mechanical record, as an increasing abnormal relaxation; and the application of stimulus induces at this point an abnormal positive response, of sudden expansion. Successive or con- tinuous stimulations, however, transform the nerve from condition A to condition B; the abnormal expansion being arrested and converted into increasing contraction. . During this stage, then, the responses to individual stimuli are trans- formed from the abnormal expansive positive to the normal contractile negative, through an intermediate diphasic. Molecular transformation is here very rapid and the re- sponses show a staircase increase (fig. 382)... An intervening period of tetanisation will now have the effect of enhanc- ing the response (fig. 383).

In the clear demonstration thus obtainable of a progressive molecular transformation, with corresponding variations of response at its different stages, we arrive at the true explanation of the change from the abnormal positive to the normal negative, in electrical response, and also of the enhancement of the normal negative after an intervening period of tetanisation (figs. 275-278). The next stage to be reached is C, where the responses are uniform. After this, we arrive at D, where fatigue-decline

begins to make its appearance. Up to this point, the nerve as a whole has been undergoing increasing contraction, the base-line of the series of records being thus tilted upwards. But after D, it begins to show relaxation, and at the stage E, the responses to individual stimuli are actually reversed, the region of transformation from diminishing to reversed response being often marked by the appearance of diphasic (figs. 396 and 400). The entire responsive cycle may thus be viewed as consisting of two halves of which one is the reverse of the other. From the state of extreme sub-tonicity at A with its abnormal positivity, the responses are transformed through diphasic to feeble normal negative at B. They here increase in a staircase manner, till they become uniform at C. After this begins the reversing process, due to fatigue, brought on by overstrain, with its diminishing normal re- sponses at D, through diphasic, to abnormal positive once

alike find their extreme case in the abolition of all response at death. The difference between the abnormal positive response of sub-tonicity and the abnormal positive response of fatigue lies in their previous history. The one is due to lack of stimulation and the other to itsexcess. For the restoration of normal response, the treatment in the two cases must be opposite. In the first, the application of stimulus is necessary ; in the second, it is its cessation, or rest, which is required.