Comparative Electro-Physiology: A Physico-Physiological Study

We have also seen that a tissue which is not in the highest tonic condition may have its tonicity increased by the action of impinging stimulus, with consequent enhance- ment of its excitability. I shall also demonstrate, in Chapter XXXIV, that the effect of an impinging stimulus on a sub-tonic tissue is a similar enhancement of con- ductivity. The result of this will be either (1) that a tissue which has already conducted a moderate intensity of

FIG. 277. Gradual Transition from Abnormal Positive, through Diphasic, to Normal Negative Responses in Frog’s Nerve stimulus to a distant point will show, after continuous stimu- lation, an enhanced power of conduction ; or (2) that in a very sub-tonic tissue, in which true excitation has at first failed to reach the responding point, the true excitatory negative is subsequently transmitted instead of the hydro- positive alone. Under actual experimental conditions, where the stimulus is applied at a distant point, the twofold effects of exaltation of excitability and conductivity under tetanisation both come into play. In normally responding nerve, the increased con-

duction of excitation, and the enhanced excitability of the responding-point, give rise to an increased amplitude of response after tetanisation, as already seen in fig. 275. Ina depressed nerve, as the transmitted effect is positive, and the tendency of the responding point itself, owing to sub-tonicity, is to the abnormal positive, the record will exhibit the ‘abnormal positive alone, as in fig. 276. But under a series of successive stimuli, the conductivity and excitability of the tissue are both gradually raised, and the effect of this is seen in the consequent gradual restoration of the normal negative response, through the intermediate diphasic (fig. 277). Or, if we do not wish to trace out the intermediate steps of transi- tion, we may tetanise the depressed nerve for a certain length of time, and record only the terminal change to the restored normal negative, as is seen in fig. 276.

Taking one of the extreme cases—say that in which the response to transmitted stimulus is positive, and is converted into normal negative after tetanisation—we see that the first result is due to inefficient conductivity, allowing only the hydro-positive effect to cause response. After this, increasing conductivity, making an increasing transmission of true exci- tation possible, gives rise to a diphasic, and ultimately to the normal negative response. This result is analogous to the three types of responses—positive, diphasic, and negative —which we have already obtained with the imperfectly con- ducting tissue of the petiole of cauliflower and the tuber of potato (figs. 47, 48). We there saw that where excitatory efficiency of transmitted stimulus was sufficiently great, it gave rise to the normal negative response. When this, how- ever, was not so great, we obtained ‘the diphasic. Finally, when the true excitatory effect could not be transmitted, only the abnormal positive response appeared. That gradation by which the transmitted stimulus was made fully, partially, or non-effective, to induce true excitation, was simply and most conclusively carried out in the case of the potato, by removing the point of stimulation to an increasing distance from the responding point. In the cases

described, then, the three types of response are exhibited by the same tissue, in indubitable relation to the variation of its effective conductivity. If, then, results exactly parallel can be demonstrated to occur in the case of nerve also, it. follows that there is no necessity there to make any such hypothetical assumption as that of the evolution of carbonic acid, suggested by Dr. Waller, in explanation of the conver- sion of abnormal response to normal.

In order to show how a varying conduction will give rise to these three types of responses, I shall now describe an Fic. 278. Abnormal Positive Response convérted through Diphasic to Normal Negative under the increasingly Effective Intensity of Stimulus, brought about by Lessening the Distance between the Responding and Stimulated Points experiment which I carried out with a frog’s nerve in some- what subtonic condition. Here, when the stimulator was placed at some distance from the responding point, the response was the abnormal positive (fig. 278). When the effective intensity of transmitted stimulus was now slightly increased by moving the point of application a little nearer, the response became diphasic ; and finally, when the stimu- lator was placed still nearer, the response became normal negative. Thus with an identical specimen we may obtain at will either negative, diphasic, or positive response, by making changes only in the effective intensity of stimulus employed. We have also seen, moreover, that if we kept

the stimulator at a certain distance from the responding point, such as at first to cause only positive response, succes- sive stimulations would then act to enhance conductivity gradually, and thus give rise to the appropriate changes, diphasic and negative in the response. The ultimate cause of these variations must therefore lie in the molecular condition of the tissue. Under varying cir- cumstances, this undergoes a cyclic change, the responsive reaction at any given moment constituting an indication of the particular molecular condition of the tissue. A more complete demonstration of this, carried out by an altogether different method, will be given in a subsequent chapter. My principal object in this chapter has been to prove the efficiency of the thermal shock as a mode of stimulation of nerve. Its wider applicability, in the case of other related investigations, will be treated in the two succeeding chapters,

Specialised conducting tissues—Isolated vegetal nerve—Method of obtaining electrical response in vegeta] nerve—Similarity of responses of plant and animal nerve: (a) action of ether—(4) action of carbonic acid—(c) action of vapour of alcohol—(d) action of ammonia—(e) exhibition of three types of response, negative, diphasic and positive—(/) effects of tetanisation of normal and modified specimens—Effect of increasing stimulus on response of modified tissue.

IT has been shown in the previous chapter that the state of excitation is transmitted to a distance in vegetable tissues. It has also been proved that such transmission is not due to the propagation of hydrostatic disturbance but to that of protoplasmic changes, precisely as in the case of animal tissues. It is obvious, further, that such transmission will be the more perfect the less the interruption of protoplasmic con- tinuity. Hence tissues like stems and petioles, which contain fibro-vascular elements, are found to be good conductors of excitation, whereas indifferent tissues, such as those of leaves and tubers, are relatively feeble as regards this power, excitation in their case remaining somewhat localised.

Even with regard to stems and petioles themselves, a contrast is found to exist in this respect between the fibro- vascular elements and the ground tissue. Thus, in the case of a petiole of cauliflower, I made two experimental prepara- tions. In the first, the ground tissue was cut away, leaving the fibro-vascular elements ; and in the second, a column of ground tissue was left outstanding, denuded of fibro-vascular elements. The former of these was found to transmit excitation to a certain distance, whereas in the latter the transmission was practically absent. In the case of a third

preparation I bifurcated the specimen, stripping away from one of the two limbs the fibro-vascular elements, and from the other most of the ground tissue. Galvanometric connec- tions were now made with the free ends of the fibro-vascular and ground tissues respectively, and stimulus was applied by means of transverse cut, or by application of a hot plate across the area of union. The transmitted effect was now perceived as galvanometric negativity, at the end of that strip which was composed of fibro-vascular elements.

In studying this | subject of conduction, I found the transmitted effect of excitation to be universally well ex- hibited in the petioles of ferns, successive re- sponses, obtained at a distance from the point of stimulation, being in their case singularly perfect and uniform. From this I was led to the conclusion that the disposition of the. conductors must here fic. 279. Frond of Fern with Conducting hie par ticularly uel} Nerves N exposed in Enlarged Figure to Right adapted to their purpose. I had long been desirous of isolating whatever elements in the vegetable tissue were to be regarded as performing the function of nerves, and it appeared to me that I had here found a good subject for this investigation ; and accordingly, on carefully breaking the hard casing of the petiole, and pulling it away in both directions, I was able to isolate the conducting fibro-vascular threads, which were long, soft, and white in colour, remarkably similar in their appearance to animal nerves (fig. 279). These threads vary in number with different species of ferns, and resemble animal nerves in general appearance. It is sometimes

possible to detach one of them having a length of 20 cm. or more. Now the essential feature of a nerve is its protoplasmic continuity, which is ensured by its fibrous structure. And in what I have called the vegetable nerve we find.the same characteristic to hold good. On viewing this structure, as it appears on making a transverse section of the petiole, we find it enclosed within sheath-like sclerenchyma. It © mainly consists in itself of a bundle of fine fibres with a few vessels in the centre. But however remarkable these external resemblances may seem, they are by no means so startling as the more fundamental similarities which are demonstrated so soon as we proceed to subject this vegetable structure to those tests of electrical response which are characteristic of animal nerve. It may be said that for the following investigation the nerves of the common maiden- hair fern (Adtantum) and Nephrodium molle were found most suitable.

In obtaining a plant nerve for purposes of experiment it is possible to dissect it out and at the same time to avoid injury. It is then placed in normal saline solution for about half an hour, so as to remove all traces of excitation due to handling. When the external temperature is not high, the excitability of the isolated plant nerve is found to remain relatively unaffected for a considerable period, but in the hot weather it undergoes rapid decline; and the only way in which I could overcome this difficulty was by placing the specimen in normal saline solution which was ice-cold. The experimental precautions to be taken are precisely the same as those observed in corresponding experiments with animal nerve; that is to say, the specimen should be placed in a moist chamber. For the process of drying is found to induce a transient increase of excitability followed by a permanent abolition of responsiveness, in the one case as in the other. In order to obtain responses, one end of the specimen may be killed by the local application of hot salt solution. The galvanometric connections are then made, one with the killed,

and the other with the unkilled portions of the specimen higher up. In order to ensure that the electrical indication be a true responsive reaction, it is well to use a non-electrical form of stimulus. One of the most perfect forms—as we have seen in the previous chapter, on excitation of animal nerve—is the thermal, and this may be applied in precisely the same manner, that is to say, by means of a platinum wire, surrounding, but not necessarily in contact with, the given area of the specimen, this wire being heated periodically in the manner previously described, by means of a metro- nome closing an electric circuit. With a good specimen, a single thermal shock, lasting for less than a second, will be found sufficient to induce a considerable electrical response, or a response of still greater amplitude may be obtained by the summated effects of several such stimuli. One of the most noticeable differences between this plant nerve and other vegetable tissues lies in its greater excitability. For example, while a single thermal shock of less than one second’s duration is sufficient, as has been said, to evoke immediate and considerable response from the isolated nerve, we find that, in order to evoke similar response from the petiole of the fern as a whole, it is necessary to submit it to the same stimulus some twenty times in succession, the response even after this taking place with relative sluggishness. |

A still further characteristic is its indefatigability. A long series of responses to uniform stimuli, such as would in the case of ordinary tissues bring about marked fatigue, will in that of nerve induce little or none. Rapidly succeeding tetanising shocks, moreover, such as in other tissues induce rapid’ decline, induce, generally speaking, but little of such an effect on the response of nerve. In the case of this vege- table nerve also the same statements hold good. A long continued series of responses shows little fatigue. After tetanisation, moreover, we find that the responses of nerve, whether animal or vegetable, become enhanced.

In the matter of the effects induced by chemical re- agents on animal and vegetable nerves, a further remarkable parallelism is to be observed. The completeness of this may be seen in greater detail in the next chapter. I shall, at the present point, confine myself to giving a few typical cases. Ether, for example, when acting on animal nerve, induces a preliminary exaltation of excitability, which is followed under its long continued action by depression. On blowing off the ether vapour again the original state of excitability is restored. In fig. 280 are seen the similar effects of this reagent on vegetable nerve, where (@) exhibits the normal response, (0) the immediate exaltation due to ether, (c) the

Fic, 280, Photographic Record of effect of Ether on the Electrical Response of Plant-nerve (a) Normal response: application of ether at point marked with ¢; (4) Enhanced response in first stage of action of ether; (c) Subse- quent depression ; (2) Restoration of normal response after blowing-oft of ether. . subsequent effect of depression, which becomes marked after continuous action during twenty-five minutes, and (d) the restoration of the original condition on the blowing-off of the ether.

Carbonic actd is known, in the case of animal nerve, to have the effect, in the first stage, or in small quantities, of inducing exaltation, which passes under its prolonged action, or, in the case of a stronger application, into depression. A similar effect is seen in fig. 281, where (a) shows the normal response of a vegetable nerve, and (0) the preliminary exalta- tion due to carbonic acid introduced into the vegetable nerve- chamber. This is seen to increase continuously for some twenty minutes in (c). But after the expiration of half an hour depression makes its appearance (¢). This becomes still more marked, after the fortieth minute, in (e), |

Fic. 281. Photographic Record of Effect of CO, on Electrical Response of Plant-nerve a, normal responses ; 4 and ¢, enhanced response during first stage of action ; @ and e, subsequent growing depression. Alcohol vapour in strong, or long-continued applications, induces marked decline of response in animal nerve. Parallel effects are seen in the case of vegetable nerve in fig. 282. The effect of ammonia on animal tissues is character- istically different, according as the subject of experiment is

Fic. 282. Photographic Record of Abolition of Response by Strong Application of Alcohol nervous or ordinary tissue. While the excitability of the muscle, for example, is but little affected by its application, that of nerve is quickly abolished. In order to see whether the same characteristic difference would be exhibited, as between ordinary vegetable tissues and vegetable nerve, I first studied its effect on the ordinary tissue of the petiole of

Fic. 283. Photographic Record of Effect of Ammonia on Ordinary Tissue of Petiole of Walnut Note that the effect of ammonia here is practically negligible. walnut. It will be seen from fig. 283 that ammonia here induced a Digs teayy no change in the excitability. But when the same reagent was applied to the isolated nerve of fern the response underwent depression, followed by total abolition, in the course of five minutes (fig. 284). One very curious charac- teristic of the _ electrical response of frog’s nerve is the occurrence, as referred to in the last chapter, of three distinct types of re- sponses, according to its

Fic. 284. Photographic Record of aye : Effect of Similar Application of condition. Thus, as_ has Ammonia on Plant-nerve already been said, while The response here is rapidly diminished highly-excitable nerve é€x- and finally abolished. sae ; hibits the normal negative response, the same nerve, when it has become sub-tonic, will give a mixed or diphasic response; and a nerve which is modified to a still greater extent will show a purely abnormal or positive electrical response. Inthe case of vegetable nerve, J find exactly the same three types of response repeated, under the same conditions. This will be seen in the three sets of records given in fig. 285. The normal responses, which are negative, are here represented as ‘up, while the abnormal positive is represented as ‘ down.’ :

Still more remarkable is the parallelism observed between the effects of tetanisation, on animal and vegetable nerve, both normal and modified. In the case of fresh frog’s nerve the responses are, as we have seen, enhanced, after Fic. 285. Photographic Record of Exhibition of Three Types of Response, Normal Negative, Diphasic, and Abnormal Positive, in Nerve of Fern under Different Conditions a period of tetanisation. The effect of tetanisation on vegetable nerve is precisely similar, as is seen in fig. 286. In the case of the modified frog’s nerve, moreover, it is-found that the abnormal positive response tends, after tetanisation, to become normal. This is seen in the abnormal response, whether positive or diphasic, being converted to the normal negative type. I have obtained exactly parallel effects in the case of modified vegetable nerve. In fig. 287 we see the abnormal diphasic response of vegetable nerve converted, after tetanisation, into normal negative.

Thus, as in the response of animal nerve, so also in that of the vegetable, tetanisation is found to have the effect of enhancing the normal, or converting the abnormal into normal response. The abnormal response of nerve we found to be due to the joint depression of conductivity and excita- bility, on account of which the positive alone, instead of the true excitatory negative, was exhibited. In experimenting with frog’s nerve we saw that abnormal! response might, at will, be converted into normal through the intermediate diphasic by appropriately increasing the effective intensity of stimulation. A simple means of effecting this was to bring the stimulator gradually nearer the responding point.

Fic, 286. Photographic Record of Effect of Tetanisation in Inducing Enhancement of Normal Negative Response in Nerve of Fern The first series of responses seen to he enhanced after intervening tetani- sation, T. In the response of vegetable nerve effects exactly parallel are to be observed. With a given specimen of vegetable nerve, the stimulator had at first been placed at a distance of 2 cm. from the proximal galvanometric contact, and the responses then taken were found to be of the abnormal positive type. The — stimulator was now brought nearer, the distance being reduced to I cm., and the next pair of responses is seen to be diphasic, consisting of a positive twitch followed by the

normal negative response. The distance was next reduced still further, namely, to ‘5 cm., with the result that the Fic. 287. Photographic Record of Conversion of the Abnormal Diphasic into Normal Negative, after Tetanisation, Tr, in Nerve of Fern Fic. 288. Photographic Record showing how the Abnormal Positive ' Response is converted through Diphasic into Normal Negative, by the Increasing Effective Intensity of Stimulus, due to Lessening the Distance between the Responding and Stimulated Points

responses now became normal negative (fig. 288). It is thus seen that there is a continuity of response in the same tissue, as between the abnormal and normal, through the intermediate diphasic. From the various experiments, then, which have been given in this chapter, it will be seen that the response of the isolated vegetable nerve is in every respect similar to the corresponding responses of animal nerve. And we shall also see how, by means of the study of this vegetable nerve, we are enabled to elucidate many obscurities in the responses of the corresponding animal tissue. We shall in the next chapter enter in detail into the question of the modifications induced in the conductivity and excitability of vegetable nerve under the action of various external agencies, and these will be found to exhibit the strictest parallel with corresponding variations induced in the animal. .

Receptivity, conductivity, and responsivity—Necessity for distinguishing these— Advantages of the Method of Balance—Simultaneous comparison of variations of receptivity, conductivity, and responsivity—The Conductivity Balance—Effect of Na,CO, on frog’s nerve—Effect of CuSO,—Effect of chemical reagents on plant nerve—Effect of CaCl, on responsivity—Responsivity variation under KCl—Comparison of simultaneous effects of NaCl and NaBr on responsivity —Effects of Na,CO, in different dilutions on conductivity—Demonstration of two different elements in conductivity, velocity, and intensity—Conductivity versus responsivity—(a) effect of KI—(4) Effect of Nal— Effect of alcohol on receptivity, conductivity, and responsivity—Comparison of simultaneous effects of alcohol—(a@) on receptivity versus conductivity—(2) on receptivity versus responsivity,

WE know that when any point in a tissue is acted on by external stimulus, it receives the stimulation and is thrown into a state of excitation. This excitation is then conducted along the length of the tissue, and may be made outwardly manifest at some distant point by means of a suitable in- dication such as motile or galvanometric response. There are thus three different aspects of the excitatory effect to be distinguished from each other, namely, first the excita- tory effect at the point of reception of stimulus, which I have elsewhere designated receptive excitability, or simply Receptivity : secondly, the power of transmission of excita- tion, or Conductivity: and thirdly, the excitatory effect evolved at the distant responding region, which I shall henceforth term Responszvity. ‘Though these three aspects of the excitatory reaction are all alike dependent upon the molecular derangement caused by stimulus, it is nevertheless important to consider them separately, since their variation is not always the same under the same circumstances. We