Comparative Electro-Physiology: A Physico-Physiological Study

We have next to consider those cases in which in the - sub-minimal region, the difference a—é is appreciable, the reversal to normal 6’>a’ taking place under higher in- tensities of stimulus. There need not in such an instance, be any second reversal. Here, then, the normal response under moderate or strong stimulus is reversed when the stimulus is sub-minimal. An example of this will be given presently. Lastly, there may be a type of response in which in the sub-minimal region the difference a—Jd is slight, and the normal 6’-<a' is reversed to a''+6" in the region of excessive stimulation. I shall be able to give an example of this also.

I have been able, by taking different specimens, to demonstrate the occurrence of these theoretical reversals of response, under sub-minimal and super-maximal stimulation. I have not yet been able to find a single specimen ex- hibiting both reversals, but it is not impossible that this exists. | | The type in which response remains normal throughout a wide range of stimulus-intensity is too numerous to require special illustration. But this normal response may be reversed under fatigue. Generally speaking, a highly

excitable tissue may be expected to show earlier or greater fatigue, than, other things being equal, a less excitable tissue.' Thus, under strong or long-continued stimulation, the excitability of the originally more excitable B may be depressed, so as to fall below that of A, with consequent reversal of response. This will be seen clearly ina typical experiment on the pulvinus of Mzmosa. Electrical con- nections were here made with the upper and less excitable surface A and the more excitable lower surface B, records being then taken of normal responses to equi-alternating

Fic. 198. Photographic Record showing Reversal of Normal Response in Pulvinus of AZzmosa due to Fatigue (az) Series of normal responses, direction of current being from more excitable lower to less excitable upper; (4) Reversed responses in same specimen, due to previous tetanisation, causing fatigue. electric shocks, The stimulus employed was of moderate intensity, the secondary being placed slightly overlapping the primary. The responses (fig. 198, a) are seen to be normal, the responsive current being from the lower to the upper. They also show signs of slight fatigue, their amplitude undergoing diminution. The secondary was then pushed over the primary, and the tissue subjected to the consequent intense stimulus for two minutes continuously.

' In this matter, the nature of the tissue must be taken into consideration, nerve, for example, being less subject to fatigue than muscle. The secondary was next brought back to its original position and a record once more taken of its successive responses to stimuli of the same intensity as before. It will be seen (fig. 197, 6) that the response is completely reversed by the relative depression of the excitability of the lower surface B. Similar reversal under fatigue will be shown in the glandular organ of Drosera in the next chapter (fig. 208). I have obtained other interesting variations of normal response induced by fatigue. Thus, taking the carpel of Dillenia indica, and making electrical contacts with its inner and outer surfaces, the responses under moderate stimulus were found to be normal—that is to say, from inner to outer —the secondary being here understood to be partially over- lapping the primary at a distance of six divisions of the scale. The secondary was now pushed home, and the tissue subjected for a short time to strong and continuous stimula- tion. Moderate fatigue was thus induced. When the secondary was now brought back to the distance of six divisions of the scale the response was found to be reversed. Thus, moderate fatigue had here been sufficient to bring about the reversal of the relative excitabilities of the two surfaces of the carpel of Dz/lenia indica, when the testing stimulus was of original intensity. But when the intensity of stimulus was increased, by pushing in the secondary to a position marked three divisions on the scale, the response became once more normal. Thus, fatigue had in this case modified the excitability of the two surfaces in such a way that an intensity of stimulus, which was formerly effective to induce greater excitation of the originally more excitable, was now ineffective ; and its greater excitation, with the restoration of normal response, could now only be evoked under stronger stimulus.

We shall next describe the reversal ot normal response under sub-minimal stimulation. For this we shall once more select the carpel of Dzd/enia indica. A record of its normal response—the direction being from inner to outer—is given in the first series of records in fig. 199. After taking this record the intensity of stimulus was reduced by pulling out the secondary further away from the primary. The responses were now found reversed, as seen in the subsequent series. | We have last to consider the reversal induced by intense stimulation. Such instances must not be confused with the effect of fatigue. The two can be distinguished by the: fact that the fatigue-reversal takes place after a series of normal responses, whereas the true reversal, due to strong intensity of stimulus, which we are now discuss- ing, is exhibited at the very beginning. Such an effect I have observed in the response of the human Fic. 199. Photographic Record showing lip. The direction of Reversal of Response in Carpel of the responsive current was

Dillenia indica, under Sub-minimal Gisiawbatins normally, under moderate The first series show normal electrical stimulus, from the epi- responses under moderate stimulus, 41,1; . responsive current from internal to thelial to : the epidermal - external surface; the second series surfaces. Under very strong exhibit reversed response under sub- ° : minimal stimulation, current from stimulus, however, this external to internal surface. normal direction was found to be reversed.

But the employment of excessively strong stimulation introduces other complicating factors. ‘The applied stimulus may be supposed to be localised only when it is of moderate intensity. With intense stimulus the subjacent tissues are liable to be involved in giving rise to excitatory response ; and it then becomes a difficult problem to discriminate how much of the observed effect is due to the superficial layer, and how much to others more deeply situated,

Consideration of the functional peculiarities of the digestive organs— Alternating phases of secretion and absorption—Relation between secretory and con- tractile responses. [Illustrated by (a) preparation of AM/zmosa; (6) glandular tentacle of Drosera—-General occurrence of contractile response—True current of rest in digestive organs—Experiments on the pitcher of Mepenthe—Three definite types of response under different conditions-—Negative and positive electrical responses, concomitant with secretion and absorption—Multiple responses due to strong stimulation— Response in glandular leaf of Drosera— Normal negative response reversed to positive under continuous stimulation —Multiple response in Drosera—Response of frog’s stomach to mechanical stimulation — Response of stomach of tortoise—Response of stomach of gecko , —Multiple response of frog’s stomach, showing three stages—negative, diphasic, and positive—Phasic variations.

HAVING now dealt with the responsive characteristics of the skin, epithelium, and glands, alike in plant and animal, the next subject to be taken up is that of the response of the digestive mucosa. And here we have to determine, first, whether or not there is, broadly speaking, any continuity between the responses of digestive organs and those which we have just been studying ; and, secondly, to what extent the functional specialisation of the tissue has acted in accentuating certain of its responsive peculiarities.

Surveying the function of digestion as a whole, we see that it consists, briefly, of two different processes—those, namely, of a previous secretion, by which food is rendered soluble, and of a subsequent absorption, by which the dis- solved foodstuffs are absorbed. In the membrane of the simplest digestive organ, then, the epithelial ‘lining, using that term in its most inclusive sense, must be endowed with the two properties of secretion and absorption under different

circumstances. The question then suggests itself, what are the circumstances which determine this outflow or inflow ? In the digestive processes, moreover, in plant and animal alike, each of the reactions referred to, whether of secretion or absorption, must be more or less long-continued. Thus these responsive actions, instead of being single and spas- modic, are likely to be multiple and long-sustained. The characteristic response to stimulus of a secreting organ is.

understood to be by secretion. Is this reaction essentially different from those fundamental processes which underlie the responses of contractile organs, or are we to regard con- traction and secretion as but different expressions of a single responsive phenomenon ? | In order to test this question, of the connection between responsive secretion and contraction, it will be well here to draw attention to certain experiments of Sachs, on the response of J/zmosa, though our inferences will be somewhat . different from those which their author intended. If we take a longitudinal slice of the lower half of the pulvinus of Mimosa and keep it in a moist chamber for some time till _ the tissue has recovered from the excitation due to section, and if we then subject it to fresh excitation, water will be found to ooze out, or undergo secretion from the excited tissue. We may explain this occurrence in either of two ways: first, that, in consequence of the molecular changes induced by stimulus, contraction and permeability-variations take place in the cells, the expulsion of water being an expression of the active process of contraction ; or, secondly, that the oozing-out of the water is a passive process, due to permeability-variation of the turgid cells alone, without con- traction.

The two theories may be distinguished broadly as those of active contraction and passive secretion. In the thin section of the Mzmosa pulvinus it is the exudation of water that is noticeable, and not any marked movement character- istic of contraction. But in the intact pulvinus, owing to,its anisotropic structure, the greater contraction of the more excitable lower half is exhibited in a marked manner by downward mechanical movement, magnified as this is by the long petiolar index. The intact organ, moreover, is invested with an impervious skin, hence the excitatory exudation, or expulsion, of water, being internal, is not seen outwardly. Thus a single identical reaction may appear from different points of view, as either secretory or contractile.

The occurrence of contraction is thus most easily demon- strable when it is accompanied by conspicuous movement. This, however, demands considerable physiological aniso- tropy, the differential contraction then giving rise to a very marked lateral movement, as in J/zmosa. Ih radial organs of plants, on the other hand, owing to balanced contractions of opposite sides, there is no marked responsive movement. Hence ordinary plant-organs have hitherto been regarded as non-contractile and insensitive. But I have shown that all these radial organs exhibit longitudinal contraction, to be detected and recorded by means of suitable magnifying devices. All motile responses are brought about, it must be remembered, by transference or redistribution of fluids. Now, in organs invested with impervious membranes the effect of fluid-transference is. manifested by mechanical movement ; whereas, in naked tissues, the fluid-transference is directly visible as secretion. |

From the very important series of researches carried out by Darwin, on the excitatory reactions in the tentacles of Drosera, we know that the pedicel, carrying the gland on its summit, is somewhat flattened, and that it is this anisotropic lower part which is alone capable of movement. The gland- cells on the head of the tentacle have been shown by Gardiner to be provided with delicate uncuticularised cell- walls, which are curiously pitted on their upper or free surfaces. The terminal organ, or head, which is radial, would thus seem to be peculiarly fitted for the exudation of liquid on excitation. In the anisotropic motile portion of the pedicel, on the other hand, the responsive reaction mani- fests itself by bending. It would thus appear that the same

excitatory reaction may exhibit itself in different parts, even of the same organ, by mechanical movement and secretion respectively, according to the facilities which one or the other portion offers. That the phenomenon of contraction is behind the ex- citatory expulsion of water in a vegetable organ, would appear highly probable from certain results obtained in the | electrical response. The stimulation of an ordinary vegetable tissue gives rise to two distinct electrical effects at a distance. The first of these is the arrival of the hydro-positive effect of galvanometric positivity, with positive turgidity-variation. The second is the wave of true excitation, with its character- istic of negative turgidity-variation and concomitant gal- vanometric negativity. The first, consisting, as this does, of a hydrostatic blow delivered at a distance, can only, it appears to me, be ascribed to an active process of contrac- tion, causing the squeezing-out of water in the excited region. A passive escape of fluid, due to mere permeability-variation, could not, as I think, originate that impulsive hydrostatic shock which is transmitted to a distance. For such a result to take place an active expulsion would seem to be requisite.

In view of these facts, is it necessary to hold the doctrine of discontinuity, or, when there is evidence in its favour, are we to believe in the continuity of these apparently different reactions? The excitatory reactions of different classes of tissues have hitherto been regarded as different, chiefly because some were looked upon as motile, and others as non-motile ; muscle, for example, was held to be typical of the first, and nerve of the second, of these classes. In this case of the nerve, it has been believed that there was no visible manifestation of the excitatory change. I shall, however, be able to show that even this supposition is incorrect, since the excitatory reaction in the nerve is in fact attended by contraction. The electrical indication of gal- vanometric negativity which is concomitant with contraction in contractile tissues, is also obtained in the case of excited

glands. The visible changes which occur under stimulation in these three types of tissues would thus appear to differ only in degree. ae Io We may now turn more especially to the question of the electrical reactions of the digestive mucosa. As regards the natural current of rest, we have seen that Rosenthal and others found that this current was ingoing—that is to say, the mucous layer was negative, as compared to the muscular coat of the stomach. Biedermann had also noticed a strong current of rest between the glandular surface of Drosera and the stalk. But it will be shown that the glandular coat of the stomach is more excitable than the muscular layer. Hence we should have expected that the natural current of rest would have been from the less excitable to the more excitable, the mucous layer in a state of rest being thus relatively galvanometrically positive. The opposite direc- tion of the current which has been observed, would rather appear to be ascribable to the excitatory after-effect of pre- paration. I have already described how the glandular foot of the snail, under conditions of perfect rest, is galvano- metrically positive. But the excitation caused by prepara- tion renders this highly excitable glandular surface negative That the fact of cutting open the stomach, to make the experimental preparation, similarly, would cause intense excitation with galvanometric negativity, was to have been expected. I shall be able, indeed, to show, by means of experiments to be described presently, that the shock con- sequent on this preparation is to give, not one, but a pro- longed series of multiple electrical responses.

I have almost invariably found, in making electrical contacts after section, with the inner and outer surfaces of frog’s stomach, that the multiple responses caused by section, persisted for more than an hour; and until these had subsided no fresh experiment could be undertaken, to obtain records of the response of the stomach to external stimulus. I have also found that many of these frogs were in the habit of swallowing stones and pebbles, the persistent

mechanical irritation of which would contribute to the negativity of the mucous lining. Finding, then, that it would be impossible to obtain the natural current, in a stomach which had to be cut open, I next turned my attention to stomachs which are naturally open. These are seen in the upper concave surface of the leaf of Drosera, for instance, which is provided with glandular tentacles. I here made electrical connections with the upper and lower surfaces respectively. But the tentacles excited by the contact of the electrode bent and clasped it round, an excitation which was seen in the galvanometer as negativity of that surface. From this may be gauged the difficulties which attend the observation of the true natural current of rest in such an excitable organ as the stomach. The demonstration, however, of the galvanometric positivity of the snail’s foot, and of the inner glandular surface of the carpel of Dzllenza indica, lead to a strong presumption in favour of the true resting-current in the stomach being from the non-mucous layers to the mucous.

Having thus seen the difficulties imposed by the high motile excitability of the tentacles of Drosera, 1 next turned my attention to other specimens. We have seen that there is a secretion of fluid at the lower end of the hollow interior of the peduncle of Uvzcls lily, and that the secreting inner layer is here galvanometrically positive in a state of rest. As this tube, however, is closed, it cannot be regarded as subserving the absorption of food-material. But the same limitation does not apply to those modified foliar structures, the pitchers of Vepenthe ' (fig. 200). These, as is well known, are open. They have a histological differentiation, moreover, of their lining membrane, actual glands being present (figs. 201, 202), which are admitted to be comparable to those of the animal digestive organ, though of a much simpler type. A fluid is secreted by these glands, and insects entrapped in the pitcher are in it dissolved or decomposed. The products

1 I have to thank the authorities of the Botanical Garden, Sibpur, for supplying me with these valuable specimens. are subsequently absorbed by the tissue, as in corresponding cases, by the stomach of animals. From the study of the responsive peculiarities of so primitive a type of stomach, we might then expect to gain much light on the action of more complex and highly specialised digestive organs. In order first to obtain the true current of rest, I took a young pitcher which had previously been kept free from all 16 disturbance. I next made electrical INN ; yl connections, by means of non-polaris- Ly ans able electrodes, with the inner (glan- i Bt ha dular) and outer surfaces of this iy pitcher respectively. As some excita- ianuil tory reaction may be induced in a 1 Mm ZN highly excitable organ, even by the wo BE contact of normal saline, the cotton ] threads in connection with these non- F polarisable electrodes were moistened Vy WE with the natural secretion of the Woe pitcher itself. On carrying out the experiment under these ideal condi- tions, I found, as I had expected, that the current of rest flowed from the outer non-glandular to the inner glandular surface, the latter thus being galvanometrically positive.

In investigating next the excitatory reaction, I obtained three different types of responses—negative, diphasic, and positive—characteristic of certain definite conditions. Before entering upon the details of these experiments, it is advisable to discuss here the probable significance of the negative and positive electrical reactions observed. We have seen that a slice of tissue from the’ pulvinus of Mimosa excretes water under excitation. The electrical reaction under these circumstances is one of galvanometric negativity. But during the process of recovery, when the tissue is absorbing water, this negativity diminishes, a change that is tantamount to that increase of positivity with which

we are already familiar, as the invariable accompaniment of a Thus, if galvanometric negativity is to be taken as the concomitant of the expulsion or secretion of fluid, it would Fic. 201. Glandular Surface of a iin of the Living Membrane of the Pitcher appear that the opposite process of absorption would be indicated by the respon- sive galvanometric tivity. Again the fresh pul- vinus of J7Zzmosa responds, when excited, by. a me- chanical fall, a negative turgidity-variation, and by galvanometric negativity.

stimulation, these normal responses are found to undergo reversal. The pulvinus expands; water must be re-absorbed, and the leaf is re-erected. The normal galvanometric nega- tivity is now reversed to positivity. It will thus be seen that while, in a fresh tissue, stimulus gives rise to expulsion of fluid—the electrical indi- cation of this being galvanometric nega- tivity—in a tissue which has already, on the other hand, been under con- tinuous stimulation there will be a tendency towards the phasic reversal of re- sponse to galvanometric positivity, indicative of the process of absorption.

In the case of motile tissues, these excitatory reactions of the outflow and inflow of fluids appear to us of little consequence, except in the form of those appropriate wu, outer surface; L, inner surface; g, glands present in the internal surface. motile responses which they occasion. But in glandular organs, they become possessed of much greater significance, since they constitute the main function of such structures. Electrical responses, in every way analogous to those which have been described, are obtained from the glandular surfaces of digestive organs. That is to say, the glandular surface when fresh exhibits responsive galvanometric nega- tivity on excitation. In Drosera, for example, under these conditions secretion is seen to take place. Thus the negative phase of response, in this as in the case of Mzmmosa, is asso- ciated with expulsion of fluid or secretion. After continuous stimulation, again, the responsive phase here, as in MWzmosa, is found to be reversed to positive, indicative, as there is every reason to believe, of absorption. From a consideration of the functions of the digestive organ, we should be prepared, as already pointed out, to expect the occurrence of two alternating processes. In the fresh state, ingestion of food, acting as a stimulus, would naturally induce excitatory secretion; and this excitatory secretion must be followed later by the absorption of dissolved food. These alternating phases of secretion and absorption indubitably occur. We shall also find, in the electrical response-records, a phasic alter- nation of negative and positive under appropriate conditions.

We have seen that. there is a continuity between the different reactions of non-glandular and glandular tissues. We have also seen that, as in the one case, so too in the other, a phasic change takes place from negative to positive. Inthe digestive organ, however, we have to deal mainly with the fluctuations of fluids—secretion and absorption—and the attendant electrical variations, which consist of two opposite phases, positive and negative. As far as I have found it possible to test the matter experimentally, it has invariably been the case that the negative electrical phase was associated with secretion ; and everything points to the probability that the converse of this—the association, namely, of. the positive electrical phase with the process of absorption—-holds equally good.

We now return to the question of the normal response of the pitcher in its three different conditions. Of these, in the youngest, no flies are present; such a specimen will be known as ‘fresh.’ Jn others, somewhat older, are a few insects. These pitchers may be regarded as moderately excited, owing either to the struggles of the insects or to the supply of food, or to both. Still another class is found, in which the glandular part of the inner surface of the pitcher is practicaily coated with captured insects, and has thus already been subjected to long- continued stimulation. The responses of these three classes of specimens are in each case, as I shall show, very characteristic.

I shall first describe experiments carried out on fresh = specimens. Records were made of their responses to equi- alternating shocks of moderate intensity, at of Normal Negative Responses of Glan- dular Surface of Wepenthe in Fresh Con- dition to Equi-alternating Electric Shocks given at Intervals of Two Minutes Responsive current from internal glandular to external non-glandular surface. Note occurrence of multiple response and trend intervals of two minutes. The responsive current was here found to flow from the internal glan- dular to the external non-