Comparative Electro-Physiology: A Physico-Physiological Study

When we come to test the electrical reactions of these tissues, we find, on making electrical con- nections with the external epi- dermis and the internal epithelium, that a natural current flows in the tissue from the external surface yc, 192, Transverse Section of to the internal: This would indi- tissue of Hollow Peduncle of : ‘ Uriclis Lily cate that the internal was the Cells of epidermis are small and more excitable of the two. This thick-walled, those of inner

: . surface large and thin-walled. conclusion is confirmed on the application of simultaneous excitation to outer and inner ; for the direction of the responsive current is found to be from the internal surface to the external. _ If, next, electrical connections be made with the.epidermal and secretory layers, a current of rest is once more observed from the external to the internal. On excitation, a very strong electrical response is given, its direction being from the highly excitable secreting layer to the less excitable epidermal. From these experiments we see that the

epidermal cells are, generally speaking, the least, and the secreting cells the most, excitable. I shall show, moreover, that all the responsive character- istics of these secretory cells are to be found repeated in those admittedly glandular layers which occur in the lining of the pitcher of Vepenthe, and cover the upper surface of the leaf of Drosera. Before, however, entering upon the consideration of these highly differentiated organs of Nepenthe and Drosera, - which are further characterised by some of the digestive functions, I shall first discuss in detail the reactions of a simpler type of vegetable organ. This is exemplified by a single unripe carpel of Dz/lenia indica, already referred to, When this is carefully removed from the inside of the pseudocarp, and opened, the inside is found to be filled with a gelatinous secretion. This is gently removed, and electrical connections are made with the inner and outer surfaces. It must be borne in mind that these vegetable organs, being not highly excitable, admit of experimental preparations being made with little or no excitatory effect of injury. Allowing now for a period of rest after making the preparation, it will always be found that the current of rest is from the outer layer to the secreting inner layer, which latter is thus, relatively speaking, galvanometrically positive. From the fact which we have generally observed, that the natural current of rest is from the less to the more excitable, it would appear, then, that the inner layer is here the more excitable. This conclusion, moreover, is in agreement with the inference already arrived at, in connection with the tissues of the Uriclis lily, that secreting cells as a rule are relatively the most excitable. This inference may, however, be subjected to the test of direct experiment.

I first tested the response of the same specimen by means of thermal shocks, applied to both surfaces simultaneously. The definite direction of the responsive current, from the inner to the outer, across the tissue, proved conclusively that * the inner surface was the more excitable, becoming as it did, galvanometrically negative in relation to the outer, A similar effect was obtained as an after-effect of equi-alternating shocks. I next took records of the direct effect of equi-alter- nating shocks. The direction of the responsive current was found, as before, from the inner to the outer.

In the fruit of water-melon we obtain another specimen whose interior cavity is filled with secretion. On making a suitable preparation of this specimen, and arranging electrical connections with the outer epidermal and inner secretory surfaces, I found the responsive current, under equi-alter- nating electrical shocks, to be from the inner secretory to the outer epidermal. Fig. 193 gives a photographic record of these responses. From the typical responsive effects thus obtained with vegetable specimens under the simplest conditions, we are enabled to see that the effect of localised stimulus depends on the characteristic response of the surface layers of the organ. When dealing with this question of the electrical reaction of epi- thelium and glands in animal tissues, Biedermann rightly Fic. 193. Photographic-Record of

: , Responses of Water-melon to came to the conclusion that it Equi-alternating Electric Shocks was the surface epithelial layer Responsive current from internal which was, in an electro-motive stale to external epidermal sense, most effective, the term, in its widest sense, including the epithelium of glands and papillz. One complicating factor present in the electrical reactions of animal epithelia and glands, but relatively absent under the simpler conditions of the plant, is the effect of injury caused by the process of isolation. The very fact of making the neces- sary section involves a stimulus of great intensity, and unless the effect of this has thoroughly subsided, the after-effect of such stimulation may be so strong as to reverse the normal current of rest, and otherwise modify the excitability of the tissue, This is seen, for example, in an experiment carried

out by Dr. Waller,’ on the isolated paw of a cat. The current of rest was found by him to flow from the surface of the pad to the section. From this he was led to the conclusion that this current could not have been due to injury, since in that case it would have flowed from the sectioned to the uninjured surface, and not in the opposite direction, as was found to be the case. the so-called ‘current of injury’ is, in fact, an after-effect of excitation. In the case under consideration, the section, acting as an intense stimulus, simply induces greater excita- tory reaction of the more excitable, which in this case happens, as we should have expected, to be the glandular surface. The injury-current here, then, is necessarily from the more excited glandular to the less excited non-glandular.

A precisely similar result was obtained in the case of anisotropic plant-organs, where excitation caused by injury of the less excitable side, becoming internally diffused, induced greater galvanometric negativity of the more ex- citable distal point (p. 162). For a typical experiment on a glandular preparation, showing the principal effects, and the complications that may arise owing to injury, we may take the detached foot of the pond-snail, the lower surface of which, as is well known, secretes a slimy fluid. On allowing ‘for the necessary period of rest, and then making electrical connections, we observe a current of rest, so-called, which flows from the glandular to the sectioned end. This is not to be mistaken for the true natural current of rest, being in fact the after-effect of a greater galvanometric negativity at the more excited glandular surface, consequent on section. An independent experiment, in support of this induction, will be described presently. On now simultaneously ex- citing the two surfaces, by equi-alternating shocks, the responsive current is found to flow from the gland inwards, the more excitable gland becoming thus galvanometrically negative. The responsive current is in this case in the same

direction as the so-called current of rest, constituting a positive variation of it. From these experiments it is clear that the responsive current is due to the greater intensity of the induced gal- vanometric negativity at the more excitable glandular surface. It must, however, be noted here that this definite understanding of the phenomenon has been arrived at by fixing our attention on the relative excitatory reactions at the two contacts. If, instead of this, we had regarded it from the usual point of view, of variations of the resting-current only, we must have interpreted it as apparently an abnormal positive variation ;! for the so-called resting-current, in such a case, on account of the excitatory after-effect of injury, must also, as we have seen, flow from the more excited gland to the less excited muscle. Great confusion, and resultant misinterpretation of observations, have arisen from not sufficiently recognising these facts, that the resting-current may be originated in either of two distinct ways, and that the excitatory effect may consequently be summated with it in different manners. The resting-current in the primary condition is, as I have demonstrated elsewhere, the natural current. This originates in the natural differences of ex- citability between different points, and is, in the intact specimen, through the tissue from the less to the more ex- citable. External stimulus now gives rise to a responsive current, which is in the opposite direction, and therefore constitutes a negative variation of this, This takes. place because the more excitable point, which was naturally positive, has now become negative. But we may have a current of rest which is due to previous excitation, or injury, such as may be caused by the shock of the pre- paration. This current, though usually regarded as the resting-current, is not the true natural current of rest. It is really, as it were, the responsive current. become persis- tent.

Succeeding stimuli, inducing responsive galvanometric 1 We shall find in Chapter XX VII. that similar misinterpretations have arisen with regard to the responsive current in the retina, p. 417. negativity of the more excitable, will now give rise to a current in the same direction as this resting-current, thus constituting a positive variation of it. It is only when fatigue has set in at the more excitable, and induced great depression of excitability there, that the response-current may undergo a reversal, its direction now being from what was. originally the less excitable, to the originally more excitable (p. 177).

An example of this I found in the sectioned foot of the large Indian garden-snail. Here the excitatory action on the glandular surface, due to the shock of preparation, was ex- tremely great, as evidenced by the profuse secretion which occurred iinmediately. Owing to this over-stimulation, fatigue was induced, with consequent great depression of ex- citability. Hence the responsive current was now found to be reversed, having, with reference to the glandular surface, become outgoing instead of ingoing.

It has been stated above that the ingoing current of rest, observed at the glandular surface under preparation, was not the true natural current, but due to the excitatory after-effect of injury. ‘This I was able to verify by observing the current of rest under natural conditions, without excitation. The snail was allowed to crawl on a glass surface, in the middle of which was a strip of linen moistened in normal saline. This brought the glandular surface into electrical connection with one of two non-polarisable electrodes. When the snail had of its own accord come to a temporary standstill on this piece of linen, the other electrode was quietly placed against the skin of the upper side of the protruding body. The natural current of rest was now found to be outgoing, as regards the glandular surface of the foot, the more excitable being thus galvanometrically positive. The absolute electro-motive difference was found to be + ‘0013 volt. The foot was now sectioned, and the difference of potential between the same points was found to have undergone a reversal. The supposed resting-current was now ingoing, through the glandular surface. Thus, owing to the excitation consequent on preparation, the more excitable surface, originally positive,

had become negative. The induced variation from the original condition, in the present case, was from + ‘OoI3 volt to — ‘0020 volt. It will thus be seen that any irritation is liable to change the natural positivity of a highly excitable glandular surface to negativity. The supposed similarities between the ingoing responsive currents of frog’s skin, and the glandular surface of the stomach, are therefore not real. That the two cases are quite different is proved indeed by the fact that local stimulation of the surface of the skin induces galvanometric positivity, whereas a similar stimulation of the glandular surface induces negativity.

In experimenting on animal tissues, it is therefore ad- visable, wherever possible, to use intact specimens. The numerous experimental difficulties with which we are in that case confronted, may be overcome by the method of simul- taneous and equi-alternating shocks which has been described. How practicable this method has been rendered will appear from the experi- ments which | have yet to describe on human subjects. We have seen that a protected sur- face is likely, other things being equal, to be more excitable than an exposed one. Partly owing to this fact, and partly also to its richer possession of imbedded glands, it appeared to me probable that the inner surface of the armpit would prove electrically more pig. 194. Photographic excitable than a corresponding area, on, Record of Electrical

Responses of Intact say, the upper and outer surface of the Human Armpit same shoulder. In the records which I Responsive current from succeeded in obtaining (fig. 194), this “Pit to shoulder. supposition was fully borne out. Equi-alternating shocks of one second’s duration were applied at intervals of one minute, and the direct effect photographically recorded. The re- sulting responses were found to be ingoing as regards the armpit, thus proving that that surface was the more excitable.

In order next to show that epithelial cells in the animal are relatively more excitable than epidermal, as we have already found to be the case in vegetable tissues, I performed the following experiment on the human lip. Here it was important that the electrical connections should be main- tained steady. A light spring-contact key was therefore made, as seen in the lower part of fig. 195. The lower. contact of this spring-clip consisted of an amalgamated plate .

of zinc leading to the lower electrode. Over this were tied four thicknesses of blotting- paper soaked in zinc sulphate solution. On this again were placed three more thicknesses of blotting-paper, soaked in normal saline. The zinc plate which formed the upper limb of the clip, in connection with the second electrode, was similarly covered with separate layers of blotting-paper, soaked in zinc sulphate and normal saline re- spectively. The protruded lower lip was now placed in the clip, as shown in the upper figure, in | such a way that the latter made : ee eS of Towne a gentle but secure contact. A

Lip. galvanometer and a source of Lower figure gives an enlarged view equi-alternating currents were oe also placed in the circuit. Of the two electrodes, the upper was in connection with the epithelial, and the lower with the epidermal surfaces. The natural current was now found to flow in the tissue, as in the corresponding cases of plant specimens, from the epidermal to the epithelial. The perfect steadiness of the contact was evidenced by the stillness of the deflected galvanometer spot of light. On now applying the alternating excitatory shock, the responsive current was found to be in the oppo-

site direction to the natural current, thus demonstrating the fact that the epithelial layer was here, as in the plant, the more excitable of the two. The regularity of this effect will be seen from the series of photographic records given below (fig. 196), in which is exhibited a slight staircase effect. We next proceed to deal with the response of the glan- dular organ, the tongue. The tongue of the frog has formed the subject of a very extended series of researches, by Engelmann and Biedermann. On_ very careful isolation, entailing as little injury as possible, it was found by these workers that the natural current was ‘entering’ that is to say, it flowed across the tongue from the upper surface to the lower. Both electrical and mechanical stimulation was found by these observers to cause a negative variation of this natural current.

As isolation of such a highly excitable organ as the tongue may, however, give rise to unknown excitatory after-effects, it ap- peared to me very desirable that an investi- gation on this subject should be carried out * pei Se prides on the intact human tongue. In connection Electrical Response with this, I must point out that both the Sere pen surfaces of the tongue are excitable. Our Responsive current inquiry, therefore, is into the relative excit- ‘Tom epithelial to

epidermal surface. abilities of its upper and lower surfaces. Here the experimental difficulty lies in this very high excitability of the organ, on account of which—except when in a quies- cent state and with a very steady contact—the galvanometer spot of light is apt to be erratic in its movements. Much of this difficulty is overcome, however, by holding the pro- truded tongue lightly clamped between the teeth. The upper and lower surfaces may then easily be held in the clip-key already described. From this double support of the clip and the teeth it is, with a little practice, possible to arrange matters in such a way that the galvanometer spot is

practically stable. The current of rest in the intact human tongue is then found to be from the upper to the lower surface, as in the frog. This, according to our previous results, would indicate that the upper surface is the less excitable. This inference finds independent verification, when we subject the organ to the stimulus of equi-alternating shocks. A very The tongue is so extremely sensitive that its characteristic response can be evoked even with very feeble stimulus. I have already explained that the alternating currents induced by speaking before a telephone are not exactly equal and opposite, the current being slightly stronger in one direction. Hence, if such currents be made to play upon an organ in which the excitability is only moderately differential, the preponderance of one of the two elements of the alternating shocks is then likely to mask the true excitatory effect. But the differential excitability of the tongue is so great that the responsive current is always from below to above, whether the exciting current be made to act in a favourable or unfavourable direction. Thus, if one speak, even in a very ordinary voice, into an exciting telephone, which is in series with the rest of the circuit, with its poles direct or reversed, a definite lingual current. is induced in response. This, as already said, is always in direction from the lower surface to the upper—surely a curious instance of the speech of one inducing lingual response in ariother, by direct, and not by provocative action !

The results which have been described are the normal effects given in response to stimulus of moderate intensity. By moderate stimulus is here meant that intensity of current which is obtained when the primary coil is slightly within the secondary. By feeble, on the other hand, is meant the intensity produced when the primary is at a distance from the secondary. Excessively strong stimulus again occurs when the primary is pushed fully within the secondary. I shall now proceed to describe occasional variations which

may be observed when the stimulus is either very feeble or excessively strong. We have seen (p. 83) that when the intensity of stimulus is below the critical degree which is sufficient to induce response, its effect is to increase the internal energy of the tissue. We have also seen that the sign of this increased internal energy is galvanometric positivity, being thus opposite to the excitatory effect. Hence, in a dif- ferentially excitable tissue, we may expect to find instances in which stimulus that falls below the threshold of true excitation will act by inducing a greater galvanometric positivity of the more excitable, whereas, under normal intensity of stimulus, the more excitable would have become galvanometrically negative. We can thus see the possibility of response being reversed under very feeble stimulus.

It must be remembered that the excitability of both the contacts is a factor in the response, which has hitherto been overlooked. A second very important factor, which has not yet been taken into consideration, is the difference between the characteristic curves of the tissues at the two different surfaces. By characteristic curve is here meant the curve which shows the relation between intensity of stimulus and response. This difference will be better understood from the diagram of the theoretical curves given below (fig. 197). This exhibits all the cases that can possibly exist. : .

Let the curve A aa’ a" represent the characteristic curve of the surface A. Let the curve B 0 0’ 6” similarly represent the characteristic curve of the surface B. Of these two surfaces, B is under moderate stimulation, normally the more excitable. In the middle portion of the curve, representing response under moderate intensity of stimulus, the induced galvanometric negativity of B is thus greater than that of A. Under moderate excitation, therefore, the current is d’>a! through the tissue in the direction from B to A. But below the threshold of true excitation, B would be positive, and A

relatively negative to it. Hence there would here be a reversal of response, the direction of the responsive current a> through the tissue being now from A to B. This current will be recorded by the galvanometer, provided the induced difference between A and B be sufficiently great. Having thus inferred the different effects possible under sub-minimal and moderate stimuli, we shall next consider excessively strong stimulus. In the middle part of the curve,

Fic. 197. Possible Variations of Responsive Current, as between Two Surfaces A and B, shown by Means of Diagrammatic ‘Representations of Characteristic Curves A, a, a’, a”, characteristic curve of surface A; B, 4, 6’, 6", that of B. Under moderate stimulation, B is the more excitable, its induced galvanometric negativity being greater, and the direction of current from 4’ Zo a', as in the middle part of the curve. Under sub-minimal and super- -maximal stimulation the direction of the responsive current is reversed to a > 6 and a" - 6” respectively.

that is to say, the excitatory effect increases very rapidly with the stimulus, in the more excitable of the two surfaces. But this increase may sooner or later reach a limit, that curve tending to become horizontal, aided in this process, possibly, by growing fatigue. The curve A a@a' a’, however, though not so steep, may yet continue to rise throughout a longer abscissa, representing increasing intensity of stimulus. In such a case, there would be a second crossing-point, and

a second reversal of normal response into a@’’>0", under excessively strong stimulation. We are thus enabled to see the theoretical possibility of the reversal of normal response under the two conditions of sub-minimal and super-maximal stimulation. All these phases may not be displayed in the same specimen; but it may be possible to find different specimens exhibiting one or the other. In some cases the difference a—d is too small to allow of an appreciable galvanometric effect, and in their higher parts the curves do not cross. In such specimens, then, there is no response under sub-minimal stimulus, and only normal response under increasing intensities, however strong. The only exception to this will take place when fatigue supervenes, a case which will be dealt with presently. I find that this type of response is the most common.