Comparative Electro-Physiology: A Physico-Physiological Study

Of great importance was the investigation carried cut by Du Bois-Reymond on the effects induced in the electrical organ by the passage of currents in different directions. Polarising-currents in the direction of the natural discharge of the organ are distinguished, in the terminology introduced by Du Bois-Reymond, as homodromous, and those in the opposite as heterodromous. Polarisation-effects in the direction of the natural discharge he distinguishes as ‘absolutely positive polarisation, and against that direction ‘as absolutely negative.’ A polarisation-current in the same direction as the polarising- current he calls ‘relatively positive,’ and in the opposite direction ‘rela- tively negative’ polarisation. It was found by him that polarising- currents of fair intensity and short duration, whether homodromous or heterodromous, would always give rise to polarisation-currents in the same direction as the natural dis- mea, re6. Mice charge. He believed this to be due

of Lamina of Vympheaalba to the occurrence in the electrical due to Transmitted Excita- % . ors. ah an Prole organ of two different polarisation- Direction of responsive current effects, positive and negative. This incaat ttl avper serace will be understood from his own dia- grammatic representation (fig. 157) of the effect which he supposed to take place immediately on the passage of the polarising-current. In the upper figure the ascending arrow represents the homodromous polarising-current. This gives rise, according to Du Bois- Reymond, to two opposite polarisation-effects. The de- flection seen in the galvanometer is the resultant of these, represented by the shaded part of the figure. The resultant of a homodromous current, then, is positive polarisation, both absolute and relative. The heterodromous current, on the other hand, induces absolutely positive and relatively

further, heterodromous shocks induce no relatively positive polarisation, or only infinitely little (see down curve in lower part of figure). from Saxton’s. machine, he obtained only the absolutely positive _ polarisation - effect. This he accounted for by supposing the relatively nega- tive polarisations in both directions to cancel each other; the heterodromous positive to be so small as to be practically negligible ; and the homodromous posi- tive. therefore to be alone effective.

excitation. in. these nomena. . What he positive polarisation has been shown by subsequent workers to be due to local polar ex- citation. But the question as to how polarising-currents in both directions could give rise to a_ single-directioned responsive effect has not up to the present, so far as I am aware, been explained fully and satisfactorily. The ex- Fic. 157. Diagrammatic Representa- tion. by Du Bois-Reymond for Ex- planation of Electrical Response in Organ of Torpedo.

’ homodromous current + (upper half -of figure) is supposed to, induce two opposite polarisations, positive » and negative. The resultant, repre- sented by shading in "figure, is absolutely and relatively positive. A heterodromous current |, on the other hand, is regarded as inducin a resultant absolutely positive and relatively negative polarisation (lower part of BEUrE ja: periments carried out on leaves, which I am sidan to describe, will, however, throw much light on this subject.

It has already been shown, from anatomico-physiological considerations, that there are certain leaves which approxi- mate to the character of single plates of such electrical organs as that of Zorpedo. One such leaf, already mentioned, was that of Pferospermum. When induction-shocks are sent in both homodromous and heterodromous directions through such a leaf, between upper and lower surfaces, the leaf being, it is understood, in a normal condition, a responsive current is found to be evoked, always in one direction—that is to say, from the lower or anterior to the upper or posterior surface. This is strictly parallel to the electrical reaction observed by Du Bois-Reymond in Zorfedo.

That this result is really due to the excitatory effect is proved by the fact that the same is found to occur when other forms of stimulation are used. Thus, if we place the leaf of Coleus aromaticus within a surrounding thermal helix, suc- cessive thermal shocks, acting simultaneously on both surfaces, give rise to responsive currents which are, as in the last case, Fic. 158. Photographic Records from the lower anterior to the ae te Thema Pee gives a series of such responses. Resultant responsive current from From the fact which has already

more excitable anterior toless been fully established, that on sa rca a the simultaneous excitation of two points the responsive current is always from the more to the less excitable, it is quite clear that in the present case it is the lower or anterior surface of the leaf which is the more excitable. These responsive currents, obtained under a non- electrical form of stimulus, and similar to those evoked by electrical shocks, completely demonstrate the fact that the result is brought about, not by polarisation, either positive or negative, but by the afferential excitability of the tissue itself. The response of electrical organs in general, then may be summarised in the following law:

The excitatory discharge is determined by the physiological anisotropy of the organ, its definiteness of direction being deter- mined by the fact that the responsive current is always from the more to the less excitable of the two surfaces. Referring once more to the definite-directioned _after- current which we have seen to be induced as the result of polarising-currents, whether homodromous or heterodromous, it is now clear that these currents act as an _ electrical form of stimulus. The intensity of the after-current here seen in the galvanometer, however, is not wholly due to the excitatory electro-motive change, but in part also to physical polarisation, which is added to it algebraically. Thus, an exciting homodromous shock gives rise to an electrical after-effect, in which the excitatory current is opposed by a counter-current of negative polarisation. Under a hetero- dromous shock, on the other. hand, the excitatory electrical change becomes summated with the negative polarisation, which is now in the same direction as itself. In these cases, though the preponderating nature of the excitatory effect determines the definite direction of the after-effect, yet it is difficult to know how much of the latter is actually due to excitatory action as such, and how much to ordinary polarisation helping or opposing this.

Very much greater complexities ensue again in practice rom the difference between anodic and kathodic actions on the two unequally excitable surfaces. In Torpedo, for instance, according to Du Bois-Reymond, the electrical organ responds better to a homodromous than to a heterodromous exciting current, while in J/alepterurus, according to Gotch, the reverse is the case, the heterodromous being more efficient than the homodromous. Such diversity of results is prob- ably to be accounted for by the considerations to which I have referred.

If we take, tor example, the simplest case, that in which the anterior surface is more excitable than the posterior, and if we suppose an induction-current of moderate intensity to be sent in a homodromous direction, we may assume that Pfliiger’s Law—the kathode excites at make, and the anode at break—will hold good. We shall here, for the sake of simplicity, neglect any effects that may accrue from anode- make and kathode-break. Under a homodromous induction- shock, then, two different excitatory electrical changes will be induced, on the lower and upper surfaces respectively, the consequent currents through the tissue being in opposite directions. On these, moreover, will be superposed again the polarisation-current. Calling the effect induced by anode- break as A, and that of kathode-make as K,, we shall obtain a resultant consisting of A,on the more excitable anterior surface, mznus K,, on the less excitable posterior, mznxus the negative polarisation-effect. Under a heterodromous shock, on the other hand, we shall have K,, on the more excitable anterior surface, mznus A’ on the less excitable posterior, plus the negative polarisation-effect.

Even this, however, does not exhaust the possibilities of complication. _ For I shall show in a subsequent Chapter, and have already shown elsewhere, that under a high E.M.F. Pfliiger’s Law does not apply. The relative excitatory values of anode and kathode may indeed undergo one or more reversals, according to the intensity of the acting electro-motive force. Thus, under a moderately high E.M.F. in what I have designated the A stage, both the anode and kathode are found to excite at make, and either kathode or anode at break. In the B stage, under a still higher E.M.F., it is the anode which excites at make, and the kathode at break.

It will thus be seen what a number of complicating factors may be present when an organ is excited by currents of varying direction and intensity. If, then, we wish to study the purely excitatory reaction of an organ, as dependent solely upon its individual characteristics, uncomplicated by defects inherent in the method of excitation, we must see first that the applied stimulus is equal on both surfaces, and, secondly, that such factors as are not excitatory—that is to say, negative or counter-polarisation—are eliminated. These ends may be accomplished by subjecting the responding organ to symmetrical and alternating equal and opposite shocks,

following each other in rapid succession. For the resultant negative polarisation will in practice be neutralised, if the primary polarising currents are similar, equal, and opposite. The stimulus applied on the two surfaces, moreover, will be equal, if the two rapidly succeeding and opposite-directioned shocks be so symmetrical as to be interchangeable. Which- ever may. be the factor of excitation will then act equally on both surfaces. The response, therefore, will now be determined solely by the natural difference of excitability as between the - two surfaces. |

It has been said that in order to accomplish these experi- mental conditions, the two opposite shocks should be equal in intensity and in point of time-relations. An ordinary make- and-break Ruhmkorff’s shock does not fulfil this condition, since the break-shock is there the quicker'and more intense of the two. Moreover, owing to the varying residual magnetisation in the iron core, successive shocks may not be equal. These defects are overcome by sending round the primary, with a constant rapidity, two equal and opposite currents in alternation. During one semi-cycle, then, the primary current varies from + C to — C,and during the next from — C to + C, and since these two changes are effected with the same rapidity, the induced currents are. symmetrical, equal, and opposite. |

Such reversal of current is accomplished by means ofa rotating reversing-key. The key R is wound up against the tension of a spring S, being maintained in this set position by the electro-magnet E, acting on the armature. When the current in the electro-magnet is broken, the alternating double shock from the induction coil I is passed through the experimental leaf L, by means of non-polarisable electrodes N, N,. In the case just described the sequence of the current through the. primary coi] was, say, right-left-right. In the next experiment, by means of the Pohl’s commutator, K,, this sequence may be made left-right-left (fig. 159).

Empioying this method, I have carried out rheotomic observations for determining the time-interval after the shock at which the E.M.F. attained its maximum. The general arrangement here is similar to that described in Chapter IV. (cf. fig. 37). C is the compensator by which any existing electro-motive difference is compensated at the beginning of each experiment. The striking-rod A breaks the current in the electro-magnet E, by which the rotating reverser R is actuated, which brings about equal and opposite shocks to the leaf. The galvanometric after-effect, at any short

Fic. 159. Experimental Arrangement for Rheotomic Observations A, B, striking-rods attached to revolving rheotomic disc; K,, key for electro-magnetic release of rotating reverser R; K,, key for unshunting the galvanometer when pressed by rod, B, for a definite period; ky, key for preliminary adjustment ; E, electro-magnet with its armature by which rotating reverser, R, is set against antagonistic spring, S ; K,, Pohl’s commutator; Cc, compensator; P, primary, and 1, the secondary, of the exciting induction-coil ; N,, N,, non-polarisable elec- trodes, making electrical contacts with posterior and anterior surfaces of leaf.

interval after excitation, is obtained by the un-shunting of the galvanometer, caused by the striker B impinging against the key K, (fig. 159). We have seen that, owing to the presence of various complicating factors, as well as to the occurrence of negative polarisation, successive responses to homodromous and heterodromous shocks are unequal. By the employment of equi-alternating induction-currents, how- ever, we obtain true excitatory effects, unmodified by any such elements of uncertainty. In order to show how perfect the results obtained by this method become, I give here (fig. 160) the records of two successive excitatory responses obtained from a leaf of Bryophyllum calycinum, the responsive current being from the lower or anterior surface to the upper or posterior. In this mode of stimulation, by equal and opposite shocks, as already said, no advantage is given to either surface over the other. Neverthe- less, I thought it well to take two successive records under shocks, in which the alternating currents in the primary circuit were first right-left-right, and then left- right-left.

In the electrical organ of TZorpedo Gotch found the maximum electromotive change to be attained in about ‘oI second after the application of the excitatory shock. In leaves, again, I find the rapidity with which the maximum effect is at- . Fic. 160. Records of Two Successive tained to depend on the Responses in Leaf of Bryophylium nature of the tissue. and calycinum under Equi-alternating exciting shock. In sluggish specimens this may be as long as ‘2 second. It should be remembered that in the case of mechanical stimulation of moderate intensity also, this period was, similarly, about *2 second (p. 51). With very vigorous leaves of Vymphaea alba, however, and employing a stronger electrical stimulus, the maximum effect was

attained in a much shorter time —that is to say, in about ‘03 second. I give below a table showing the rheotomic observations made on such a leaf at gradually increasing intervals after the exciting shock. It should be remembered that the recording galvanometer was un-shunted for ‘ol second, The curve given in fig. 161 has been plotted from these results. The maximum electro-motive change took place, as already pointed out, in ‘03 second after the application of stimulus. This curve shows multiple apices, as was also the case, it will be remembered, after a strong mechanical stimulation (cf. fig. 40). This point will be referred to in greater detail in the next chapter. In the course of half a second after the shock, the . excitatory electro-motive change had subsided to about one-twelfth of the maximum, Fic. 161. Response-curve from It has been said that the scp teeer epee onLeaf excitatory current depends for Ordinate represents galvanometer its definiteness of direction on deflection; abscissa,: time in the physiological anisotropy of hundredths of a second. the organ. In_ those leaves in which the physiological differentiation of the upper and lower surfaces is not strongly marked, the differential

excitability of the two is liable to undergo reversal, under the changing conditions of age, season, and fatigue induced by previous stimulation. In the leaf of Pzerospermum, however, which I have here taken as the type corresponding with Zorpedo, the normal differential excitability is generally very persistent. Here the excitability of the posterior surface, which is leathery, is slight, and practically negligible. But the anterior surface, with its rich and prominent venation, is highly excitable, The excitatory discharge of such a leaf is thus from the anterior to the posterior. I give in fig. 162 a series of its responses to equi-alternating electric shocks, It will be seen that these are very uniform, and exhibit practically no signs of fatigue. We have thus found a vegetable organ whose re- sponses are exactly parallel to those of a single plate of the electrical organ of Zorpedo and its type. We shall next ens study the responsive pecu- TiC; 162. Series of Responses, given liarities of the vegetable organ folium to Stimulus of Equi-alternat- Base responses correspon d ng eectneal Shocks at Intervals ot with those of the organ of Malepterurus. \t has been mentioned that the posterior surface of each single element of this electrical organ is regarded as consisting of a glandular, rather than a muscular, modification. Among corresponding leaf-organs, then, the carpellary leat of Dzllenta indica might, as we also saw, be taken as the

‘type, its posterior surface: being glandular. Or the analogy will be still more perfect if we take as the vegetal type the pitcher of Vepenthe. Here the internal or posterior surface is richly provided with glands. -The next point to be deter- mined is whether, in these cases also, on excitation the responsive current is from the posterior surface to the anterior, as in the electrical element of Madepterurus. And on sub- jecting them to equi-alternating electrical shocks, I found

this to be the case. The responsive current here flowed from the glandular posterior to the non-glandular anterior surface. From this experiment we see that a glandular surface is exceptionally excitable, a conclusion which will be found to be supported by the numerous experiments on glandular organs in general, to be described in Chapter XXIV. I give in fig. 163 a series of photographic records, obtained on excitation of Dzllenza indica. In the next record (fig. 164) are seen the responses given by the pitcher of Wepenthe.

; ic Record of Re- ; saat Fic. 163. suis ge Gy rae Pitcher of Mepenthe, under Equi- sponses 0 a alternating Electric Shocks Natural current from posterior to an- ‘ ene terior, and responsive current from sie scare aca woes harden anterior to posterior surfaces. hg nae he donee glandular surface. Note ten- An interesting fact to be noticcd in the latter is the tendency to multiple response. ' Similar results were also obtained on taking any single scale of the bulb of Uvzclzs lily about the time of flowering. In each of these the lower or outer surface is invested with a more or less dry and glistening membrane, while the upper or concave is moist and glanduloid. The moisture observed inside each scale is in fact exuded from this inner surface. On subjecting one of these scales, then, to the electrical

excitation already described, it is found that a very strong responsive current is obtained, whose direction is, as in the last case, from the glanduloid to the non-glanduloid surface. The effect of the serial arrangement, again, in enhancing the electro-motive force—as seen in the pile-like arrangement of the electrical organ of fishes—may be exemplified, in the parallel instance of the plant-organ, by means of the superposed scales of the bulb, as found in nature. The bulb may be divided longitudinally into halves, of which the right-hand half is mounted, for experiment, with the scales vertical. It will be understood that all the glanduloid surfaces here face the left, while the non-glanduloid are turned to the right. Thus the left aspect of this pile corresponds to the head aspect of the organ of JZalepterurus. The latter, on excitation, responds by a current in the direction of head to tail—that is to say, from glandular to non-glandular ; and similarly, in the pile-like half-bulb of Uricls lily, the responsive current is from glanduloid to non- glanduloid—that is to say, from left to right.

Another interesting way to perform the same experi- ment is without making any section of the bulb. We take a bulb of Uvzclzs, with the peduncle rising out of the middle. When this hollow peduncle is cut across, it allows of an electrical connection being made with the centre of the interior of the bulb. An equatorial belt makes the second, or outer, connection. On subjecting this to equi-alternating shocks, the resulting response will be found to be from the inner surface to the outer, through the numerous intermediate scales, the individual effect in each being concordant and additive: .

We have thus seen how the response of a leaf gives us an insight into the action of a plate of an electrical organ ; how the differential excitabilities of the two surfaces give rise on stimulation to an induced E.M.F. as between the two; how a nervous and indifferent-tissued surface will give rise toa response in one direction, and a glandular and non-glandular in the other; and finally how, by a serial arrangement, the terminal electro-motive effect becomes enhanced. The light thus thrown on the two types of response, known to occur in the electrical organs of fishes, is evident. Further con- siderations, relating to the theory of electrical organs, will be given in detail in the next chapter.

Existing theories—Their inadequiacy—The ‘blaze-current’ so called— Response uni-directioned, to shocks homodromous or heterodromous, characteristic of electric organs—Similar results with inorganic specimens — Uni- directioned _response due to differential excitability—Electrical response of pulvinus of | Mimosa to equi-alternating electric shocks—Response of petiole of Musa— Of plagiotropic stem of Cucurbita—Of Eel—The organ-current of electric fishes—- Multiple responses of electrical organ — Multiple responses of Biophytum.

whether the activity of such organs is specific—that is to say, peculiarly characteristic of them—or-falls into line with the other electro-motive reactions observed in animal tissue. Many arguments have been brought forward for and against the identity of these phenomena with the excitatory reactions of the nerve and muscle. : however, it would already appear that such reactions as these of the electrical organ are not specifically characteristic, even of the animal structure, but may equally well be observed in plant tissues. It is therefore essential, if we are to determine that basic reaction which is common to all alike, that we should find a wider generalisation than has hitherto been contemplated. This basic reaction, as we have already seen, depends upon the differential excitability of an anisotropic organ, and this aspect of the case we are now. about to study in greater detail. Before doing this, however, we shall briefly glance at various theories which have been suggested, but are generally admitted to be inadequate. |