Comparative Electro-Physiology: A Physico-Physiological Study

effects—namely, a negative or a positive variation of the injury-current. : I give below a short summary of the diversities of response which may occur when either the natural, or the injury-current, is taken as the current of reference. Fic. 117. Typical Cases of Variation of Current of Rest and Action- Current. Specimen originally isotropic (a) A, end slightly injured and negative; Cc, current of injury; R, action- current, a negative variation of Cc. (4) A, end killed and positive ; C, current of injury; R, action-current, a positive variation of c.

First—we take the case where the point A is slightly injured (fig. 117,@). The current of injury, Cc, is A> B, and the responsive current, Rk, is B- A, constituting a negative variation. But when A is killed, the current of injury C is B> A, the responsive current is also BA, constituting a positive variation (fig. 117, 0). Second —we take an instance where, owing to some physio- logical difference, an intermediate point A is less excitable than B or B’ (fig. 118, a). The primary natural current will here be from the less to the more excitable : that is to say, C will

right, an identical excitatory current R, flowing away from the excited point from right to left, will cause seemingly opposite effects: that is to say, a negative variation of A+B’ and a positive variation of A> B. Fic. 118. Typical Cases of Variation of Current of Rest and Action- Current ; intermediate point naturally less or more excitable than either of terminal. (a) Intermediate A, less excitable, shown by vertical shading ; current or rest A>B and A->B’; when right-hand point x excited, action-current R from right to left, gives rise to negative variation of As’, and positive variation of A>B. (4) Intermediate B, more excitable, shown by horizontal shading ; current of rest A>B and A’->B ; action-current on excitation at x, from right to left, giving rise to positive variation of a’/—>B and negative variation of AB.

_ Again, we ‘may have the intermediate point B naturally more excitable than A’ or A. The natural current C will be A>B and A’>B (fig. 118, 4). Stimulation at x will now give rise to an excitatory current R, from right to left. The results here will, however, appear to be exactly the reverse of those in the last case: that is to say,.an identical current, R, will give rise to a positive variation of A’ > B, and negative variation of A>B. Instances of these effects will be given in Chapter XVIII.

And, lastly, we may have a typically anisotropic tissue, composed of two halves, which are unequally excitable—as, for instance, the upper and lower halves of the pulvinus of Mimosa, or the muscle and gland in a muscle-and-gland preparation. Under normal conditions the primary or natural current C is from the less excitable A to the more excitable B, represented by A > B (fig. 119, a). The action-current R, being in the opposite direction B > A, constitutes a negative variation. - But owing to the after-effect of excitation, such as may occur in isolating the specimen by section, the normal resting current C is reversed toB > A (fig. 119, 4). Here the end B may still be the more excitable of the two, hence the action-current B- A will constitute a positive variation of the current of rest. But when B becomes fatigued, its excitability is reduced below that of A; hence the action-current is from the relatively more to less excitable, ze. A>B. In this case,

Fic. 119. Typical Cases of Variation of Current of Rest and Action- Current. Anisotropic organ, B end originally more excitable than A - (a) Current of rest A->B ; action-current, R, in opposite direction ; response by negative variation. (4) Owing to excitatory after-effect, current of rest reversed to B—>A; B nevertheless more excitable than A; action- current, R, is B—>A; response by positive variation. (c) Current of rest reversed BA; action-current also reversed AB, by depression of excitability of B, owing to fatigue ; response by negative variation.

on account of the reversal of both the current of rest and action-current, the latter appears to constitute a negative variation of the former (fig. 119, c). It will thus be seen how intricate and diverse are the responsive variations of the resting current, induced by stimulus. Sometimes negative, sometimes positive, it would appear as if there were no guiding principle to regulate these phenomena. The so-called explanations hitherto attempted have consisted in assigning the positive variation to a hypothetical process of assimilation, and the negative to dissimilation. Such explanatory phrases:reach the climax of absurdity when we find ourselves compelled to ascribe | one identical excitatory reaction now to assimilation and then to dissimilation.

Indeed it must be said that, however suggestive the general theory of assimilation and dissimilation may have been found, its abuse has often stood in the way of physio- logical inquiry. The inquirer, when faced with any difficulty, instead of attempting to surmount it by patient inquiry, was tempted rather to evade it by invoking the aid of an hypothesis which could be made with equal ease to explain a given fact or its direct opposite. We must remember that in the — investigation of obscure problems, the danger is always, instead of seeking an underlying law, to become satisfied with the mere registration of phenomena, and by naming these to imagine that they have been explained. The resulting chaos in the present case has served to deepen the impression that vital phenomena must always remain capricious and mystical.

But when we come to survey the facts that have been described, we find the phenomena of response, however diverse they may at first sight appear, to be in no way governed by chance or caprice. ‘They are, on the contrary, definite and uniform under definite conditions. As regards the so-called current of rest in a naturally isotropic tissue, of which one end has been subjected to injury, we must remember that the effect of injury is one of excitation, its sign, within limits, being contraction and galvanometric negativity. But we have seen that when a point is over-stimulated, fatigue-changes appear which give rise to a reversal of its normal sign of response, from con- traction to expansion, from negative to positive (cf fig. 64) The change at death, in which contractile rigor passes into post-mortem relaxation, is analogous to this. ‘Thus when one end of the specimen is merely injured, that end becomes more or less persistently galvanometrically negative, the current flowing away from it. But when the same end is actually killed, the electrical change may be reversed, to one of galvanometric positivity.

In an isotropic tissue, then, we may, by moderate injury, bring about a state of anisotropy, under which the uninjured end is rendered relatively the more excitable, and galvano- metrically positive, compared with the inexcitable injured end. In a naturally anisotropic organ, we have a state of things which is analogous. In this case, in the primary condition, the more excitable surface is galvanometrically positive. But under the excitation due to preparation, or accidental dis- turbance, this more excitable surface becomes the more excited, and, relatively to the other, gaivanometrically negative. These varying changes in the direction of the so-called resting current, or current of reference, are the cause of the existing anomalies in the interpretation of response by the positive or negative variation.

But the direction of the action-current under normal conditions is always the same. On diffuse stimulation it is always from the more excited B to the less excited A. The differential excitability or anisotropy, may be either natural, or artificially induced, as by injuring one end of an isotropic tissue. There are two different conditions under which the normal effect may undergo reversal, those, namely, of great sub-tonicity or excessive fatigue. But the statement that the responsive current is always from the more excited to the less excited, remains universally true. Numerous illustra- tions, in verification of the cases laid down, will be met with in the course of subsequent chapters.

General observation of effect of temperature on plant—FEffect of fall and rise of temperature on autonomous response of Desmodium —Effect of frost in abolition of electrical response—After-etfects of application of cold, in EZucharis, Ivy and Holly—Effect of rise of temperature in diminishing height of response— This not probably due to diminution of excitability—Similar effect in auto- nomous motile response of Desmodium—Enhanced response as after-effect of cyclic variation of temperature—Abolition of response at a critical high temperature.

WE have now seen that the physiological activity of a living tissue may be gauged by means of its electrical response. We know further that the influence of temperature is of importance in the maintenance of a proper physiological _condition. There is a certain range of temperature which is favourable to this, and above or below these limits physio- logical efficiency is diminished. If the plant be kept too long at or above a certain maximum temperature, it is liable to undergo death. Similarly, there is a minimum point at which physiological activity is arrested,and below which death is apt tooccur. The plant has thus two death-points, one above the maximum and the other below the minimum temperature. Some can resist these extremes better than others, and length of exposure is also a factor which should not be for- gotten in the question of the ultimate survival of the plant under the given unfavourable conditions. Certain species are hardy, while others succumb easily.

An unmistakable indication of the effect of temperature on physiological activity is found in the variations induced by it in the autonomous motile pulsations of the telegraph plant, Desmodium gyrans. Here, too great a lowering of the temperature abolishes the pulsation. In fig. 120 are seen (1) the records of normal pulsations ; (2) their arrest under the application of ice-cold water; and (3) their revival, as the plant regains the temperature of the room. In fig. 121 is shown the effect on similar pulsations of a rising tempera- ture. The records in this case were obtained with a different

Fic, 120. Photographic Record showing Effect of Rapid Cooling, by Ice-cold Water, on Pulsations’of Desmodium gyrans Normal pulsations recorded to the left. Effect of application of ice-cold water is seen in the production of diminished amplitude and abolition of pulsation. Gradual return to the temperature of the room revives the pulsation in a staircase manner, the period remaining approximately constant. Note that cooling displaces the pulsation in a downward or contracted direction. Gradual warming, conversely, is seen to produce the opposite displacement towards expansion. Up-records represent the fall of the leaflet, down-records its rise.

specimen, and it is seen that the pulsations are diminished | in amplitude while their period is quickened, with rise of temperature. When the temperature is raised still higher, they come to a stop altogether. _ We shall next proceed to observe the effect of temperature on the electrical response of plants. As regards the influence of cold, for example, I have found, during the course of a research carried out in England, that after frosty weather,

the electrical responses undergo an almost complete aboli- tion. During a certain week, for instance, the temperature was 10° C., and the electrical responses then obtained from radish (Raphanus sativus) were considerable, giving an E.M. response which varied from ‘o5 to ‘1 volt. Two or three days afterwards, however, as the effect of frost, I found the electrical response of this plant to have practically dis- appeared. A few specimens were found nevertheless which were somewhat resistant. But even in these the average E..M. response had only a value of ‘003 volt, instead of the normal mean of ‘075 volt. That is to say, their average sensitiveness had been reduced to one twenty-fifth. On now

Fic. 121. Photographic Record of Pulsations of Desmodium during Continuous Rise of Temperature from 30° C. to 39° C. warming these radishes to 20° C. there was an appreciable revival, as shown by their increasing response. But in those specimens which had been frost-bitten, warming effected no restoration. From this it would appear that frost killed some, which could not be subsequently revived, whereas others were reduced to a condition of torpidity from which, on warming, there was a revival.

I have also investigated the effect of an artificial lowering of temperature on the electrical response of plants. The Eucharis lily is particularly sensitive to cold. In this case I took the petiole, and obtained response at the ordinary temperature of the room, which was at the time 17°C. I then placed it for 15 minutes in a cooling chamber at a temperature of —2°C. On now again trying to obtain response, it was found that it had practically disappeared. The same specimen was next warmed to 20° C., and this induced a revival of response (fig. 122).

I was next desirous of studying the after-effect of lowered temperatures on different plants. For this purpose I chose three specimens (1) the petiole of Eucharts Lily, (2) the stem of Ivy, and (3) Holly. I took their normal responses at 17° C.,, and after- wards placed them in an ice-chamber at a temperature of o° C. for 24 hours. The specimens were then taken out, and their responses under stimu- lation once more re- corded (fig. 123). From these it will be seen that while the respon- siveness of the delicate (h) Eucharis Lily was com- | | pletely abolished, that eagnss of the hardier plants, 110,128, Diminuton of Response in Holly and Ivy, exhibited (z) Normal response at 17° C. complete revival. (6) The response almost disappears when plant

; : f is subjected to —2° C. for fifteen minutes. One interesting fact (c) Revival of response on warming to 20° C. is that when a plant approaches its death-point, by reason of excessively high or low temperature, not only is its re- sponse, of galvanometric negativity, diminished to zero, but it is even occasionally reversed to positive. This effect is due to the unmasking of the positive, by the abolition of the true excitatory component. We shall next study the effect on the electrical response of the plant of a rise of temperature. The great difficulty of this investigation lies in raising the plant-chamber to the various determinate temperatures required. I was able, however, to accomplish this by means of electric heating. <A spiral of german silver wire was placed in the plant-chamber (cf. fig. 21), and by varying the intensity of the current the temperature was then regulated at will. In the process — of this electric heating a complicating factor was found in the excitatory action of any sudden variation of temperature. But no such excitatory disturbance occurs if the rise of tempera- ture be not fluctuating, but gradual .and continuous. I was able to secure this, by selecting at the beginning of the

Holly Eucharis Fic. 123. After-effect of Cold on Ivy, Holly, and Zucharis Lily _ a, The normal response ; 4, response after subjection to freezing temperature for twenty-four hours. experiment, a suitable strength of current, such as to raise the temperature of the chamber continuously, at an approxi- mately uniform rate. Care had also to be taken that thermal radiation from the wire should not strike the specimen, since such radiation, as we shall see later, acts as a stimulus. The interposition of a sheet of mica, however, obviated this diff- culty, mica being opaque to thermal radiation.

While, under these conditions, the temperature was being raised, uniform vibrational stimuli were applied at intervals, and responses recorded, the temperature of the chamber at the moments of stimulation being carefully noted. In this way I obtained the following photographic record, with a petiole of Hucharis lily, affording a general idea of the effect of temperature on response. It will be seen that while the temperature was rising from 20° C. to 22° C. the amplitude of the response was increased. After this, however, it fell rapidly in height with rise of temperature and became very small, at or near 60° C.- On allowing the temperature to: fall, however, the responses revived, with this peculiarity, that during the cooling, as compared with those given during the ascent of temperature, they were markedly en- hanced (fig. 124).

The heating arrange- ments in this case were such that the temperature was made to rise some- what rapidly. It will be noticed that response had not here disappeared, even at 65° C., though, as we shall see in the next chapter, the death- point is at about 60° C., This apparent anomaly is due to the fact that the plant, which is a bad conductor, was not al- lowed time fully to attain the temperature of its surroundings. We shall Fic. 124. Photographic Record of Responses in Hucharis Lily during Rise and Fall of Temperature

Stimulus constant, applied at intervals of one minute. The temperature of plant- chamber gradually rose on starting current in the heating coil; on breaking current, the temperature fell gradually. Tem- perature corresponding to each record is given below. the specimen is not too thick, excitatory response disappears with the approach of death, at a temperature very near 60° C, I give below a record of the effect of temperature fatigue, and I have found that this plant is but little subject to it. It will be noticed how the response is continuously depressed, as the temperature rises from 30° C. upwards. In this case, the thermal ascent took place at the rela- tively slow rate of 1° C. in 1I°5 minute, so that there was time for the plant to attain the temperature of its sur- roundings. A tabular statement is given below, showing the effect of temperature on amplitude of response, in two - different specimens of Amaranth :

The same fall in amplitude of response, when a certain point has been reached in the thermal ascent, to which | have already referred, in the case of Eucharzs lily, has also been noticed in that of muscle. From this it might be concluded that rise of temperature beyond 30° C. or so, induced depression of excitability. But here we are met by an anomaly. 45° For growth, which I have 50° shown to be a phenomenon of excitatory response, in- - ad : creases, in the case of . Fic. 125. Diminished Amplitude of the plant, throughout the Response with Rising Temperature. thermal ascent up to an

35° C. Conductivity, again, which is, to a certain extent, correlated with excitability, undergoes enhancement with rising temperature. Thus in a certain specimen of Szo- phytum, for example, while the velocity of transmission at 30° C. was 3°77 mm. per second, it became enhanced to 9'I mm.—that is, nearly three times—when the temperature was raised to 37°C. From these considerations, it would appear that the diminution in amplitude of electrical response, under a rising temperature below the rigor-point, might not always be due to a diminution of excitability, but to some other cause. |

In connection with this, it must be borne in mind that two factors are included in the process of response: namely, the external stimulus which induces contraction, or galvano- metric negativity, and the internal factor which brings about recovery. For I have already shown that whereas the action of stimulus induces one effect—the contraction, for example, of an excited tissue with galvanometric negativity—an increase of internal energy causes exactly the opposite —that is to say, the expansion of the tissue and galvanometric positivity. External stimulus and internal energy thus act antagonisti- cally. A steady rise of temperature causes, as we have seen, an increase of internal energy. Hence, increased energy due to rise of temperature, enhancing the force of recovery, may cause a diminution of response, which is not due to diminution of excitability.

The inference that it is the increased internal energy due to rise of temperature which, by augmenting the force of recovery, diminishes the amplitude of response, appears the more probable from certain characteristics observed in the autonomous pulsation of Desmodium gyrans. If rise of temperature increased the force of recovery, we should expect, conversely, that a fall of a few degrees would have the effect of diminishing this force of recovery, and con sequently enhancing the response. That this actually occurs will be seen in fig. 126, in the first part of which is given a series of responses at the temperature of the room, which © was 29° C. When the temperature of the plant-chamber was now lowered to 25° C., the force of recovery would appear

to be diminished, since the amplitude of response was con- siderably enhanced. That the general excitability of the plant was not increased by the lowering of temperature, is seen from the fact that the frequency of pulsation was reduced on cooling to about two-thirds of its original value. The diminution of response with rising temperature may thus be due to an increase of internal energy, which tends Fic. 126. Photographic Records of Autonomous Pulsations in Des- modium, showing Increase of Amplitude and Decrease of Frequency, with Lowering of Temperature