Comparative Electro-Physiology: A Physico-Physiological Study

For the purposes of such a comparison, a new balancing arrangement has to be employed (fig. 305). Here, two electro-thermic stimulators are in series, so that ex- citations may be produced at two different points simultaneously. The gal- vanometer contacts E’ and Fic. 305. Diagrammatic Repre- sentation of Experimental Ar- rangement for Demonstration of RECEPTIVITY versus CON- FIG. 306. RECEPTIVITY versus RESPON- DUCTIVITY, or of RECEPTIVITY SIVITY under Alcohol versus RESPONSIVITY

s and s’ are exciting thermal loops to the left R’, and the responsive point in series ; R and R’, the enclosed to the right E. The formula was receptive points ; C and Cc’, con- R'aic.Eatc, The photographic record ducting arms; E and E’, the shows the relative enhancement of responsive points. receptivity. E are made with two points intermediate between the stimu- lators. The distance of one of the two stimulators is kept constant, at, say, 2 cm. to the left of E’, while the other is moved nearer to, or further from, E, until a balance is obtained. A 1 per cent. solution of alcohol is then applied to the left receptive point, R’, and the right conducting area, C, the formula now being R’,.C,,... The fact that the receptive excitability is heightened by this reagent, and conductivity depressed, receives independent confirmation “from the upset of the balance, giving rise to a downward response.

The next experiment consists of a comparison of the simultaneous variations of receptivity and_ responsivity. Alcohol is applied at R’ and E, the formula thus being RlacEace .And we find here in confirmation. of our previous results that, on account of the opposite effects of this agent on the receptive and responsive excitabilities, the resultant response is downwards (fig. 306), showing that the receptivity has been relatively exalted. Thus the experi- ments which I have here described show that the same agent may have different effects on receptive and _ responsive excitability, and thus accentuate the necessity of clearly distinguishing between the two.

Effect of temperature in inducing variations of conductivity : (2) by Method of Mechanical Response ; (4) by Method of Electric Balance—Effect of cold— Effect of rising temperature—The Thermal Cell—After-effect of stimulation on conductivity—The Avalanche Theory—Determination of the after-effect of stimulus on conductivity by the Electrical Balance—After-effect of moder- ate stimulation—After-effect of excessive stimulation. IN studying the effect of temperature in inducing variations of conductivity, we may use either of two different methods— in the first place the method of mechanical, or in the second that of electrical response. For the first of these it is neces- sary to have what is generally known as a ‘sensitive’ plant, the leaves or leaflets of which afford conspicuous motile indications of the arrival of the excitatory wave from a distance. In such a case the time-interval between the application of stimulus and the response of a leaflet at a known distance gives us a measure of the velocity of con- duction ; and if we carry out successive experiments at different temperatures we have a means of determining the effect of temperature on conductivity. Employing this method, I have elsewhere given a, determination of the effect of temperature on the velocity of transmission in Biophytum sensitivum. It was there shown that lower- ing of temperature reduced the velocity of transmission even to the extent of abolition, when the cooling was suf- ficiently intense. With moderate cooling the velocity was found to be decreased to about one-third. ‘The effect of rise of temperature was, on the contrary, an increase of

velocity. When it rose from 30° C. to 35° C., for example, the velocity was doubled. By employing the electrical method of response, however, we are rendered independent of the use of sensitive plants, and by means of the Conductivity Balance we are enabled to demonstrate the slightest variation of conductivity, as between the left arm of the balance, which is maintained at standard temperature, and the right, which is subjected to the given | change. ,

Thus in a definite experiment on a nerve of fern the temperature of the room was 30°C. After first obtaining the balanced record, the temperature of a portion of the right arm of the balance was lowered. This one-sided cooling was effected by supporting the right arm of the nerve, through a certain length, in the concavity of a U-tube through which cold water at 15° C. was passed. Stimuli were now applied at intervals of one minute. Previously, as will be understood, such stimuli, owing to balance, had induced no resultant effect. But now, on account of the depression of conductivity on the right side, brought about by cooling, the balance was disturbed, and the resultant down-response seen in fig. 307 shows the diminished con- ductivity of the right arm. On the ‘gessation of the flow of cold water the balance was gradually restored, in concomit- ance with the return to the original temperature.

I next investigated the results of a rise of temperature, and here I specially desired to observe the conductivity variations, not at any one degree, but throughout a graduated and con+ tinuous rise. I was confronted at the outset of this investi- gation by the difficulty arising from the fact that there was no convenient and satisfactory means for the local variation of the temperature of the nerve, in definite and known degrees. In connection with this there was also the further difficulty that a sudden variation of temperature will, in itself, act as a stimulus. Hence, in studying the effects of temperature per se, it is essential that there should be no such sudden variation. These difficulties were overcome by

the employment of an electrical arrangement to bring about the graduated and continuous rise of temperature. A certain length of the vegetable nerve on the right arm of the Conductivity Balance was thus raised continuously in temperature, and its conductivity compared with that of the left arm of the balance, the latter being maintained at the temperature of the room, which happened at the time to be 33°C. The device by means of which this was accomplished

Fic, 307. Photographic Record showing Effect of Cooling on Con- ductivity of Plant-nerve Balance was obtained at starting, when temperature of both arms was 30° C. On cold being applied on right arm, the balance was dis- turbed, showing diminished conductivity on that side. On restoration of normal temperature, the balance is seen at the end of the record to be again restored. will be understood from fig. 308. A piece of cork has a small chamber cut into it measuring I cm. each way. In this is placed moist blotting-paper, which keeps it damp, and across it passes a. length of 1 cm. of the right arm of the vegetable nerve in the Conductivity Balance. This cork- chamber has inlet and outlet tubes ¢ and #.. The first of these contains a spiral, H, of platinum, which can be heated to a suitable degree by means of an electrical current, the

intensity of which is capable of careful adjustment. The cork chamber is closed with a cover, through which passes a thermometer, T, for the indication of the temperature within, The tube Z’ is connected with an aspirator, and air is thus sucked in by 4, and, passing through the platinum spiral, is warmed, and raises the temperature of the nerve in the chamber. This rise of tem- perature is adjusted (1) by regu- + lating the electrical current which | heats the spiral, and (2) by con- trolling the inflow of air... As regards the first of these two processes, the electrical heating- circuit has a carbon rheostat interposed, by which the rate of rise of temperature may be regulated. The movement of the current of air, on the other hand, is controlled by adjusting the stopcock of the aspirator. By the joint manipulation of both these the rate of rise of temperature t inside the chamber may be made perfectly uniform, and in my yoga ee Rouing ate experiments this rate was approxi- Temperature of one Armof mately 1° C. per minute. ene As already said, I selected a A and B, the two halves of the - , chamber ; T, thermometer ; piece of vegetable nerve and took at ia cap aerate a balanced record. After this the heating. temperature of the thermal cell on the right-hand side was raised continuously, the response-record being taken at each degree of the rise, till a temperature of 50° C. had beenat tained. From the record given in fig. 309 it will be seen that the conductivity was always greater at temperatures up to 47° C. than it was on the left-hand side, which was all the time maintained at the constant temperature of 33°C. At 48°C,

however, the reversal of response showed that the conduc- tivity was now being depressed. And at still higher tem- peratures it was found to undergo a very great depression, as is seen by the abrupt downward movement of the curve. It is thus seen that, by means of the Method of Balance, this very difficult problem of the variation of conductivity under variation of temperature is made capable of exact study. I shall next describe the results of an investigation into the after-effects of stimulus on conductivity and excitability,

Fic. 309. Photographic Record Showing Effect of Rising Péapelatons on Conductivity Balance obtained at starting at 33° C. Successive responses recorded at each degree C. of rise of temperature. Record shows increasing conductivity up to 472 C. A depression of conductivity is seen by reversal of curve to set in at 48° C., and this becomes extremely pro- nounced at 50° €. a subject of much difficulty and of considerable theoretical importance. It has been found in Animal Physiology that the sciatic nerve of a frog, for instance, is not equally excitable throughout its length. When such a nerve, with its attached terminal muscle, is cut off from the spinal cord, it is seen to be more excitable the further from the muscle is the point on the nerve that is subjected to stimulation. From this fact that excitation increased with the distance of the point excited from the motor organ, Pfliiger was led to

the ‘ Avalanche Theory,’ namely, that during the passage of excitation down the nerve it. actually gathers strength. But it is clear that this cannot be true, since we have seen that, other things being equal, excitation is always greater the nearer the point of stimulation to the responding region, and on this fact have depended all those experiments already described, which involved a delicate balance of equal excitations. It follows that the observed enhancement of excitability of a point on the nerve which is distal from the muscle, and in the neighbour- hood of a section, must be ascribed to some other cause. In reference to this Heidenhain, indeed, explained the greater excitability of higher tracts of divided nerve by the proximity of the artificial section. For the lower end of the nerve at once exhibits the same marked activity as the upper end if a section be made lower down. Excitability is, in fact, raised near the section, wherever the section may be. The distance travelled by the excitation could not, therefore, be the determining factor in the magnitude of effect. For so far from increasing it, this, as a matter of fact, causes diminution. It is to be remembered that though the excitability is increased near the point of section, yet at the section itself it is almost abolished, otherwise there could not have been any response by'so-called negative variation. The question now arises, Why should the excitability be raised near: the point of section ?

It has been supposed that this was due to certain electri- cal changes induced by section, which in turn gave rise to electro-tonic variations of excitability. We shall see, in Chapter XL, that the passage of an electrical current through a living tissue induces changes of excitability. And this phenomenon is known as the electro-tonic effect.. Now any ‘injury, such as a mechanical or thermal section, is known to induce galvanometric negativity, or anodic change, at or near the point of section. But it is the kathode-effect which is excitatory. And the observed greater excitability of the nerve near a point of section is supposed to be due to kat- electrotonus, produced within a certain tract from the cross-

section by internal short-circuiting of the nerve-current. That this explanation, however, does not meet all the requirements of the case will appear from certain experi- ments which I shall describe, where, under exactly similar electrotonic changes due to section, a result directly the opposite of this, that is to say, of depression, is seen to be induced. . All these various facts will be found fully reconcilable, however, on the basis of a proposition which I shall establish, namely that zz a nerve, moderate stimulation enhances ex- citability and conductivity, while excessive stimulation has the opposite effect of bringing about the depression of both. It is indeed natural to expect that while moderate stimulation, by increasing molecular mobility, would bring about one effect, excessive stimulus, by inducing overstrain, would result in exactly the opposite. Before proceeding to give an experi- mental demonstration of this hypothesis, we shall first consider the explanation which it affords of the peculiar excitatory changes observed in the case of cross-sectioned nerve. In the first place we know that a cut acts as a stimulus. And since we found that the effect of stimulus decreases with the distance from the point of stimulation, it would appear that at the section itself the stimulation would be excessive ; moderately strong at a certain distance from it; and practically negligible when very far away. In complete accordance with this is the resulting increase of excitability which has been observed near the point of section, while at the point itself the nerve is relatively inexcitable.

The fact that stimulation, when not excessive, increases the conductivity and excitability, we found illustrated in the staircase increase of electrical response, and in the enhance- ment of amplitude after tetanisation, in vegetable and animal nerves (figs. 275 and 286). The same fact will be demonstrated later by means of the mechanical response of nerve. I shall now describe certain experiments which demonstrate it once more in a new and interesting manner. 2

A vegetable nerve was adjusted for balance, with the ends projecting some distance beyond the electrodes. In order to show that the effect of injury is due to stimulus as such, and not to any particular form of it, I now made a thermal instead of mechanical section, by applying salt solution heated to about 60° C. in the region A, at a distance of I cm. to the right of E (fig. 310). The effect of this stimulation was to induce a moderate excitation of the right | arm of the balance, relatively to the left. If this moderate stimulation were to induce any increase of excitability and conductivity, that fact would be demonstrated by

Fic. 311. Photographic Record Showing Effect of Moderate Stimulation in Enhancing Conductivity and Excita- Fic. 310. Experimental Arrange- ment for Studying After-effect of Stimulus on Conductivity and The stimulator adjusted to obtain balance between Eandk’. Stimulus of moderate or strong intensity is applied to a point on the right of E. Upsetting of the balance in an _upward direction shows an en- hancement, and in a downward direction, depression, of con- ductivity and excitability.

otted line at beginning shows the resting-current, as a per- sistent effect of stimulation. The upsetting of the balance upwards constitutes a positive variation of the resting- current, and indicates en- hanced conductivity and excitability. the upsetting of the balance, the resultant response being upwards. That this is what actually occurs will be seen from the records in fig. 311. It will be noticed that in consequence of stimulation to the right of E, that point became, more or less permanently, galvanometrically negative. This is represented by the dotted line upwards at the beginning of the record. It must be remembered that before the application of the thermal section, the right and left hand excitations, proceeding from the electro-thermically stimu-

lated point in the middle, were exactly equal and balanced. The fact that after this application, however, there are resultant responses which are upwards, shows, as already said, that by the moderate stimulation of the right-hand side, both excitability and conductivity have been increased. The resultant upward response here is, then, in the same direc- tion as the so-called ‘ current of injury, and forms, as it were, a positive variation of it.

In another experiment, in which I wished to try the effect of excessive stimulation, instead of applying a hot solution at 60° C., I produced greater injury and consequent excessive stimulation, by scorch- ing the nerve at the same point as before, witha red-hot platinum wire. In this case resultant response was downwards, show- ing that the excitability and conductivity of the right-hand side of the balance had been showing Effect of Excessive ; P Stimulation in Depressing Ex- depressed by over-stimulation. citability and Conductivity

: . ing-current due to after-effect of demonstrating that the excita- stimulation. The upsetting of bility of the over-stimulated or balance in a downward direction excited point undergoes depres- of the resting-current, and shows sion. For this purpose I took depression of conductivity and excitability. a fresh specimen and first ob- tained a state of balance. Similar excitation of E’ and E produced a balanced or null effect. The point E was then injured by touching it with a hot platinum wire. On now proceeding to take records, it is seen that the responses were downwards, showing the depression of excitability at the injured E (fig. 312).

The fact that galvanometric negativity had been induced at E, by reason of injury, is demonstrated at the beginning of the record as an up-line. The subsequent resultant responses due to simultaneous excitation of E and E’ are -seen to be a negative variation of the resting-current due to injury. It is thus seen that while simultaneous-excita- tions of two normally excitable points E and E’ are prevented by balance from giving rise to any response, the excitatory response becomes manifest when the balance is disturbed by the injury of either point of galvanometric contact ; and that, under these circumstances, the response is a negative variation of the current of injury. This experiment is | important as giving a theoretical insight into the so-called response by negative variation. It also shows how limited is the applicability of the assumption that response is always by negative variation. For, in the similar experiment, previously described, under moderate injury, the response was by positive variation of the resting-current.

It is further seen from these experiments that the enhancement of excitability, under the stimulation due to moderate injury, could not be caused by the suggested electrotonic effect. For the same anodic change induced by injury at E causes, in the case of moderate injury, an enhancement, and under greater injury a depression, of excitability. It is thus clear that the modifying influence is the effective intensity of stimulation. This fact, that moderate stimulation enhances, and excessive stimulation depresses excitability, will be further demonstrated ina future chapter, by the independent method in which the effects of electrotonus are completely eliminated. )

Current assumption of non-motility of nerve—Shortcomings of galvanometric modes of detecting excitation—Mechanical response to continuous electric shocks—Optical Kunchangraph—Effect of ammonia on the mechanical response of nerve—Effect of morphia—Action of alcohol—Of chloroform— Abnormal positive or expansive response converted into normal contractile through diphasic, after tetanisation—Similar effects in mechanical response of vegetable nerve—Mechanical response due to transmitted effects of stimulation—Determination of velocity of transmission—Indeterminateness of velocity in isolated nerve—Kunchangraphic records on smoked glass— Oscillating recorder—Mechanical response of afferent nerve—Record of mechanical response of nerve due to transmitted stimulation, in gecko— Fatigue of conductivity—Conversion of normal contractile response into abnormal expansive, through diphasic, due to fatigue.

I HAVE already referred to the distinctions which are com- monly insisted on, as between the reactions of different animal tissues. Certain of these are regarded as motile and others as. non-motile. From an evolutionary point of view, however, it is difficult to conceive of such a hard-and- fast distinction. It would be easier, believing -in continuity, to suppose that a certain responsive reaction, characteristic of the simplest living substance, had become accentuated in some tissues, and not so accentuated in others, according to their different functional requirements. Thus the belief held so implicitly by physiologists that nerves exhibit no motile response whatsoever’ becomes questionable, and is seen to require investigation. After submitting it to this, moreover, one finds it difficult to understand how such an

‘ ‘Nerves are irritable; when they are stimulated, a change is produced in them ; this change is propagated along the nerve, and is called a nervous impulse ; there is no change of form in the nerve visible to the highest powers of the- microscope.’ (Kirke’s Handbook of Physiology, 15th edition, p. 105.). idea ever gained currency, unless, indeed, it was due to the tyranny imposed on our thought by these arbitrary classifi- cations themselves. Before entering, however, on the question whether the excitatory reaction in nerve finds motile expression or not, we shall first examine the only method at present available for the detection of the condition of excitation. Since ex-

cited nerve has hitherto been supposed to exhibit no visible change, it followed that the only method possible for the detection of the excitatory change was the electrical. In- vestigations on nerve, therefore, had perforce to be carried out by this means, through the medium either of the capillary electrometer or of the sensitive galvanometer. But the elec- trical method labours under certain inherent disadvantages, and first of these is the objection which it raises to the free employment of the most convenient form of stimulus, that, - namely, by induction shocks. For we have seen that unless extraordinary precautions are taken, we have here, owing to the possible escape of current, an element of error and un- certainty in the results. If, on the other hand, it should become possible to obtain mechanical response from the _ nerve, this particular form of stimulation might be employed without misgiving. |

The second limitation which the electrical mode of detection imposes upon us is that arising from the differ- ential character of the response which it indicates. For stimulus induces electrical changes at both the contacts— proximal and distal-—the record made being finally due to the algebraical summation of the two. It is true that the excitability of one contact is artificially depressed by injury. But it is often difficult to say how far this injury has been effective in completely abolishing the excitability of this point. The depression of excitability, due to partial injury, will sometimes disappear to a certain extent, with lapse of time, and much uncertainty sometimes occurs as to whether a certain curious variation in the response of the nerve— negative followed by positive—is due to this or some other

cause. With mechanical response, however, provided this could be rendered practicable, no such difficulty need arise. For in that case it would be the direct effect of the exci- tatory change, uncomplicated by any other disturbance, which would be recorded. Finally, as regards the detection of the excitatory change itself, the galvanometer is unable to indicate any change below a certain high intensity of excitation. Thus it gives no indication when excitation is due to one or to a few shocks: it can only detect an excitatory effect which is much stronger than this, having been brought about by the super- posed effects of tetanic shocks of a certain duration. In order to obtain even such effects, a galvanometer of very high’ sensitiveness is necessary. That of a fairly delicate instrument, detecting a current of about ‘ooI ampere, will have to be exalted some ten millions of times before it can give efficient indications of excitatory effects in nerves ; and in such a degree of galvanometric sensitiveness we approach a limit which cannot be very much exceeded.