Comparative Electro-Physiology: A Physico-Physiological Study

have seen, for example, that a rise of temperature, by in- creasing molecular mobility, enhances conductivity. But this increase of molecular mobility and internal energy also goes to augment the force of recovery, and, owing to this, the amplitude of excitatory response may be decreased. Thus, while a rise of temperature increases conductivity, it may appear to decrease responsive excitability. So much for the necessity of a distinction between conductivity and | responsivity. The term ‘excitability’ is commonly used for receptivity and responsivity indifferently. But I shall show in the course of the present chapter that it is important to make a distinction between these, since ‘the same external agent may effect the two differently.’ In the ‘following investigation, receptive excitability, or receptivity, will be represented by R, conductivity by cC, and responsive ex- citability, or responsivity, by E.

In determining the effect of any external condition such as the application of a chemical reagent on responsive ex- citability, in the case of animal nerve, it is usual to take a series of normal responses, and then to record the modified responses after the application of the reagent. By com- paring a number of such series of records, representing the action of various reagents on different specimens, the relative effect of each chemical may be inferred. The drawback to this method lies, first, in the fact that by the addition of the chemical reagent the resistance of the electrical circuit undergoes an unknown change, thus inducing a variation in the amplitude of response, which is not necessarily due to the excitatory electromotive change fer se. It is true that this difficulty may to a greater or less extent be obviated by interposing a high external resistance in the circuit, but this, by reducing the deflection, necessarily reduces the sensitive- ness of the method also. Different specimens again cannot but be characterised by slight individual peculiarities, and the experimental arrangements therefore can only be considered to be perfect when we are able to compare the effects of two

agents on an identical specimen. Again, in a series of chemical compounds which differ but slightly in effect from one another, an arrangement has to be devised by which the most minute excitatory variations will be conspicuously displayed. The same delicacy of experimental adjustment also becomes necessary when we wish to investigate the varying effects of time and quantity in the application. Similar considerations are involved when we attempt to observe the effects of various agents on conductivity and receptivity ; and still more complicated are the difficulties to be overcome when we have to study the property of con- ductivity versus responsivity or receptivity, or of receptivity versus responsivity, under the action of the same external agent. The methods hitherto available are neither perfect nor delicate enough for a complete and satisfactory determina- tion by their means of the various problems which arise in this connection. I' shall now, however, describe a very perfect and delicate method carried out by an experimental arrangement which I have devised, and shall designate as the Conductivity-Balance, by which the variation of an affected region may be continuously compared with a normal area as regards each of the three different aspects of the ex- citatory reaction, namely, receptivity, conductivity, and responsivity. In this method, moreover, the result is un- affected by any variation of resistance in the circuit that may be induced by changed conditions. It also enables us to solve the various difficulties encountered in comparing the relative changes induced in conductivity with those induced in receptivity or responsivity, or in the two last in respect to each other, under the influence of a given reagent.

In fig. 289 is given a diagrammatic representation of the principal parts of this Conductivity-Balance. The thermal stimulator produces stimulation of the enclosed area of the specimen. The excitatory wave travels along both arms of the balance, through the conducting region C and c’, and induces excitatory electromotive effects at the two responsive points Eand E’. The excitatory electrical effects at E and E’ are opposed, and when these are equal, and balance each other, the galvanometer indication is then reduced to zero. E and E’ are usually at a distance of about 4 cm. from each other. When the stimulator is brought too near to the left contact E’, the excitatory effect of galvanometric negativity which is induced there is relatively greater than at E. The balance is thus dis- turbed, and the resultant responsive deflection is then, say, downwards. When the stimulator is placed, on the other hand, too near the contact E, to the right, the resultant galvano- metric deflection will be up.! By suitable movement of the stimulator, to and fro between these two ex- tremes, a point may be found where the excitatory effects at E and E’ will exactly balance each other. I give here (fig. 290) a record taken

Fic. 290. Photographic Re- cord made during Pre- liminary Adjustment for Balance of Nerve of Fern The first two down-responses show over-balance, when S is too near the left, E’ being relatively more s, thermal stimulator; c and c’, the indicate over - balance conducting arms of the balance ; caused by s_ being too E and E’, responding points. Dif- much to the right. The ferential excitatory electrical effects horizontal record shows at E and E’ recorded by galvano- attainment of — exact meter, G. balance.

during this preliminary stage of adjustment. The first two down-responses were obtained when the stimulator was too far away from the balancing-point to the left. The next two 1 It is to be understood that what is said here refers to nerve in a normal condition of conductivity. up-responses were obtained when it-was contrariwise too far to the right. More careful adjustment reduced this up- movement, as seen in the next two responses, and finally, when the exact balancing-point was reached, the effect was null, as seen in the horizontal record.

In studying the question of the variation of responsive excitability induced by any given reagent, the agent is applied at the point E to the right. Any variation of excit- ability will then upset the balance. If the reagent be of a stimulatory character we shall obtain a resultant up-response, but if it be of a depressing nature, E will be rendered rela- tively the less excitable of the two points, and the response will consequently be down. It will thus be seen that that upsetting of the balance by which either up- or down-responses are induced is due simply to the relatively excitatory or depressing effect of the reagent, and is completely inde- pendent of any variation of resistance which might be brought about by its application. In the course of the following investigation, it is to be understood that the elec- trical connections are so made that the greater excitation of the right-hand contact is always represented by up-response, and vice versd. If it be desired to make a comparison between the excitatory reactions of two reagents, then the two are applied simultaneously, one at E and the other at E’. The resulting record then affords us a continuous graphic illustration of the relative and varying effects of the two.

If, again, it is the influence of any agent on conductivity that is to be studied, we first take a balanced record and then apply the given reagent on an area of about I cm. at Con the conductingarm. In this case, the responsive excitabilities of the two points E and E’ are the same, but if the effect of the agent have been to induce increased conductivity of Cc, then the excitation transmitted to the right-hand side, E, will be greater, and the response caused by the upsetting of the balance will be upwards. Conversely, a down-response will indicate that the effect of the agent has been to depress the conductivity. Again, we can compare the relative effects in

conductivity-variation induced by two different agents which are applied simultaneously, one on the arm C and the other enc, It is possible again to compare the variation of con- ductivity with that of responsivity, by applying one agent at a responding region, say E, and the other on the opposite arm of the balance atc’. The mode of investigation of receptivity changes will be described presently. : In fig. 291 we have the complete apparatus. The animal or vegetal nerve, N N, rests on non-polarisable electrodes of

The nerve N supported on electrodes E,, E,;. The two other electrodes E, E, are not used in this experiment, but are employed for experiments on electrotonus; T, thermal stimulator, the relative lengths of the arms of the balance being adjusted by the slide s. a U-shape. For the present experiments, two electrodes, E, and £,, are sufficient, their mutual distance being capable of any variation by movement along a sliding-bar. The same apparatus might be used for experiments on electrotonus, in which two additional electrodes would be required. The position of the electrothermic stimulator T is capable of very careful adjustment for purposes of balance, by means of the sliding-rod s. A glass cover, not shown in the figure, fits into the groove which is represented by a double dotted line sur- rounding the apparatus, and thus enables the chamber con- taining the nerve to be kept in a properly humid condition.

In all these experiments by balance, it is to be borne in mind that adjustment is always made for perfect balance at the beginning of the record, and represented by a short, more or less horizontal, line. te In order to show the typical effects of induced variations of excitability, in upsetting the balance, I shall first give records of experiments carried out on the nerve of frog. Dilute sodium carbonate is known to be an agent which enhances excitability. A long-continued application, or the application of a stronger dose, may, however, bring about a depression. When a dilute solution of Na,CO, was ap-

Fic. 292. Effect of Na,CO, Solution on Responsive Excitability of Frog’s Nerve In this and following records the hori- zontal line at the beginning indicates exact balance. The upsetting of the balance in the up-direction repre- sents either the enhanced respon- sivity of the right-hand responding eg _E, = the Leer a con- uctivity of the right-hand arm c, Riowascueeal eee ont correspond- Fic. ASS SEM Se SRR On Et0g's ing absolute or relative depressions. Nerve Na,CO, applied to E is seen to exalt The down record shows depression of the responsivity of that point. excitability.

plied at the responsive point E on the right side, the up- setting of the balance upwards immediately indicated the greater excitability induced by the reagent. The long- continued action of this reagent, however, showed that the enhanced excitability was undergoing a gradual decline (fig. 292). In order to exhibit the characteristic upset caused by a depressing agent, I employed on another specimen a toxic solution of copper sulphate, applying it at E on the right. The previous state of equilibrium is seen by the horizontal line at the beginning of the record, and the

subsequent depression of excitability at E is shown by the upsetting of the balance downwards (fig. 293). I shall next take up the determination of the changes induced by chemical agents on the excitability of plant nerve and shall begin by describing the different effects which occur on the application of calcium and potassium salts. For this purpose, deci-molecular solutions were employed. Fig. 294 shows the effect of CaCl, on. vegetable nerve, the solution being applied at E on the -right-hand side. It will be noticed that this caused an upset of the balance, showing an increase of excitability that becomes considerable after the expiration of five minutes. In the case of KCl, however, this effect was re- versed, that is to say, a de- pression was induced. This , is seen in fig. 295, where the _ Fic. 294. Photographic Record show- balanced record gives way, ing Enhancement of Responsivity Gat te diphasic, and :atter-

by Application of CaCl, CaCl, applied to E is seen to exalt wards to a down-response, | pression at E. These two experiments show the effect of the basic moiety in in- ducing changes of responsive excitability. I shall next describe experiments by which the simul- taneous effects of two different reagents on the responsivity of a given tissue may be compared. For this purpose, one agent is applied at one end of the balance E, the other being administered at E’. In the case of animal nerve, it was shown by Griitzner, that both NaCl and NaBr induce ex- citatory effects, that induced by NaBr being relatively the greater. But the continued action of either of these reagents

induces depression, which sets in earlier in the case of NaBr. The effect of these two reagents on vegetable nerve is pre- Fic. 295. Photographic Record showing Depression of Responsive Excitability by Application of KCl cisely the same, as will be seen from an inspection of the record given in fig. 296. The NaBr was applied on the Fic. 296. Photographic Record exhibiting Comparative Effects of NaCl and NaBr on Responsivity NaCl was applied on E’ and NaBr on £, the formula being E’yaciEnapr- The record shows the greater and earlier effect of NaBr at E in causing relative excitation followed by relative depression.

right-hand side E, and NaCl on the left-hand E’,a process which is expressed, for the sake of brevity, by the formula E'wacil’wapr =0Lhe greater and earlier. excitatory effect of NaBr, applied on the right-hand side, is shown by the resultant up-responses. But after a time, E being now depressed by. the continued, action of NaBr, the effect of NaCl, applied on the left, becomes relatively predominant, a fact demonstrated by the upset of the balance in the oppo- site direction, with concomitant down-responses.

We shall next take up the subject of variations induced in conductivity. We have seen that dilute solutions of Na,CO, have the effect of exalting responsive excitability. FIG. 297. Photographic Record of Effect of Dilute (+5 per cent.) Solution of Na,CO, on Variation of Conductivity Reagent applied on right arm c. Record shows immediate enhancement of conductivity giving rise to up-curves, followed by depression, seen in down-curves. Note the appearance of a down-twitch at the be- ginning of the sixth response due to the later arrival of excitation at E. Note further the replacement of up- by increasing down-responses.

Long-continued applications, or strong solutions, however, have the effect of inducing a depression. Similarly, I find that this reagent has the effect of enhancing conductivity, provided the solution is sufficiently dilute. In the case of the petioles of ferns, a 2 per cent. solution was found to induce a preliminary exaltation of excitability, followed by a depression (p. 136). In dealing with the conductivity- variation in certain isolated vegetable nerves, however, a 2 per cent. solution was found to induce a depression of con- ductivity, but a °5 per cent. solution caused an enhancement

of conductivity, followed, after long-continued action, by depression. These facts are illustrated in an extremely interesting manner in the records given in figs. 297 and 298. In both these cases the solution was applied on the right arm of the balance at C, the difference being only that in the first experi- ment the strength of solution was ‘5, and in the second 2 per cent. An inspection of fig. 297 shows that the application of the first induced a great and immediate enhancement of conductivity, causing resultant up-responses, which were par- ticularly marked during the first four minutes. This increased

Fic. 298. Photographic Record of Effect of Stronger Dose (2 per cent.) of Na,CO, Solution on Conductivity. The solution was applied on the right arm of the balance c. Note grow- ing depression and appearance of diphasic effect. conductivity is then seen to undergo a continuous decrease and reversal into growing depression, as seen in the substi- tution of increasing down-responses. This record deserves special attention, inasmuch as it affords us an insight into a phenomenon which could not otherwise have been suspected. Greater conductivity is usually associated with increased velocity of transmission. It would appear, however, that the term conductivity really covers two different phenomena which may not always be concomitant. That is to say, an increase of conductivity may mean either a greater speed of transmission of excitation or a greater intensity of the

excitation transmitted. In the first four records of the present series the induced enhancement of conductivity is shown by the occurrence of up-responses only. The fifth record, how- ever, shows a marked preliminary twitch in the negative direction, followed by an up-response of some amplitude. This shows that the excitatory effect reached the. right end E later than the left, though the intensity still remained greater. The continued action of the reagent subsequently ~ reduced the intensity also, so that this diphasic ultimately became converted into a purely monophasic down-response, gradually increasing to a maximum. In fig. 298 we observe the depression of conductivity by a stronger dose of 2 per cent. solution of Na,CO,, applied on the right-hand side at C. Here, again, we can see the separated effects of the two elements of conductivity—that is to say, the intensity of the effect transmitted and the speed of transmission. In the first ‘few responses of this series we see the diminished intensity of transmission to the right giving rise to resultant responses which are entirely downwards. Later, this transmission of enfeebled excitation becomes delayed also, and by the phase- difference thus induced we obtain the growing diphasic effects which have already been fully explained on p. 144, fig. 100. Owing now to this growing difference of phase, the two opposed effects no longer neutralise each other to the same extent as before, and we obtain increasing amplitude of both the constituent phases. The down-curve in the diphasic response represents the earlier arrival, and relatively greater intensity, of effect at the left contact E’. And the up-curve shows the later arrival of the less intense effect at the right- hand contact: E.

It is thus clearly seen that conductivity includes two different elements of speed and intensity which may not in all cases be coincident. I shall next describe experiments which will demonstrate the variation of conductivity versus that of responsive excit- ability under the action of the same reagent. In animal nerve responsive excitability is diminished by the action of strong solutions of neutral salts, and potassium salts induce greater depression than corresponding sodium salts. But neutral salts, generally speaking, affect conductivity to a much slighter extent than responsivity. There is, however, a very curious exception to this rule in the case of animal nerve, where 6'1 per cent. of Nal is found to affect the conductivity to a much greater extent than the responsive excitability. I find a remarkable parallelism to these effects in the case of vegetable nerve, which is capable of striking demonstration by the comparative method of simultaneous variations of conductivity and excitability already de- scribed. In order to demonstrate these con- trasted effects of KI and Nal on conductivity and excitability, I shall here give an account of two different experi- ments. In the first, after obtaining the pre- liminary balance, KI was applied at ton Fic. 299. KESPONSIVITY versus

ie pia ; CONDUCTIVITY under KI SHE Sie eee, SOS saat This photographic record shows the effect of reagent being also ap- KI on responsivity and conductivity when Tiead th ee reagent applied at £’ and C simultaneously. pile at € end E o The formula is E’x;Cx;. Record shows the left arm, this pro- greater depression of responsivity than of by the formula E’,..C,,.. The record seen in fig.'299 shows, by its resultant up-responses, that a greater depression of responsivity at E’ than of conductivity at c has been induced. In the next experiment (fig. 300) Nal was applied instead of KI, on C to the right, and E’ to the left, the formula thus being E’,..C.., The resultant responses were now down- wards, showing that there was a relatively greater depression

of conductivity than of responsivity. In respect of conduc- tivity and responsivity, therefore, the effects of these two drugs, KI and Nal, are seen to be opposite. In order to observe the effect of alcohol on nervous tissue, by means of the conductivity balance, I first experimented on the nerve of frog. A 5 per cent. solution was applied at the responding point E. This is seen (fig. 301) to induce a depression of responsivity. A more dilute solution generally induces a preliminary exaltation followed by depression.

We said in the previous chapter that when alcohol vapour was passed into the chamber of the vegetable nerve the responses underwent a rapid abolition. This result, however, The formula in this case is E’ya1Cnar. Photographic record shows an effect opposite to that of KI as now a relatively greater de- Upsetting of the balance in the pression of conductivity than of downward direction shows responsivity. depression. was due to the joint action of the variations of receptivity, conductivity, and responsivity, some of which may possibly have been in the positive and others in the negative direction. In order to determine the effect of each of these we must, then, perform separate experiments. Such a deter- mination I have made, using the method of the so-called ‘negative variation, in which the proximal galvanometric

contact was on an unkilled, and the distal on a killed area. The first of these experiments was on variation of receptivity. The thermal stimulator was provided with mica shields, so that the receptive area was strictly circumscribed at the centre of the thermal platinum loop. Normal responses were first taken; the receptive area was next touched with I per cent. solution of alcohol, and the modified responses were recorded. The results are seen in fig. 302, which gives

Fic, 302, Photographic Record of Effect of Alcohol Vapour on Receptivity The three normal responses to the left are seen to be exalted after applica- tion of ether onsthe receptive point. a striking demonstration of the increased receptivity induced by dilute alcohol. The effect on conductivity, however, is in curious con- trast to this, On applying I per cent. solution in the conducting region between the stimulator and the proximal contact, a very great diminution of the conducting power is observed, as seen in fig. 303. It may be stated here that a similar enhancement of receptive excitability, and depression of conductivity, are found to be the result of the action of alcohol in animal nerve also.. In the next ex- periment, it is the variation of responsivity under the action of dilute alcohol which is tested. After taking the normal

responses as usual, a I per cent. solution of alcohol was applied at the proximal contact. It will be seen from the record in fig. 304 that the immediate effect was a depression Fic. 303. Photographic Record of Effect of Alcohol on Conductivity The three large responses to the left show the normal effect of transmitted excitation. Responses almost abolished, as seen on the right, by depression of conductivity. of the amplitude of response. This subsequently becomes converted into a diphasic response, consisting of a preliminary positive followed by the normal negative; and finally the

Fic. 304. Photographic Record showing Effect of Alcohol on Responsivity a, normal responses, depressed, after application of alcohol, to d@; and converted later to abnormal positive responses c. response was totally reversed to positive, by the abolition of the true excitatory effect. It is thus seen that while dilute alcohol exalts the recep- tive excitability, it induces a depression of both conductivity and responsivity. I shall now describe further experiments by which the. relative effects of alcohol are compared, as between conductivity and receptivity, and as between recep- tivity and responsivity. :