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same cell. Such a view would regard the phenomena of transmission in cells and nerve-fibers as essentially an expression or consequence of electrical effects resulting from local activity. Let us now inquire if the general peculiarities of the bioelectric variations in such a tissue as nerve-which is the best for illustration because it is primarily conducting in function-are in fact consistent with such an hypothesis.

The transmission of the excitation-state in living tissues must have at least this in common with the other innumerable instances of transmission of physical changes in nature—that the change taking place at one region of the transmitting medium or system in some manner produces or determines a similar change at adjoining regions. By a repetition of this effect the state of activity, whatever its special nature may be, is propagated from region to region. Our present problem is: what is the essential physico-chemical nature of the process which takes place at the excited region of a conducting element (like a nerve-fiber) and causes excitation in adjoining resting regions? We have seen that the electrical variation accompanying activity is the only observable change (in nerve at least) which is known to be capable of stimulating a resting part of the same tissue. We are thus led to form the hypothesis that the bioelectric variation at the active region is the direct cause of stimulation at the adjoining resting region. If this can be shown to be the case, transmission is accounted for, since all portions of the tissue are equally sensitive to electrical stimulation. In his Croonian Lecture, delivered in 1912, Keith Lucas formulates the problem with his customary clearness and exactitude. He reviews the various facts indicating that the bioelectric variation is an inseparable feature of the " propagated disturbance," i. e., of the excitation-wave or nerveimpulse, and proceeds as follows:

Up to the present the available evidence does not contradict the proposition that the electric response is a constant concomitant of the propagated disturbance. But for the purpose of any hypothesis as to the physico-chemical nature of the latter the mere stringent proof of this proposition would not be enough. The important point for any such hypothesis is whether they are identical, i. e., whether the disturbance of electric potential at one point in a nerve is the actual and direct cause of the same phenomenon in a neighboring part. Any hypothesis must be prepared to state whether the electric phenomena play the essential part in propagation or are to be relegated to the position of a mere by-product. Evidently this distinction is an important one to bear in mind, for it might well be that the electrical variation accompanying ▲ Proc. Roy. Soc., 1912, Vol. 85, B, p. 507.

the excitation-wave is merely a sign or index of some other underlying process (which determines both the local change and the transmission), and is in itself of no functional importance.

It is necessary first to inquire under what conditions an electric current led into the tissue from outside causes stimulation, and then to inquire whether the current produced by the tissue in its own activity has those characteristics-of intensity, duration, rate of variation, and direction-which will enable it to stimulate the adjoining resting portions of the same tissue. If this is found to be the case, the view that the normal transmission is due to this current will be substantiated; for if the bioelectric current can thus excite the inactive areas, and especially if the rate of this excitation is sufficient to account for the observed velocity of the excitation-wave, there is no reason to seek other causes for the transmission.

In order that an electric current passed through a nerve or other irritable tissue shall cause stimulation, it must have a certain minimal intensity and duration, it must reach that intensity with sufficient rapidity, and it must have a certain direction relatively to the surface of the irritable elements. Too weak a current will not stimulate, nor will one which lasts for too short a time or which rises from zero to its full intensity gradually instead of suddenly. We know, however, from simple observation that in these last respects the current produced by an active muscle or nerve is adequate to stimulate the resting tissue; this is shown by the experiment with the rheoscopic muscle or nerve, in which, e. g., the action current of one muscle is made to stimulate another muscle. The question of the direction of the current relatively to the cell-surface at the region of excitation is more important. In general, electrical stimulation is a polar phenomenon; the current stimulates only at the region where the positive stream leaves the cell or nervefiber; where it enters the cell it causes the reverse effect of inhibition. This is the so-called "law of polar stimulation," which is of very wide if not of universal application in irritable tissues. If the current is led into a nerve by non-polarizable electrodes it can be shown that the excitation-wave arises at the negative electrode or cathode; if the positive electrode is interposed between the cathode and the muscle the excitation-wave may fail to reach the latter because of the inhibitory or "blocking" influence at the anodal region of the nerve. Similar facts apply to muscle; here also where the positive stream leaves the tissue it excites, and where it enters it inhibits. The physiologVOL. VII.-30.

ical effects produced by breaking the current illustrate the same law, since in this case the reverse polarization-current is to be regarded as the cause of stimulation.

Now when we study the peculiarities of the bioelectric current produced by the conducting tissue in its own activity we are at once struck with the fact that this current has all of those characteristics of intensity, duration, rate of change and direction which would be required if the current were actually designed to stimulate the tissue at the adjacent resting regions and bring it again to rest at the active region. All of the conditions required for the automatic transmission of the excitation-state from active to resting regions are in fact present. This may be seen from a consideration of the diagram (Fig. 1).

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FIG. 1. Diagram of the momentary conditions in a frog's motor nerve at 20°. The shaded region marked A, between R2 and R3, is occupied at the instant under consideration by the excitation-wave, which is regarded as advancing in the direction of the large arrow at the rate of 30 meters per second. Its length, assuming the total duration of the local process (as indicated by the duration of the local bioelectric variation) to be .002 second, is 6 cm. The excitation-process is just beginning at R3, has reached its maximum at A10, and has just subsided at R2. The curve represents the variation from the resting potential at different points in the active region; the maximum P.D., at A10, is ca. 40 millivolts. The regions marked R are in the resting state. The small arrows indicate the direction of the bioelectric current (positive stream) in a portion of the active-resting circuit. Between R and R1 its intensity is sufficient to excite the nerve; excitation is thus always being initiated at a distance 3 cm. in advance of the wave-front (i. e., up to R). For a somewhat similar distance RR in the wake of the excitation-wave the nerve is refractory to stimulation.

The region which is in a state of activity is electrically negative to the regions which are in a state of rest; i. e., there is a circuit in which the positive stream flows in the external medium from inactive to active regions; the current enters the cell-surface at the active region and leaves it at the inactive regions. An external current from a battery having this direction would tend to excite the tissue at R and tend to inhibit an already existing excitation at A (see Fig. 2). There is no apparent reason why a self-generated current should have different physiological effects from one which reaches the tissue from an outside source. Hence we are forced to conclude that the local bioelectric current, as soon as it originates, tends to excite

or activate the resting regions immediately adjoining the active area, and to cut short or inhibit activity in the active area itself. We may compare the active and the adjacent inactive areas with a pair of electrodes applied to the surface of the tissue, with the obvious difference that these areas are continually changing position, keeping pace with the excitation-wave as it advances. The consequence of this is that as each successive

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FIG. 2 (inserted for comparison) indicates the effects of passing an external current through a nerve from a battery by non-polarisable electrodes. At the entrance of the current (region corresponding to A in Fig. 1) the effect is inhibitory; at its exit (corresponding to R) excitatory.

region becomes active, excitation is automatically induced at adjoining regions, and automatically cut short at the active region itself. What we actually observe, quite apart from hypothesis, is that a wave of activity, accompanied by a local electrical circuit, travels in either direction from the original point of excitation. Since this electrical circuit is an essential component of the activation-wave, it is not surprising that the latter produces the characteristic physiological effects of an electric current-excitation at one pole, inhibition at the other -at every region which it traverses in its course. Both the self-propagating and the self-limiting character of the local excitation-process may thus be understood. It should again be noted that excitation at one region, in constant association with simultaneous inhibition at another, is a frequent phenomenon in living organisms, not only in the central nervous system, where it is well-known under Sherrington's term of reciprocal inhibition, but also in locally controlled muscular movements like peristalsis (myenteric law), and even in growth processes and amoeboid movement. The condition observed during the passage of the excitation-wave along a nerve-fiber, where the region in advance of the wave-front and that immediately behind are oppositely affected, being respectively stimulated and inhibited, is thus in no sense unique or unexampled. A fundamental property of protoplasm, that of being influenced in a polar manner by the electrical current, is apparently the essential factor in all of these phenomena.

Why the excitation-wave continues its progress in the one

direction, and does not strike back, so to speak, may seem to require explanation. Why should not any region of the tissue, when it has once returned to rest, be again excited by the active area which is receding from it, just as it was previously excited by the active area advancing toward it? This effect, however, is rendered impossible by the fact that the tissue always becomes temporarily inexcitable for a brief period (the socalled refractory period) immediately following excitation. The excitation-wave thus leaves behind itself a trail or wake of inexcitable tissue, and by the time any single local region has recovered its normal excitability the region of activity is already at too great a distance in advance to exert any stimulating influence at the recovered region.

It would thus seem that when we take into consideration the fact of an electrical circuit between active and inactive areas, and apply the law of polar stimulation, the wave-like transmission of the active state along the nerve-fiber may be accounted for, if we assume that the intensity of the bioelectric current traversing the resting region at a certain distance in advance of the active region is sufficient for stimulation, and that its duration and time of development at that point meet the chronological requirements of the tissue. It can, however, be shown that this is almost certainly the case. The potential-difference between excited and unexcited areas can be measured, and in a frog's nerve has a value of 30 or 40 millivolts; two platinum electrodes with the same P.D., placed 3 or 4 centimeters part in contact with the nerve, will give a current sufficiently intense for excitation. The normal bioelectric current passing between an active region and an inactive region at a similar distance apart (say 3 cm.) should have the same stimulating efficacy as that between the platinum electrodes, since the P.D. and the conditions of resistance are similar in the two circuits; and there are other facts indicating that the current of the local bioelectric circuit has a sufficient intensity at a distance of two or three centimeters from the active area to stimulate the resting tissue. We know too the duration of the local current and its rate of development; these may be ascertained by measuring the time-relations of the bioelectric variation with the string galvanometer. By this means the local variation of potential at the excited area is shown to occupy a certain time which in a given tissue is definite and characteristic,-e. g., in frog's motor nerve, ca. .002 seç. at 20°, according to Garten's observations; i. e., the variation rises from zero to its maximum and

5 Cf. Garten, Winterstein's "Handbuch der vergleichenden Physiologie," 1910, Vol. 3, Part 2, pp. 135 seq.

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