Chapter 22

ELECTRIC RESPONSE

Conditions for obtaining electric response--Method of injury--Current of injury--Injured end, cuproid: uninjured, zincoid--Current of response in nerve from more excited to less excited--Difficulties of present nomenclature--Electric recorder--Two types of response, positive and negative--Universal applicability of electric mode of response--Electric response a measure of physiological activity--Electric response in plants.

Unlike muscle, a length of nerve, when mechanically or electrically excited, does not undergo any visible change. That it is thrown into an excitatory state, and that it conducts the excitatory disturbance, is shown however by the contraction produced in an attached piece of muscle, which serves as an indicator.

But the excitatory effect produced in the nerve by stimulus can also be detected by an electrical method. If an isolated piece of nerve be taken and two contacts be made on its surface by means of non-polarisable electrodes at A and B, connection being made with a galvanometer, no current will be observed, as both A and B are in the same physico-chemical condition. The two points, that is to say, are iso-electric.

If now the nerve be excited by stimulus, similar disturbances will be evoked at both A and B. If, further, these disturbances, reaching A and B almost simultaneously, cause any electrical change, then, similar changes taking place at both points, and there being thus no relative difference between the two, the galvanometer will still indicate no current. This null-effect is due to the balancing action of B as against A. (See fig. 2, _a_.)

#Conditions for obtaining electric response.#--If then we wish to detect the response by means of the galvanometer, one means of doing so will lie in the abolition of this balance, which may be accomplished by making one of the two points, say B, more or less permanently irresponsive. In that case, stimulus will cause greater electrical disturbance at the more responsive point, say A, and this will be shown by the galvanometer as a current of response. To make B less responsive we may injure it by means of a cross-sectional cut, a burn, or the action of strong chemical reagents.

#Current of injury.#--We shall revert to the subject of electric response; meanwhile it is necessary to say a few words regarding the electric disturbance caused by the injury itself. Since the physico-chemical conditions of the uninjured A and the injured B are now no longer the same, it follows that their electric conditions have also become different. They are no longer iso-electric. There is thus a more or less permanent or resting difference of electric potential between them. A current--the current of injury--is found to flow _in the nerve_, from the injured to the uninjured, and in the galvanometer, through the electrolytic contacts from the uninjured to the injured. As long as there is no further disturbance this current of injury remains approximately constant, and is therefore sometimes known as 'the current of rest' (fig. 2, _b_).

A piece of living tissue, unequally injured at the two ends, is thus seen to act like a voltaic element, comparable to a copper and zinc couple. As some confusion has arisen, on the question of whether the injured end is like the zinc or copper in such a combination, it will perhaps be well to enter upon this subject in detail.

If we take two rods, of zinc and copper respectively, in metallic contact, and further, if the points A and B are connected by a strip of cloth _s_ moistened with salt solution, it will be seen that we have a complete voltaic element. A current will now flow from B to A in the metal (fig. 3, _a_) and from A to B through the electrolyte _s_. Or instead of connecting A and B by a single strip of cloth _s_, we may connect them by two strips _s s'_, leading to non-polarisable electrodes E E'. The current will then be found just the same as before, i.e. from B to A in the metallic part, and from A through _s s'_ to B, the wire W being interposed, as it were, in the electrolytic part of the circuit. If now a galvanometer be interposed at O, the current will flow from B to A through the galvanometer, i.e. from right to left. But if we interpose the galvanometer in the electrolytic part of the circuit, that is to say, at W, the same current will appear to flow in the opposite direction. In fig. 3, _c_, the galvanometer is so interposed, and in this case it is to be noticed that when the current in the galvanometer flows from left to right, the metal connected to the left is zinc.

Compare fig. 3, _d_, where A B is a piece of nerve of which the B end is injured. The current in the galvanometer through the non-polarisable electrode is from left to right. The uninjured end is therefore comparable to the zinc in a voltaic cell (is zincoid), the injured being copper-like or cuproid.[2]

If the electrical condition of, say, zinc in the voltaic couple (fig. 3, _c_) undergo any change (and I shall show later that this can be caused by molecular disturbance), then the existing difference of potential between A and B will also undergo variation. If for example the electrical condition of A approach that of B, the potential difference will undergo a diminution, and the current hitherto flowing in the circuit will, as a consequence, display a diminution, or _negative_ variation.

#Action current.#--We have seen that a current of injury--sometimes known as 'current of rest'--flows in a nerve from the injured to the uninjured, and that the injured B is then less excitable than the uninjured A. If now the nerve be excited, there being a greater effect produced at A, the existing difference of potential may thus be reduced, with a consequent diminution of the current of injury. During stimulation, therefore, a nerve exhibits a negative variation. We may express this in a different way by saying that a 'current of action' was produced in response to stimulus, and acted in an opposite direction to the current of injury (fig. 2, _b_). The action current in the nerve _is from the relatively more excited to the relatively less excited_.

#Difficulties of present nomenclature.#--We shall deal later with a method by which a responsive current of action is obtained without any antecedent current of injury. 'Negative variation' has then no meaning. Or, again, a current of injury may sometimes undergo a change of direction (see note, p. 12). In view of these considerations it is necessary to have at our disposal other forms of expression by which the direction of the current of response can still be designated. Keeping in touch with the old phraseology, we might then call a current 'negative' that flowed from the more excited to the less excited. Or, bearing in mind the fact that an uninjured contact acts as the zinc in a voltaic couple, we might call it 'zincoid,' and the injured contact 'cuproid.' Stimulation of the uninjured end, approximating it to the condition of the injured, might then be said to induce a cuproid change.

The electric change produced in a normal nerve by stimulation may therefore be expressed by saying that there has been a negative variation, or that there was a current of action from the more excited to the less excited, or that stimulation has produced a cuproid change.

The excitation, or molecular disturbance, produced by a stimulus has thus a concomitant electrical expression. As the excitatory state disappears with the return of the excitable tissue to its original condition, the current of action will gradually disappear.[3] The movement of the galvanometer needle during excitation of the tissue thus indicates a molecular upset by the stimulus; and the gradual creeping back of the galvanometer deflection exhibits a molecular recovery.

This transitory electrical variation constitutes the 'response,' and its intensity varies according to that of the stimulus.

#Electric recorder.#--We have thus a method of obtaining curves of response electrically. After all, it is not essentially very different from the mechanical method. In this case we use a magnetic lever (fig. 4, _a_), the needle of the galvanometer, which is deflected by the electromagnetic pull of the current, generated under the action of stimulus, just as the mechanical lever was deflected by the mechanical pull of the muscle contracting under stimulus.

The accompanying diagram (fig. 4, _b_) shows how, under the action of stimulus, the current of rest undergoes a transitory diminution, and how on the cessation of stimulus there is gradual recovery of the tissue, as exhibited in the return of the galvanometer needle to its original position.

#Two types of response--positive and negative.#--It may here be added that though stimulus in general produces a diminution of current of rest, or a negative variation (e.g. muscles and nerves), yet, in certain cases, there is an increase, or positive variation. This is seen in the response of the retina to light. Again, a tissue which normally gives a negative variation may undergo molecular changes, after which it gives a positive variation. Thus Dr. Waller finds that whereas fresh nerve always gives negative variation, stale nerve sometimes gives positive; and that retina, which when fresh gives positive, when stale, exhibits negative variation.

The following is a tabular statement of the two types of response:

I. _Negative variation._--Action current from more excited to less excited--cuproid change in the excited--e.g. fresh muscle and nerve, stale retina.

II. _Positive variation._--Action current from less excited to more excited--zincoid change in the excited--e.g. stale nerve, fresh retina.[4]

From this it will be seen that it is the fact of the electrical response of living substances to stimulus that is of essential importance, the sign _plus_ or _minus_ being a minor consideration.

#Universal applicability of the electrical mode of response.#--This mode of obtaining electrical response is applicable to all living tissues, and in cases like that of muscle, where mechanical response is also available, it is found that the electrical and mechanical records are practically identical.

The two response-curves seen in the accompanying diagram (fig. 5), and taken from the same muscle by the two methods simultaneously, clearly exhibit this. Thus we see that electrical response can not only take the place of the mechanical record, but has the further advantage of being applicable in cases where the latter cannot be used.

#Electrical response: A measure of physiological activity.#--These electrical changes are regarded as physiological, or characteristic of living tissue, for any conditions which enhance physiological activity also, _pari passu_, increase their intensity. Again, when the tissue is killed by poison, electrical response disappears, the tissue passing into an irresponsive condition. Anæsthetics, like chloroform, gradually diminish, and finally altogether abolish, electrical response.

From these observed facts--that living tissue gives response while a tissue that has been killed does not--it is concluded that the phenomenon of response is peculiar to living organisms.[5] The response phenomena that we have been studying are therefore considered as due to some unknown, super-physical 'vital' force and are thus relegated to a region beyond physical inquiry.

It may, however, be that this limitation is not justified, and surely, at least until we have explored the whole range of physical action, it cannot be asserted definitely that a particular class of phenomena is by its very nature outside that category.

#Electric response in plants.#--But before we proceed to the inquiry as to whether these responses are or are not due to some physical property of matter, and are to be met with even in inorganic substances, it will perhaps be advisable to see whether they are not paralleled by phenomena in the transitional world of plants. We shall thus pass from a study of response in highly complex animal tissues to those given under simpler vital conditions.

Electric response has been found by Munck, Burdon-Sanderson, and others to occur in sensitive plants. But it would be interesting to know whether these responses were confined to plants which exhibit such remarkable mechanical movements, and whether they could not also be obtained from ordinary plants where visible movements are completely absent. In this connection, Kunkel observed electrical changes in association with the injury or flexion of stems of ordinary plants.[6] My own attempt, however, was directed, not towards the obtaining of a mere qualitative response, but rather to the determination of whether throughout the whole range of response phenomena a parallelism between animal and vegetable could be detected. That is to say, I desired to know, with regard to plants, what was the relation between intensity of stimulus and the corresponding response; what were the effects of superposition of stimuli; whether fatigue was present, and in what manner it influenced response; what were the effects of extremes of temperature on the response; and, lastly, if chemical reagents could exercise any influence in the modification of plant response, as stimulating, anæsthetic, and poisonous drugs have been found to do with nerve and muscle.

If it could be proved that the electric response served as a faithful index of the physiological activity of plants, it would then be possible successfully to attack many problems in plant physiology, the solution of which at present offers many experimental difficulties.

With animal tissues, experiments have to be carried on under many great and unavoidable difficulties. The isolated tissue, for example, is subject to unknown changes inseparable from the rapid approach of death. Plants, however, offer a great advantage in this respect, for they maintain their vitality unimpaired during a very great length of time.

In animal tissues, again, the vital conditions themselves are highly complex. Those essential factors which modify response can, therefore, be better determined under the simpler conditions which obtain in vegetable life.

In the succeeding chapters it will be shown that the response phenomena are exhibited not only by plants but by inorganic substances as well, and that the responses are modified by various conditions in exactly the same manner as those of animal tissues. In order to show how striking are these similarities, I shall for comparison place side by side the responses of animal tissues and those I have obtained with plants and inorganic substances. For the electric response in animal tissues, I shall take the latest and most complete examples from the records made by Dr. Waller.

But before we can obtain satisfactory and conclusive results regarding plant response, many experimental difficulties will have to be surmounted. I shall now describe how this has been accomplished.[7]

FOOTNOTES:

[2] In some physiological text-books much wrong inference has been made, based on the supposition that the injured end is zinc-like.

[3] 'The exciting cause is able to produce a particular molecular rearrangement in the nerve; this constitutes the state of excitation and is accompanied by local electrical changes as an ascertained physical concomitant.'

'The excitatory state evoked by stimulus manifests itself in nerve fibres by E.M. changes, and as far as our present knowledge goes by these only. The conception of such an excitable living tissue as nerve implies that of a molecular state which is in stable equilibrium. This equilibrium can be readily upset by an external agency, the stimulus, but the term "stable" expresses the fact that a change in any direction must be succeeded by one of opposite character, this being the return of the living structure to its previous state. Thus the electrical manifestation of the excitatory state is one whose duration depends upon the time during which the external agent is able to upset and retain in a new poise the living equilibrium, and if this is extremely brief, then the recoil of the tissue causes such manifestation to be itself of very short duration.'--_Text-book of Physiology_, ed. by Schäfer, ii. 453.

[4] I shall here mention briefly one complication that might arise from regarding the current of injury as the current of reference, and designating the response current either positive or negative in relation to it. If this current of injury remained always invariable in direction--that is to say, from the injured to the uninjured--there would be no source of uncertainty. But it is often found, for example in the retina, that the current of injury undergoes a reversal, or is reversed from the beginning. That is to say, the direction is now from the uninjured to the injured, instead of the opposite. Confusion is thus very apt to arise. No such misunderstanding can however occur if we call the current of response towards the more excited _positive_, and towards the less excited _negative_.

[5] 'The Electrical Sign of Life.... An isolated muscle gives sign of life by contracting when stimulated.... An ordinary nerve, normally connected with its terminal organs, gives sign of life by means of muscle, which by direct or reflex path is set in motion when the nerve trunk is stimulated. But such nerve separated from its natural termini, isolated from the rest of the organism, gives no sign of life when excited, either in the shape of chemical or of thermic changes, and it is only by means of an electrical change that we can ascertain whether or no it is alive.... The most general and most delicate sign of life is then the electrical response.'--Waller, in _Brain_, pp. 3 and 4. Spring 1900.

[6] Kunkel thought the electric disturbance to be due to movement of water through the tissue. It will be shown that this explanation is inadequate.

[7] My assistant Mr. J. Bull has rendered me very efficient help in these experiments.