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.
[Illustration: FIG. 2.--ELECTRIC METHOD OF DETECTING NERVE RESPONSE
(_a_) Iso-electric contacts; no current in the galvanometer.
(_b_) The end B injured; current of injury from B to A: stimulation
gives rise to an action current from A to B.
(_c_) Non-polarisable electrode.]
#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]
[Illustration: FIG. 3.--DIAGRAM SHOWING THE CORRESPONDENCE BETWEEN
INJURED (B) AND UNINJURED (A) CONTACTS IN NERVE, AND Cu AND Zn IN A
VOLTAIC ELEMENT
Comparison of (_c_) and (_d_) will show that the injured end of B in
(_d_) corresponds with the Cu in (_c_).]
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.
[Illustration: FIG. 4.--ELECTRIC RECORDER
(_a_) M muscle; A uninjured, B injured ends. E E' non-polarising
electrodes connecting A and B with galvanometer G. Stimulus produces
'negative variation' of current of rest. Index connected with
galvanometer needle records curve on travelling paper (in practice,
moving galvanometer spot of light traces curve on photographic
plate). Rising part of curve shows effect of stimulus; descending
part, recovery.
(_b_) O is the zero position of the galvanometer; injury produces a
deflection A B; stimulus diminishes this deflection to C; C D is the
recovery.]
#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.
[Illustration: FIG. 5.--SIMULTANEOUS RECORD OF THE MECHANICAL (M) AND
(E) ELECTRICAL RESPONSES OF THE MUSCLE OF FROG. (WALLER.)]
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.