Response in the Living and Non-Living | Page 6

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