Response in the Living and Non-Living | Page 5

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.