Response in the Living and Non-Living | Page 4

place in too short a time for
detailed observation by ordinary means. A myographic apparatus is
therefore used, by means of which the changes in the muscle are
self-recorded. Thus we obtain a history of its change and recovery from
the change. The muscle is connected to one end of a writing lever.
When the muscle contracts, the tracing point is pulled up in one
direction, say to the right. The extent of this pull depends on the
amount of contraction. A band of paper or a revolving drum-surface
moves at a uniform speed at right angles to the direction of motion of
the writing lever. When the muscle recovers from the stimulus, it
relaxes into its original form, and the writing point traces the recovery
as it moves now to the left, regaining its first position. A curve is thus
described, the rising portion of which is due to contraction, and the
falling portion to relaxation or recovery. The ordinate of the curve
represents the intensity of response, and the abscissa the time (fig. 1).
[Illustration: FIG. 1.--MECHANICAL LEVER RECORDER The
muscle M with the attached bone is securely held at one end, the other
end being connected with the writing lever. Under the action of
stimulus the contracting muscle pulls the lever and moves the tracing
point to the right over the travelling recording surface P. When the
muscle recovers from contraction, the tracing point returns to its
original position. See on P the record of muscle curve.]
#Characteristics of the response-curve: (1) Period, (2) Amplitude, (3)
Form.#--Just as a wave of sound is characterised by its (1) period, (2)
amplitude, and (3) form, so may these response-curves be distinguished
from each other. As regards the period, there is an enormous variation,
corresponding to the functional activity of the muscle. For instance, in
tortoise it may be as high as a second, whereas in the wing-muscles of
many insects it is as small as 1/300 part of a second. 'It is probable that
a continuous graduated scale might, as suggested by Hermann, be
drawn up in the animal kingdom, from the excessively rapid

contraction of insects to those of tortoises and hibernating dormice.'[1]
Differences in form and amplitude of curve are well illustrated by
various muscles of the tortoise. The curve for the muscle of the neck,
used for rapid withdrawal of the head on approach of danger, is quite
different from that of the pectoral muscle of the same animal, used for
its sluggish movements.
Again, progressive changes in the same muscle are well seen in the
modifications of form which consecutive muscle-curves gradually
undergo. In a dying muscle, for example, the amplitude of succeeding
curves is continuously diminished, and the curves themselves are
elongated. Numerous illustrations will be seen later, of the effect, in
changing the form of the curve, of the increased excitation or
depression produced by various agencies.
Thus these response records give us a means of studying the effect of
stimulus, and the modification of response, under varying external
conditions, advantage being taken of the mechanical contraction
produced in the tissue by the stimulus. But there are other kinds of
tissue where the excitation produced by stimulus is not exhibited in a
visible form. In order to study these we have to use an altogether
independent method, the method of electric response.
FOOTNOTES:
[1] Biedermann, Electro-physiology, p. 59.
CHAPTER II
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