INORGANIC RESPONSE--FATIGUE, STAIRCASE, AND MODIFIED RESPONSE
Fatigue in metals--Fatigue under continuous stimulation--Staircase
effect--Reversed responses due to molecular modification in nerve
and metal, and their transformation into normal after continuous
stimulation--Increased response after continuous stimulation.
[Illustration: FIG. 69.--FATIGUE IN MUSCLE (WALLER)]
[Illustration: FIG. 70.--FATIGUE IN PLATINUM]
#Fatigue.#--In some metals, as in muscle and in plant, we find instances
of that progressive diminution of response which is known as fatigue
(fig. 69). The accompanying record shows this in platinum (fig. 70). It
has been said that tin is practically indefatigable. We must, however,
remember that this is a question of degree only. Nothing is absolutely
indefatigable. The exhibition of fatigue depends on various conditions.
Even in tin, then, I obtained the characteristic fatigue-curve with a
specimen which had been in continuous use for many days (fig. 71).
While discussing the subject of fatigue in plants, I have adduced
considerations which showed that the residual effect of strain was one
of the main causes for the production of fatigue. This conclusion
receives independent support from the records obtained with metals.
[Illustration: FIG. 71.--FATIGUE SHOWN BY TIN WIRE WHICH HAD BEEN
CONTINUOUSLY STIMULATED FOR SEVERAL DAYS]
In this connection the important fact is that the various typical
fatigue effects exhibited in living substances are exactly reproduced in
metals, where there can be question neither of fatigue-product producing
fatigue effects, nor of those constructive processes by which they might
be removed. We have seen, both in muscles and in plants, that if
sufficient time for complete recovery be allowed between each pair of
stimuli, the heights of successive responses are the same, and there is
no apparent fatigue (see page 39). But the height of response diminishes
as the excitation interval is shortened. We find the same thing in
metals. Below is given a record taken with tin (fig. 72). Throughout the
experiment the amplitude of vibration was maintained constant, but in
(_a_) the interval between consecutive stimuli was 1', while in (_b_)
this was reduced to 30". A diminution of height immediately occurs. On
restoring the original rhythm as in (_c_), the responses revert to their
first large value. Thus we see that when the wire has not completely
recovered, its responses, owing to residual strain, undergo diminution.
Height of response is thus decreased by incomplete recovery. If then
sufficient time be not allowed for perfect recovery, we can understand
how, under certain circumstances, the residual strain would
progressively increase with repetition of stimulus, and thus there would
be a progressive diminution of height of response or fatigue. Again, we
saw in the last chapter that increase of strain necessitates a longer
period of recovery. Thus the longer a wire is stimulated, the more and
more overstrained it becomes, and it therefore requires a gradual
prolongation of the interval between the successive stimuli, if recovery
is to be complete. This interval, however, being maintained constant,
the recovery periods virtually undergo a gradual reduction, and
successive recoveries become more and more incomplete. These
considerations may be found to afford an insight into the progressive
diminution of response in fatigued substances.
[Illustration: FIG. 72.--DIMINUTION OF RESPONSE DUE TO SHORTENING THE
PERIOD OF RECOVERY
The stimulus is maintained constant. In (_a_) the interval between two
successive stimuli is one minute, in (_b_) it is half a minute, and
in (_c_) it is again one minute. The response in (_b_) is feebler
than in either (_a_) or (_c_).]
#Fatigue under continuous stimulation.#--Fatigue is perhaps best shown
under continuous stimulation. For example, in muscles, when fresh and
not fatigued, the top of the tetanic curve is horizontal, or may even be
ascending, but with long-continued stimulation the curve declines. The
rapidity of this decline depends on the nature of the muscle and its
previous condition.
In metals I have found exactly parallel instances. In tin, so little
liable to fatigue, the top of the curve is horizontal or ascending; or
it may exhibit a slight decline. But the record with platinum shows the
rapid decline due to fatigue (fig. 73).
[Illustration: FIG. 73
(_a_) The top of response-curve under continuous stimulation in tin
is horizontal or ascending as in figure.
(_b_) In platinum there is rapid decline owing to fatigue.]
Taking any of these instances, say that in which fatigue is most
prominent, it is found that short period of rest restores the original
intensity of response. This affords additional proof of the fact that
fatigue is due to overstrain, and that this strain, with its sign of
attendant fatigue, disappears with time.
#Staircase effect.#--We shall now discuss an effect which appears to be
the direct opposite of fatigue. This is the curious phenomenon known to
physiologists as 'the staircase' effect, in which successive uniform
stimuli produce a series of increasing responses. This is seen under
particular conditions in the response of certain muscles (fig. 74, _a_).
It is also observed sometimes even in nerve, which otherwise, generally
speaking, gives uniform responses. Of this effect, no satisfactory
theory has as yet been offered. It is in direct contradiction to that
theory which supposes that each stimulus is followed by dissimilation or
break-down of the tissue, reducing its function below par. For in these
cases the supposed dissimilation is followed not by a decrease but by an
increase of functional activity. This 'staircase effect' I have shown to
be occasionally exhibited by plants. I have also found it in metals. In
the last chapter we have seen that a wire often falls, especially after
resting for a long time, into a state of comparative sluggishness, and
that this molecular inertness then gradually gives place to increased
mobility under stimulation. As a consequence, an increased response is
thus obtained. I give in fig. 74, _b_, a series of responses to uniform
stimuli, exhibited by platinum which had been at rest for some time.
This effect is very clearly shown here. So we see that in a substance
which has previously been in a sluggish condition, stimulation confers
increased mobility. Response thus reaches a maximum, but continued
stimulation may afterwards produce overstrain, and the subsequent
responses may then show a decline. This consideration will explain
certain types of responses exhibited by muscles, where the first part
of the series exhibits a staircase increase followed by declining
responses of fatigue.
[Illustration: FIG. 74.--'STAIRCASE' EFFECT
(_a_) in muscle (after Engelmann).
(_b_) in metal.]
#Reversed response due to molecular modification and its transformation
into normal after continuous stimulation (1) in nerve.#--Reference has
already been made to the fact that a nerve which, when fresh, exhibited
the normal negative response, will often, if kept for some time in
preservative saline, undergo a molecular modification, after which it
gives a positive variation. Thus while the response given by fresh nerve
is _normal_ or negative, a stale nerve gives _modified_, i.e. reversed
or positive, response. This peculiar modification does not always occur,
yet is too frequent to be considered abnormal. Again, when such a nerve
is subjected to tetanisation or continuous stimulation, this modified
response tends once more to become normal.
It is found that not only tetanisation, but also CO_2 has the power of
converting the modified response into normal. Hence it has been
suggested that the conversion under tetanisation of modified response to
normal, in stale nerve, is due to a hypothetical evolution of CO_2 in
the nerve during stimulation.[16]
#(2) In metals.#--I have, however, met with exactly parallel phenomena in
metals, where, owing to some molecular modification, the responses
became reversed, and where, under continuous stimulation, though here
there could be no possibility of the evolution of CO_2, they tended
again to become normal.
If after mounting a wire in a cell filled with water, it be set aside
for too long a time, I have sometimes noticed that it undergoes a
certain modification, owing to which its response ceases to be normal
and becomes reversed in sign. I have obtained this effect with various
metals, for instance lead and tin, and even with the chemically inactive
substance--platinum.
[Illustration: FIG. 75.--ABNORMAL POSITIVE (UP) RESPONSE IN NERVE
CONVERTED INTO NORMAL (DOWN) RESPONSE AFTER CONTINUOUS STIMULATION T
(WALLER)
The galvanometer is not dead-beat, and shows after-oscillation.]
The subject will be made clearer if we first follow in detail the
phenomenon exhibited by modified nerve, giving this abnormal response.
The normal responses in nerve are usually represented by 'down' and the
reversed abnormal responses by 'up' curves. In the modified nerve, then,
the abnormal responses are 'up' instead of the normal 'down.' The record
of such abnormal response in the modified nerve is shown in fig. 75. It
will be noticed that in this, the successive responses are undergoing a
diminution, or tending towards the normal. After continuous stimulation
or tetanisation (T), it will be seen that the abnormal or 'up' responses
are converted into normal or 'down.'
I shall now give a record which will exhibit an exactly similar
transformation from the abnormal to normal response after continuous
stimulation. Here the normal responses are represented by 'up' and the
abnormal by 'down' curves. This record was given by a tin wire, which
had been molecularly modified (fig. 76). We have at first the abnormal
responses; successive responses are undergoing a diminution or tending
towards the normal; after continuous stimulation (T), the subsequent
responses are seen to have become normal. Another record, obtained with
platinum, shows the same phenomenon (fig. 77).
[Illustration: FIG. 76 AND FIG. 77
Abnormal 'down' response in tin (fig. 76) and in platinum (fig. 77)
transformed into normal 'up' response, after continuous
stimulation, T.]
On placing the three sets of records--nerve, tin, and platinum--side by
side, it will be seen how essentially similar they are in every
respect.[17]
This reversion to normal is seen to have appeared in a pronounced manner
after rapidly continuous stimulation, in process of which the modified
molecular condition must in some way have reverted to the normal.
[Illustration: FIG. 78.--THE GRADUAL TRANSITION FROM ABNORMAL TO NORMAL
RESPONSE IN PLATINUM
The transition will be seen to have commenced at the third and ended at
the seventh, counting from the left.]
Being desirous to trace this change gradually taking place, I took a
platinum wire cell giving modified responses, and obtained a series of
records of effects of individual stimuli continued for a long time. In
this series, the points of transition from modified response to normal
will be clearly seen (fig. 78).
[Illustration: FIG. 79.--THE NORMAL RESPONSE _a_ IN NERVE ENHANCED TO
_b_ AFTER CONTINUOUS STIMULATION T (WALLER)
The normal response in nerve is recorded 'down.']
[Illustration: FIG. 80.--ENHANCED RESPONSE IN PLATINUM AFTER CONTINUOUS
STIMULATION T]
#Increased response after continuous stimulation.#--We have seen that
responses to uniform stimuli sometimes show a staircase increase,
apparently owing to the gradual removal of molecular sluggishness.
Possibly analogous to this is the increase of response in nerve after
continuous stimulation or tetanisation, observed by Waller (fig. 79).
Like the staircase effect, this contravenes the commonly accepted theory
of the dissimilation of tissue by stimulus, and the consequent
depression of response. It is suggested by Waller that this increase of
response after tetanisation may be due to the hypothetical evolution of
CO_2 to which allusion has previously been made.
[Illustration: FIG. 81.--ENHANCED RESPONSE IN TIN AFTER CONTINUOUS
STIMULATION T]
But there is an exact correspondence between this phenomenon and that
exhibited by metals under similar conditions. I give here two sets of
records (figs. 80, 81), one obtained with platinum and the other with
tin, which demonstrate how the response is enhanced after continuous
stimulation in a manner exactly similar to that noticed in the case of
nerve.
The explanation which has been suggested with regard to the staircase
effect--increased molecular mobility due to removal of sluggishness by
repeated stimulation--would appear to be applicable in this case also.
It would appear, then, that in all the phenomena which we have studied
under the heads of 'staircase' effect, increase of response after
continuous stimulation, and fatigue, there is a similarity between the
observations made upon the response of muscle and nerve on the one hand,
and that of metals on the other. Even in their abnormalities we have
seen an agreement.
But amongst these phenomena themselves, though at first sight so
diverse, there is some kind of continuity. Calling _all_ normal response
_positive_, for the sake of convenience, we observe its gradual
modification, corresponding to changes in the molecular condition of the
substance.
Beginning with that case in which molecular modification is extreme, we
find a maximum variation of response from the normal, that is to say, to
_negative_.
Continued stimulation, however, brings back the molecular condition to
normal, as evidenced by the progressive lessening of the negative
response, culminating in reversion to the normal _positive_. This is
equally true of nerve and metal.
In the next class of phenomena, the modification of molecular condition
is not so great. It now exhibits itself merely as a relative inertness,
and the responses, though positive, are feeble. Under continued
stimulation, they increase in the same direction as in the last case,
that is to say, from less positive to _more positive_, being the reverse
of fatigue. This is evidenced alike by the staircase effect and by the
increase of response after tetanisation, seen not only in nerve but also
in platinum and tin.
The substance may next be in what we call the normal condition.
Successive uniform stimuli now evoke uniform and equal positive
responses, that is to say, there is no fatigue. But after intense or
long-continued stimulation, the substance is overstrained. The responses
now undergo a change from positive to _less positive_; fatigue, that is
to say, appears.
Again, under very much prolonged stimulation the response may decline to
zero, or even undergo a reversal to _negative_, a phenomenon which we
shall find instanced in the reversed response of retina under the
long-continued stimulus of light.
We must then recognise that a substance may exist in various molecular
conditions, whether due to internal changes or to the action of
stimulus. The responses give us indications of these conditions. A
complete cycle of molecular modifications can be traced, from the
abnormal negative to the normal positive, and then again to negative
seen in reversal under continuous stimulation.
FOOTNOTES:
[16] 'Considering that we have no previous evidence of any chemical or
physical change in tetanised nerve, it seems to me not worth while
pausing to deal with the criticism that it is not CO_2, but "something
else" that has given the result.'--Waller, _Animal Electricity_, p. 59.
That this phenomenon is nevertheless capable of physical explanation
will be shown presently.
[17] In order to explain the phenomena of electric response, some
physiologists assume that the negative response is due to a process of
dissimilation, or breakdown, and the positive to a process of
assimilation, or building up, of the tissue. The modified or positive
response in nerve is thus held to be due to assimilation; after
continuous stimulation, this process is supposed to be transformed into
one of dissimilation, with the attendant negative response.
How arbitrary and unnecessary such assumptions are will become evident,
when the abnormal and normal responses, and their transformation from
one to the other, are found repeated in all details in metals, where
there can be no question of the processes of assimilation or
dissimilation.