EXCITATION IN ANIMAL NERVE.
I shall now take up the question whether an electric current induced
any selective variation of conductivity in the animal nerve, similar to
that induced in the conducting tissue of the plant.
THE METHOD OF EXPERIMENT.
In the experiments which I am about to describe, arrangements were
specially made so that (1) the excitation had not to traverse the polar
region, and (2) the point of stimulation was at a relatively great
distance from either pole. The fulfilment of the latter condition
ensured the point of stimulation being placed at the neutral region.
In the choice of experimental specimens I was fortunate enough to
secure frogs of unusually large size, locally known as “golden frogs”
(_Rana tigrina_). A preparation was made of the spine, the attached
nerve, the muscle and the tendon. The electrodes for constant current
were applied at the extreme ends, on the spine and on the tendon
(Fig. 48). The following are the measurements, in a typical case, of
the different parts of the preparation. Length of spine between the
electrode and the nerve = 40 mm. length of nerve = 90 mm. length of
muscle = 50 mm. length of tendon = 30 mm. Stimulus is applied in all
cases on the nerve, midway between the two electrodes this point being
at a minimum distance of 100 mm. from either electrode. The point of
stimulation is, therefore, situated at an indifferent region.
[Illustration: FIG. 48.--Experimental arrangement for study of
variation of conductivity of nerve by the directive action of an
electric current. _n n′_, nerve; S, point of application of stimulus
in the middle or indifferent region.]
Great precautions have to be taken to guard against the leakage of
current. The general arrangement for the experiment on animal nerve is
similar to that employed for the corresponding investigations on the
plant. The choking coil is used to prevent the stimulating induction
current from getting round the circuit of constant current. The
specimen is held on an ebonite support, and every part of the apparatus
insulated with the utmost care.
VARIATION OF VELOCITY OF TRANSMISSION.
In the case of the conducting tissue of the plant a very striking
proof of the influence of the direction of current on conductivity was
afforded by the induced variation of velocity of transmission. Equally
striking is the result which I have obtained with the nerve of the frog.
[Illustration: FIG. 49.--Effect of heterodromous and homodromous
current in inducing variation in velocity of transmission through
nerve. N, normal record, upper record shows enhancement, and lower
record retardation in velocity of transmission under heterodromous and
homodromous currents, respectively.]
_Experiment 44._--The experiments described below were carried out
during the cold weather. The following records (Fig. 49), obtained by
means of the pendulum myograph, exhibit the effect of the direction
of current on the period of transmission through a given length of
nerve. The latent period of muscle being constant, the variations in
the records exhibit changed rates of conduction. The middle record is
the normal, in the absence of any current. The upper record, denoted
by the left-hand arrow, shows the action of a heterodromous current in
shortening the period of transmission and thus enhancing the velocity
above the normal rate. The lower record, denoted by the right-hand
arrow, exhibits the effect of a homodromous current in retarding the
velocity below the normal rate. I find that a very feeble heterodromous
current is enough to induce a considerable increase of velocity,
which soon reaches a limit. For inducing retardation of velocity, a
relatively strong homodromous current is necessary. I give below a
table showing the results of several experiments.
TABLE V--EFFECT OF HETERODROMOUS AND HOMODROMOUS CURRENT OF FEEBLE
INTENSITY ON VELOCITY OF TRANSMISSION.
+---------+-------------+-------------+------------+-------------+
| |Intensity of | |Intensity of| |
|Specimen.|heterodromous|Acceleration |homodromous |Retardation |
| | current. |above normal.| current. |below normal.|
+---------+-------------+-------------+------------+-------------+
| | microampère | per cent. |microampères| per cent. |
| | | | | |
| 1 | 0.35 | 16 | 1 | 20 |
| 2 | 0.7 | 13 | 1.5 | 19 |
| 3 | 0.8 | 18 | 2.0 | 14 |
| 4 | 0.8 | 11 | 2.0 | 13 |
| 5 | 1.0 | 18 | 2.5 | 12 |
| 6 | 1.5 | 15 | 3.0 | 40 |
+---------+-------------+-------------+------------+-------------+
VARIATION OF INTENSITY OF TRANSMITTED EXCITATION UNDER HETERODROMOUS
AND HOMODROMOUS CURRENTS.
In the next method of investigation, the induced variation of intensity
of transmitted excitation is inferred from the varying amplitude of
response of the terminal muscle. Testing stimulus of sub-maximal
intensity is applied at the middle of the nerve, where the constant
current induces no variation of excitability. Stimulation is effected
either by single break-shock or by the summated effects of a definite
number of equi-alternating shocks, or by chemical stimulation
_Experiment 45._--Under the action of feeble heterodromous current the
transmitted excitation was always enhanced, whatever be the form of
stimulation. This is seen illustrated in Fig. 50. Homodromous current
on the other hand inhibited or blocked excitation (Fig. 51).
[Illustration: FIG. 50.--Ineffectively transmitted salt-tetanus
becoming effective under heterodromous current, denoted by
down-pointing arrow.]
_Complication due to variation of Excitability of Muscle_.--In
experiments with the plant, there was the unusual advantage in having
both the point of stimulation and the responding motile organ in the
middle or indifferent region. Unfortunately this ideally perfect
condition cannot be secured in experiments with the nerve-and-muscle
preparation of the frog. It is true that the point of stimulation in
this case is chosen to lie on the nerve at the middle or indifferent
region. But the responding muscle is at one end, not very distant from
the electrode applied on the tendon. It is, therefore, necessary to
find out by separate experiments any variation of excitability that
might be induced in the muscle by the proximity of either the anode or
the cathode, and make allowance for such variation in interpreting the
results obtained from investigations on variation of conductivity.
In the experimental arrangement employed, the heterodromous current
is obtained by making the electrode on the spine cathode and that on
the tendon anode. The depressing influence of the anode in this case
may be expected to lower, to a certain extent, the normal excitability
of the responding muscle. Conversely, with homodromous current, the
tendon is made the cathode and under its influence the muscle might
have its excitability raised above the normal. These anticipations are
fully supported by results of experiments. Sub-maximal stimulus of
equi-alternating induction shock was directly applied to the muscle
and records taken of (_1_) response under normal condition without any
current, (_2_) response under heterodromous current, the tendon being
the anode, and (_3_) response under homodromous current, the tendon
being now made the cathode. It was thus found that under heterodromous
current the excitability of the muscle was depressed, and under
homodromous current the excitability was enhanced.
The effect of current on response to direct stimulation is thus
opposite to that on response to transmitted excitation, as will be seen
in the following Table.
TABLE VIII.--INFLUENCE OF DIRECTION OF CURRENT ON DIRECT AND
TRANSMITTED EFFECTS OF STIMULATION.
+---------------------+-----------------------+-------------------+
|Direction of current.|Transmitted excitation.|Direct stimulation.|
+---------------------+-----------------------+-------------------+
|Heterodromous current|Enhanced response |Depressed response |
|Homodromous current |Depressed response |Enhanced response |
+---------------------+-----------------------+-------------------+
The passage of a current, therefore, induces opposing effects on the
conductivity of the nerve and the excitability of the muscle, the
resulting response being due to their differential actions. Under
heterodromous current a more intense excitation is transmitted along
the nerve, on account of induced enhancement of conductivity. But
this intense excitation finds the responding muscle in a state of
depressed excitability. In spite of this the resulting response is
enhanced (Fig. 50). The enhancement of conduction under heterodromous
current is, in reality, much greater than is indicated in the record.
Similarly, under homodromous current the depression of conduction in
the nerve may be so great as to cause even an abolition of response, in
spite of the enhanced excitability of the muscle (Fig. 51). The actual
effects of current on conductivity are, thus, far in excess of what are
indicated in the records.
AFTER-EFFECTS OF HETERODROMOUS AND HOMODROMOUS CURRENTS.
On the cessation of a current there is induced in the plant-tissue a
transient conductivity change of opposite sign to that induced by the
direct current (_cf. Expt. 43_). The same I find to be the case as
regards the after-effect of current on conductivity change in animal
nerve. Of this I only give a typical experiment of the direct and
after-effect of homodromous current on salt-tetanus.
[Illustration: FIG. 51.--Direct and after-effect of homodromous
current. Transmitted excitation (salt-tetanus T,) arrested under
homodromous current denoted by up-pointing arrow; on cessation of
current represented by dotted line there is a transient enhancement
above the normal.]
_Experiment 46._--In this experiment sufficient length of time was
allowed to elapse after the application of the salt on the nerve, so
that the muscle, in response to the transmitted excitation, exhibited
an incomplete tetanus T. The homodromous current was next applied,
with the result of inducing a complete block of conduction, with
the concomitant disappearance of tetanus. The homodromous current
was gradually reduced to zero by the appropriate movement of the
potentiometer slide. The after-effect of homodromous current is now
seen in the transient enhancement of transmitted excitation, which
lasted for nearly 40 seconds. After this the normal conductivity was
restored. Repetition of the experiment gave similar results (Fig. 51).
The results that have been given are only typical of a very large
number, which invariably supported the characteristic phenomena that
have been described.
It will thus be seen that with feeble or moderate current, conductivity
is enhanced against the direction of the current and depressed or
blocked with the direction of the current. Under strong current the
normal effect is liable to undergo a reversal.
It has thus been shown that a perfect parallelism exists in the
conductivity variation induced in the plant and in the animal by the
directive action of the current. No explanation could be regarded as
satisfactory which is not applicable to both cases. Now with the plant
we are able to arrange the experimental condition in such a way that
the factor of variation of excitability is completely eliminated. The
various effects described about the plant-tissue are, therefore, due
entirely to variation of conductivity. The parallel phenomena observed
in the case of transmission of excitation in the animal nerve must,
therefore, be due to the induced change of conductivity.
The action of an electrical current in inducing variation of
conductivity may be enunciated under the following laws, which are
equally applicable to the conducting tissue of the plant and the nerve
of the animal:--
LAWS OF VARIATION OF NERVOUS CONDUCTION UNDER THE ACTION OF ELECTRIC
CURRENTS.
1. The passage of a current induces a variation of conductivity,
the effect depending on the direction and intensity of current.
2. Under feeble intensity, heterodromous current enhances, and
homodromous current depresses, the conduction of excitation.
3. The after-effect of a feeble current is a transient conductivity
variation, the sign of which is opposite that induced during the
continuation of current.
SUMMARY.
The variation of conductivity induced by the directive action of
current has been investigated by two different methods:--
(1) The method in which the normal speed and its induced variation
are automatically recorded;
(2) That in which the variation in the intensity of transmitted
excitations is gauged by the varying amplitudes of resulting
responses.
The great difficulty arising from leakage of the exciting induction
current into the polarising circuit was successfully overcome by the
interposition of a choking coil.
The following summarises the effects of direction and intensity of an
electric current, on transmission of excitation through the conducting
tissue of the plant.
The velocity of transmission is enhanced against the direction of a
feeble current, and retarded in the direction of the current.
Feeble heterodromous current enhances conductivity, homodromous
current, on the other hand, depresses it.
Ineffectively transmitted excitation becomes effectively transmitted
under heterodromous current. Effectively transmitted excitation, on
the other hand, becomes ineffectively transmitted under the action of
homodromous current.
The after-effect of a current is a transient conductivity change,
the sign of which is opposite to that induced during the passage
of current. The after-effect of a heterodromous current is, thus,
a transient depression, that of homodromous current, a transient
enhancement of conductivity.
The characteristic variations of conductivity induced in animal nerve
by the direction and intensity of current are in every way similar to
those induced in the conducting tissue of the plant.
These various effects are demonstrated by the employment of not
one, but various kinds of testing stimulus, such as the excitation
caused (1) by a single break-induction shock or (2) by a series of
equi-alternating tetanising shocks or (3) by chemical stimulation.
VIII.--EFFECT OF INDIRECT STIMULUS ON PULVINATED ORGANS
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS, L.M.S.
The leaf of _Mimosa pudica_ undergoes an almost instantaneous fall
when the stimulus is applied directly on the pulvinus which is the
responding organ. The latent period, _i.e._, the interval between the
application of stimulus and the resulting response is about 0.1 second.
Indirect stimulus, _i.e._, application of stimulus at a distance from
the pulvinus, also causes a fall of the leaf; but a longer interval
will elapse between the incidence of stimulus and the response; for it
will take a definite time for the excitation to be conducted through
the intervening tissue. I have already shown that this conduction of
excitation in plant is analogous to the transmission of nervous impulse
in animal.
The power of conduction varies widely in different plants. In the
petiole of _Mimosa pudica_ the velocity may be as high as 30 mm. per
second. In the stem the velocity is considerably less, _i.e._, about
6 mm. per second in the longitudinal direction; but conduction across
the stem is a very much slower process. In the petiole of _Averrhoa_
the longitudinal velocity is of the order of 1 mm. per second.
DUAL CHARACTER OF THE TRANSMITTED IMPULSE.
The record of the transmitted effect of stimulus is found to exhibit
a remarkable preliminary variation. This was detected by my delicate
recorders, which gave magnifications from fifty to hundred times. I
shall give a detailed account of a typical experiment carried out with
_Averrhoa carambola_, which will bring out clearly the characteristic
effects of Indirect Stimulus.
[Illustration: FIG. 52.--Effect of indirect Stimulus on leaflet of
_Averrhoa carambola_. Stimulus was applied at the short vertical
line. Successive dots at intervals of one second. Note the _positive_
response preceding the _negative_.]
_Experiment 47._--Stimulus of electric shock applied at a point on the
long petiole of _Averrhoa_ causes successive fall of pairs of leaflets.
In the experiment to be described one of the leaflets of the plant
was attached to the recorder. Stimulus was applied at a distance of
50 mm. The successive dots in the record are at intervals of a second.
It will be noticed that two distinct impulses--a _positive_ and a
_negative_--were generated by the action of Indirect Stimulus. The
positive impulse reached the responding organ after 1.5 second and
caused an erectile movement. The velocity of the positive impulse in
the present case is 33 mm. per second. The normal excitatory negative
impulse reached the motile organ 44 seconds after the application of
stimulus, and caused a very rapid fall of the leaflet, the fall being
far more pronounced than the positive movement of erection (Fig. 52).
In this and in all subsequent records, the positive and negative
responses offer a great contrast. The movement in response to positive
reaction is slow, whereas that due to negative reaction is very abrupt,
almost ‘explosive,’ the successive dots being now very wide apart. As
regards the velocity of impulse the relation is reversed, the positive
being the quicker of the two. In the present case, the velocity of the
excitatory _negative_ impulse is 1.1 mm. per second, as against 33 mm.
of the _positive_ impulse.
The negative impulse is due to the comparatively slow propagation of
the excitatory protoplasmic change, which brings about a diminution
of turgor in the pulvinus and fall of the responding leaflet. The
erectile movement of the leaflet by the positive impulse must be due
to an increase of turgor, brought on evidently, by the forcing in of
water. This presupposes a forcing out of water somewhere else, probably
at the point of application of stimulus. It may be supposed that an
active contraction occurred in plant cells under direct stimulus, in
consequence of which water was forced out giving rise to a hydraulic
wave. On this supposition the positive impulse is to be regarded as
hydro-mechanical. I have, however, not yet been able to devise a
direct experimental test to settle the question.
EFFECT OF DISTANCE OF APPLICATION OF STIMULUS.
In the last experiment the stimulus was applied at the moderate
distance of 50 mm. Let us now consider the respective effects, first,
of an increase, and second, of a decrease of the intervening distance.
In a tissue whose conducting power is not great, the excitatory
impulse is weakened, even to extinction in transmission through a long
distance. Thus the negative impulse may fail to reach the responding
organ, when the stimulus is feeble or the intervening distance long or
semi-conducting. Hence, under the above conditions, stimulus applied at
a distance will give rise only to a positive response.
A reduction of the intervening distance will give rise to a different
result. As the negative response is the more intense of the two, the
feeble positive will be masked by the superposed negative. The separate
exhibition of the two responses is only possible by a sufficient lag
of the negative impulse behind the positive. This lag increases with
increase of length of transmission and decreases with the diminution of
the length. Hence the application of stimulus near the responding organ
will give rise only to a negative response, in spite of the presence of
the positive, which becomes masked by the predominant negative.[P]
[P] _Cf._ BOSE--“Plant Response,” p. 535; “Comparative
Electro-Physiology,” p. 64; “Irritability of Plants,” p. 196.
These inferences have been fully borne out by results of experiments
carried out with various specimens of plants under the action of
diverse forms of stimuli. In all cases, application of stimulus at
a distance causes a pure positive response; moderate reduction of
the distance induces a diphasic response--a positive followed by a
negative; further diminution of distance gives rise to a resultant
negative response, the positive being masked by the predominant
negative.
From what has been said it will be understood that the exhibition of
positive response is favoured by the conditions, that the transmitting
tissue should be semi-conducting, and the stimulus feeble. It is thus
easier to exhibit the positive effect with the feebly conducting
petiole of _Averrhoa_ than with the better conducting petiole of
_Mimosa_. It is, however, possible to obtain positive response in the
_Mimosa_ by application of indirect stimulus to the stem in which
conduction is less rapid than in the petioles.
TABLE IX.--PERIODS OF TRANSMISSION OF POSITIVE AND NEGATIVE IMPULSES IN
THE PETIOLE OF _AVERRHOA_ AND STEM OF _MIMOSA_.
+---+----------+-----------+---------------+------------+------------+
| | | | |Transmission|Transmission|
|No.| Specimen |Distance in| Stimulus |period for |period for |
| | | mm. | |positive |negative |
| | | | |impulse. |impulse. |
+---+----------+-----------+---------------+------------+------------+
| 1 |_Averrhoa_| 70 |Thermal | 22 secs | 65 secs. |
| 2 | " | 130 | " | 40 " | 95 " |
| 3 | " | 10 |Induction-shock| 6 " | 20 " |
| 4 | " | 20 | " | 14 " | 48 " |
| 5 | " | 35 |Chemical | 21 " | 50 " |
| 6 | _Mimosa_ | 5 |Induction-shock| 0.5 " | 12 " |
| 7 | " | 10 | " | 0.6 " | 9.4 " |
| 8 | " | 20 | " | 1.1 " | 10 " |
| 9 | " | 60 | " | 2 " | 29 " |
|10 | " | 35 |Chemical | 5 " | 17 " |
+---+----------+-----------+---------------+------------+------------+
EFFECTS OF DIRECT AND INDIRECT STIMULUS.
From the results given in course of the Paper we are able to formulate
the following laws about the effects of Direct and Indirect Stimulus on
pulvinated organs:--
1. Effect of all forms of Direct stimulus is a diminution of
turgor, a contraction and a negative mechanical response.
2. Effect of Indirect stimulus is an increase of turgor, an
expansion and a positive mechanical response.
3. Prolonged application of indirect stimulus of moderate intensity
gives rise to a diphasic, positive mechanical response followed
by the negative.
4. If the intervening tissue be highly conducting, the transmitted
positive effect becomes masked by the predominant negative.
The laws of Effects of Direct and Indirect stimulus hold good not
merely in the case of sensitive plants, but universally for all plants.
This aspect of the subject will be treated in fuller detail in later
Papers of this series.
IX.--MODIFYING INFLUENCE OF TONIC CONDITION ON RESPONSE
_By_
SIR J. C. BOSE
_Assisted by_
GURUPRASANNA DAS.
In experiments with different pulvinated organs, great difference is
noticed as regards their excitability. If electric shock of increasing
intensity from a secondary coil be passed through the pulvini of
_Mimosa_, _Neptunia_, and _Erythrina_ arranged in series, it would be
found that _Mimosa_ would be the first to respond; a nearer approach
of the secondary coil to the primary would be necessary for _Neptunia_
to show sign of excitation. _Erythrina_ would require a far greater
intensity of electric shock to induce excitatory movement. Organs of
different plants may thus be arranged, according to their excitability,
in a vertical series, the one at the top being the most excitable.
The specific excitability of a given organ is different in different
species.
In addition to this characteristic difference, an identical organ may,
on account of favourable or unfavourable conditions, exhibit wide
variation in excitability. Thus under favourable conditions of light,
warmth and other factors, the excitability of an organ is greatly
enhanced. In the absence of these favourable tonic conditions the
excitability is depressed or even abolished. I shall, for convenience,
distinguish the different tonic conditions of the plant as _normal_,
_hyper-tonic_ and _sub-tonic_. In the first case, stimulus of moderate
intensity will induce excitation; in the second, the excitability being
exceptionally high, very feeble stimulus will be found to precipitate
excitatory reaction. But a tissue in a _sub-tonic_ condition will
require a very strong stimulus to bring about excitation. The
excitability of an organ is thus determined by two factors: the
specific excitability, and the tonic condition of the tissue.
THEORY OF ASSIMILATION AND DISSIMILATION.
A muscle contracts under stimulus; this is assumed to be due to some
explosive chemical change which leaves the tissue in a condition less
capable of functioning, or in a condition below par. Herring designates
this as a process of _dissimilation_. The excitability of the muscle
is restored after suitable periods of rest, by the opposite metabolic
change of _assimilation_. “Assimilation and Dissimilation must be
conceived as two closely interwoven processes, which constitute the
metabolism (unknown to us in its intrinsic nature) of the living
substance. Excitability diminishes in proportion with the duration of
D-stimulus, or, as it is usually expressed, the substance _fatigues_
itself. It is perfectly intelligible that a progressive fatigue and
decrement of the magnitude of contraction must ensue. The only point
that is difficult to elucidate is the initial staircase increment of
the twitches, more especially in excised, bloodless muscle, which seems
in direct contradiction with the previous theory.”[Q]
[Q] BIEDERMANN--Electro-Physiology (English Translation), Vol 1, pp.
83, 84, 85; Macmillan & Co.
With reference to Herring’s theory given above, Bayliss in his
“Principles of General Physiology” (1915), page 377 says, “In the
phenomenon of metabolism, two processes must be distinguished, the
building up of a complex system or substance of high potential energy,
‘anabolism,’ and the breaking down of such a system, ‘catabolism,’
giving off energy in other forms. The tendency of much recent work,
however, is to throw doubt on the universality of this opposition of
anabolism and catabolism as explanatory of physiological activity in
general.”
The results obtained with the response of plants to stimulus may
perhaps throw some light on the obscurities that surround the subject.
They show that the two processes may be present simultaneously, and
that the ‘down’ change induced by stimulus may, in certain instances,
be more than compensated by the ‘up’ change.[R] I shall, for
convenience, designate the physico-chemical modification, associated
with the excitatory negative mechanical and electrical response of
plants, as the “D” change; this is attended by run down of energy. The
positive mechanical and electrical response must therefore connote
opposite physico-chemical change, with increase of potential energy.
This I shall designate as the “A” change, which by increasing the
latent energy, enhances the functional activity of the tissue. That
stimulus may give rise simultaneously to both A, and D, effects, finds
strong support in the dual reactions exhibited in plant-response. Under
indirect stimulus, the two responses are seen separately, the more
intense negative following the feeble positive. When by the reduction
of the intervening distance, stimulus is made direct, the resultant
response, as previously stated, is negative; and this is due not to
the total absence of the positive but to its being masked by the
predominant negative. Let us next consider the question of unmasking
this positive element in the resultant negative response.
[R] In the response of inorganic matter I have obtained records
of positive, diphasic and negative responses. It would perhaps be
advisable to refer the ‘A’ and ‘D’ effects, to physico-chemical change.
The simultaneous double reaction, combination and decomposition, is of
frequent occurrence in many chemical changes.
UNMASKING OF THE POSITIVE EFFECT.
Under favourable conditions of the environment, the excitability of
the organs is at its maximum. A given stimulus will bring about an
intense excitation, and the ‘down’ D-change will therefore be very much
greater than the A-change. Let us now consider the case at the opposite
extreme where, owing to unfavourable condition, the excitability is
at its lowest. Under stimulus the excitatory D-change will now be
relatively feeble compared to the A-change, by which the potential
energy of the system becomes increased. In such a case successive
stimuli will increase the functional activity of the tissue, and bring
about staircase response. Biedermann mentions the staircase response of
_excised bloodless muscle_ as offering difficulty of explanation. It
is obvious that the physiological condition of the excised muscle must
have fallen below par. The staircase response in such a tissue is thus
explained from considerations that have just been adduced.
The results obtained with _Mimosa_ not only corroborate them, but add
incontestable proof of the simultaneous existence of both A and D
changes. The physiological condition of a plant, _Mimosa_ for example,
is greatly modified by the favourable or unfavourable condition of
the environment. In a hyper-tonic condition its excitability becomes
very great; in this condition the plant responds to its maximum even
under very feeble stimulus. Here the D-change is relatively great, and
successive responses are apt to show sign of fatigue.
[Illustration: FIG. 53.--Record showing the effect of stimulus
modifying tonicity and producing staircase effect. (_Mimosa_)]
But the plant in a sub-tonic condition will exhibit feeble or no
excitation. The D-change will be absent while the A-change will take
place under the action of stimulus. This, by increasing the potential
energy, will enhance the functional activity of the tissue.
_Staircase response in Mimosa: Experiment 48._--The theoretical
considerations will be found experimentally verified in the record
obtained with a specimen of _Mimosa_ in a sub-tonic condition
(Fig. 53). Owing to the lack of favourable ‘tone’ the leaf was relaxing
as seen in the first part of the curve. The stimulus of electric shock,
applied at the thick dot in the curve slanting downwards, gave no
response but raised the tone of the tissue by arresting the growing
relaxation. Subsequent stimuli gave rise to staircase responses.
Stimulus has, through the A-effect, raised the functional activity of
the tissue to a maximum.
ARTIFICIAL DEPRESSION OF TONIC CONDITION AND MODIFICATION OF RESPONSE.
It has been shown that while favourable tonic condition has the effect
of raising the excitability and enhancing the negative response with
the associated D-change, a condition of sub-tonicity, on the other
hand, induces depression of excitability, a diminution of negative
response and of the attendant D-change. In this condition the positive
element in the response with the A-change will come into greater
prominence. These considerations led me to experiment with specimens
exhibiting increasing sub-tonicity, with a view of unmasking the
positive element in the response, _i.e._, the A-change. In the last
experiment a specimen was found which happened to be in a sub-tonic
condition on account of the unfavourable condition of its surroundings.
I was next desirous of securing specimens in which I could induce
increasing sub-tonicity at will.
I have shown (_Expt. 23_) that a detached branch of _Mimosa_ can be
kept alive for several days with the cut end immersed in water. In this
condition the pulvinus retains its sensitiveness for more than two
days. The excitability undergoes a continuous decline and is abolished
about the fiftieth hour. Isolation from the parent organism thus causes
a continuous depression of the tonic condition of the specimen. The
case is somewhat analogous to the depression of excitability in an
excised bloodless muscle. It is thus possible to secure specimens of
varying degrees of sub-tonicity. A specimen that has been detached for
six hours will exhibit a slight amount of depression, while a different
specimen isolated for twenty-four hours will occupy a very much lower
position in the scale of tonicity.
_Experiment 49._--The staircase response of _Mimosa_ given in figure
53 was obtained with the stimulus of induction shock. In order to
establish a wider generalisation I now used the stimulus of light given
by an arc lamp. There may be a difficulty on account of the diurnal
movement of _Mimosa_; the leaf, generally speaking, has a movement in
a downward direction from morning till noon, after which there is a
comparative state of rest. It is better to choose the time of noon for
experiment. In any case the response to stimulus is very abrupt and in
strong contrast with the slow diurnal movement. A horizontal pencil of
light was thrown upwards by means of a small mirror and made to fall
on the lower half of a pulvinus of the _Mimosa_ leaf. The excitatory
down movement is followed by recovery on the cessation of light. The
intensity of stimulus can be modified by varying the intensity of
light. I took for my first series of experiments a specimen that had
been isolated for six hours. Stimulation was caused by successive
applications of light for 25 seconds at intervals of 3 minutes.
Figure 54 shows how the functional activity of the sub-tonic specimen
is enhanced by stimulus, the successive responses thus exhibiting the
staircase effect.
[Illustration: FIG. 54.--Staircase response in sub-tonic _Mimosa_.]
[Illustration: FIG. 55.--Positive, diphasic and negative response under
successive stimulation.]
POSITIVE RESPONSE IN SUB-TONIC SPECIMEN.
_Experiment 50._--A still lower degree of sub-tonicity was ensured by
keeping the specimen in an isolated condition for 12 hours. Stimulus of
light for 20 seconds’ duration was applied at intervals of 2 minutes.
In the record (Fig. 55) the first two responses, not shown, were purely
positive. The third exhibited a positive A-effect, followed by the
negative response D-effect. The A-effect is thus seen fully unmasked.
In subsequent responses the A-effect became more and more overshadowed
by the D-effect. At the third response the masking is complete and the
excitatory negative response is at its maximum. The record of staircase
effect (Fig. 54) also exhibits a preliminary positive twitch at the
beginning of the series, which disappeared after the second response.
The modifying influence of tonic condition on response I find to be
of universal occurrence. In vigorous specimens the electric response
to stimulation is _negative_; but tissues in sub-tonic condition
give _positive_ response and after long-continued stimulation the
abnormal positive is converted into the normal _negative_. It is very
interesting that under condition of sub-tonicity diverse expressions
of physiological reaction exhibit similar change of sign of normal
response. Thus in my measurement of the velocity of transmission of
excitation in the conducting tissue of _Mimosa_, I find that, when
the tissue is in an optimum condition, exhibiting high velocity of
transmission, excessive stimulus has the effect of diminishing the
conducting power. But in a depressed condition of the tissue the
effect is precisely the opposite. Thus in a given case the velocity of
transmission was low; strong electric stimulation enhanced the rate by
33 per cent. In extreme cases of sub-tonicity, where the conducting
power was in abeyance, the excessive stimulus caused by wound not
only restored the power of conduction but raised the velocity of
transmission to 25 mm. per second (_Expt. 37_).
SUMMARY.
The excitability of a plant is found to be modified by its tonic
condition.
A sub-tonic specimen of _Mimosa_, like an excised bloodless muscle,
shows a preliminary staircase response. Stimulus induces simultaneously
both “A” and “D” effects, with their attendant positive and negative
reactions.
A tissue in optimum condition exhibits only the resultant negative
response, the comparatively feeble positive being masked by the
predominant negative. With decline of tone, the “D” effect diminishes
and we get “A” effect unmasked.
In extreme sub-tonic specimen, we get first only the “A” effect,
with its positive response. Successive stimulation converts the pure
positive into diphasic and ultimately into normal negative response.