Thomas Gregor Brodie.

The essentials of experimental physiology, for the use of students online

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must be greater for both cases. This tends to confirm the previous
result, viz. that it is muscle fibre which is being stimulated, and not
nerve fibre, and in that case shows that MUSCLE IS LESS EXCIT-
ABLE THAN NERVE.

Experiment 3. — Xuhne's Sartorius Experiment. — Carefully dissect
out a sartorius, and to the tendon which attaches it to the tibia tie a fine
thread. The upper end of the muscle may be freed from its attachment
to the symphysis. Suspend the muscle with its upper end hanging down-
wards, and bring up under it a drop of glycerine in a watch glass until the
end of the muscle just touches the glycerine. No contraction results. Cut
off the end which has touched the glycerine, and note that the muscle con-
tracts under the mechanical excitation. Again touch the cut surface with
glycerine. If only about 1 mm. of the end has been cut off there is a^ain
no contraction. Cut off a fresh miUimetre of muscle and repeat as before.
It will be found that when about 3 to 4 mm. of the upper (pelvic) end has
been cut away, on contact of the freshly exposed end with the glycerine, the
muscle shows irregular twitchings, and is at last thrown into a state of incon-
plete tetanus. This experiment should in the next place be supplemented by
showing that if a gastrocnemius nerve muscle preparation be made, and the
cut end of the nerve dipped into glycerine, the gastrocnemius is thrown
into a similar series of irregular twitchings. Nerve fibre is therefore excit-
able to glycerine.

The explanation of the preceding experiment becomes clear by the



78 EXPERIMENTAL PHYSIOLOGY

light of a knowledge of the distribution of the nerve fibres in the
sartorius. Kuhne showed that the upper 4 to 5 mm. of the
sartorius contained no nerve fibres, nor could nerve-endings of any
kind be traced in this part. The same holds good for the lower 2
to 3 mm. Hence in the first part of the experiment only muscle
fibre was being exposed to the glycerine ; and as the muscle did not
contract, it follows that muscle fibre is not excitable to glycerine. As
soon, however, as the lower part had been cut away, some of the nerve
fibres became exposed, and as they are excitable to glycerine the
muscle was thrown into a series of irregular twitchings.

'Experiment 4. — Upon the sartorius of the opposite limb perform an exactly
similar experiment, but use a salt solution (0*65 per cent.) containing a drop
of ammonia solution. The muscle will be found to be extremely sensitive to
this, even the vapour of NH 3 from it being quite sufficient to throw it into
tetanus. Nerve, on the other hand, is not excited by it. Prove this by dip-
ping the freshly cut end of the sciatic nerve of a gastrocnemius preparation
into the solution, taking care that the muscle is thoroughly protected from
the vapour by folds of blotting-paper moistened with normal saline solution.
It is best, too, to hold the watch-glass of ammonia solution above the level of
the muscle, and to lift up the nerve by a fine glass rod bent into the form of a
hook, and thus dip the cut end of the nerve into the solution. No contrac-
tion of the gastrocnemius residts, but the nerve is not unaffected, for it will be
found that the part which has been exposed to the fluid, if tested by electrical
stimuli, has been killed.

Experiment 5. — Xiihne's Curare Experiment. — Pin down two strips of
copper foil upon a flat plate of cork with their ends about 4 cm. apart, and join
them by a strip of blotting-paper moistened with 0*65 per cent. NaCl solution.
Connect the copper strips to the secondary coil of an inductorium arranged for
tetanising. Prepare a sartorius and cut it transversely into five pieces of equal
length, and arrange these in series upon the strip of moist blotting-paper.
Starting with the secondary coil at some distance from the primary, send teta-
nising shocks through the preparation, gradually increasing the strength of the
stimulation until at last one is found which causes the three middle pieces of
the sartorius to contract while the upper and lower ends remain quiescent.
Increase the strength of the stimulation still further, when the two terminal
pieces are also thrown into contraction. If a curarised sartorius be employed
all five pieces go into tetanus at once, viz. when the stronger stimulus which
was required to tetanise the two terminal pieces in the first experiment is
reached.

The difference in behaviour of the five pieces is due to the fact
that the two ends contain no nerve terminals, while the three
centre-pieces do, and as was seen from a previous experiment (ex-
periment 2) muscle is less excitable than nerve.

In addition to these experiments other facts are known tending
to show the inherent excitability of muscle. Nerve, for instance, is
not excited by stimuli which are arranged at right angles to the
direction of the fibres, it being necessary that the stimulus or part of
it should pass in the same direction as that of the fibres. Muscle, on



INDEPENDENT MUSCULAR EXCITABILITY 79

the contrary, is quite as excitable to stimuli in a direction transverse
to the muscle fibres as to one parallel to them. Nerve again is
especially sensitive to currents of very short duration, whereas muscle
will not respond unless the duration be sufficiently prolonged. A
curarised muscle is much less sensitive to shocks of short duration
than a non-curarised one.

In a further direction Kuhne showed that chemical stimuli which
were particularly irritant to muscle (such as NH 3 or very weak HC1)
were equally effective to both curarised and non-curarised muscles.
If a strong constant current be sent through a nerve, it is found that
the excitability of the nerve in the part immediately surrounding the
anode is very greatly diminished. Kuhne utilised this to lower the
excitability of the nerve fibres in a sartorius by throwing a constant
current into its nerve, placing the anode close to the muscle. A muscle
thus treated was found to be just as excitable to ammonia or weak
hydrochloric acid, whilst those forms of stimuli, such as glycerine,
which act on nerve only, are now without effect, or only produce one
when the excitation becomes excessive.

If the nerve supplying a muscle be cut and allowed to degenerate
for some days, the response of the muscle to electrical stimuli becomes
considerably modified ; while the intra-muscular nerve endings are
intact the muscle responds more readily to induced shocks than to
the constant current, whereas when these terminals have degenerated
the reverse is found to be the case. This change of condition is
explicable on the fact already tested in experiment 2, which shows
that muscle is much less excitable to currents of short duration than
nerve.



EXCITATION OF MUSCLE BY A CONSTANT CURRENT

In our experiments up to this point, we have as a rule employed
an induced shock whenever we wished to stimulate a muscle or its
nerve, but we have also seen that a muscle is excitable to thermal,
mechanical, and chemical stimuli as well as to electrical. We have
now to consider the response of muscle to electrical stimuli other than
induced currents. We found that muscle was less excitable to induced
currents than nerve, and this is found to be due to the very short
duration of these currents.

If a constant current of sufficient strength be sent through a muscle
a contraction occurs at the instant of make of the current and again
at the instant of break, but no effect is as a rule produced during the
passage of the current. These two contractions are different, not only



80



EXPERIMENTAL PHYSIOLOGY



in amount, but in that they start from different points. Calling the
electrode at which the current enters the muscle the anode, and that
at which it leaves, the kathode, it is found that on make of the current
the contraction starts from the kathode and thence spreads over the
muscle, but that on break the contraction starts from the anode.
This very important point in the response of muscle to an electrical
current can be shown by the following experiments : —



Experiment 6. — Dissect out a sartorius and fix it to record its move-
ments as shown in fig. 71. The muscle is lightly clipped between two
pieces of cork c and d at a point near its tibial end. A fine thread is tied
round the tendon at that end and attached to a crank lever, l. Two pins are
passed through the corks c d, and by these the muscle is fixed to the myo-
graph plate. The remaining longer piece of the muscle is connected to two
electrodes (unpolarisable, see p. 83) a and b, which are connected to a con-
stant current through a Pohl's commutator with cross wires, so that the
direction of the current may be reversed. The muscle is clamped so tightly
by the corks that it prevents any movement at a or b pulling on the piece of
sartorius s to the left of the cork clamp, and so moving the lever. It is not,
however, so tightly clamped as to prevent a wave of contraction passing
across from the piece s 1 to the piece s. If now a contraction start at one
instant from a, and passes as a wave along the muscle to s, a longer interval
will elapse before s begins to contract and raises the lever than if the con-
traction started at b and had only a short piece of muscle to travel along

before it reached s. The
experiment now consists
in measuring the latent
periods of four curves :
two, one at make, the
other at break of a con-
stant current when the
current passes from a to
b, i.e. when the anode is
at a, and two when the
current travels from b to
a, i.e. when the anode is
at b. To record the in-
stants of opening and closing the current a signal included in the primary
circuit is arranged to record its movements directly beneath the myograph
lever. This does not give us an absolutely accurate measurement of the
latent period because there is a latency in the signal ; but as this is the same
in all four cases this does not matter, for we only require to measure differ-
ences in the latent period.




Fig. 71.— Method of studying Polar Excitation of
a Muscle.



It will be found that make of the ascending current (anode at b)
and break of the descending current (anode at a) have practically the
same latent period, and both are greater than those on break of the
ascending current or make of the descending current, which in their
turn are practically equal in value.

Hence it is argued that the contraction on make of an ascending



POLARISATION 81

current starts from a, and of a descending current from b, in both
cases the kathode ; and, on the other hand, that the contraction on
break starts from B with an ascending, and from a with a descending,
current, these two points being the anodes in the two cases. The
differences are more clearly seen if the muscle be fatigued.

Experiment 7. — Engelmann's experiment. Curarise a frog, dissect out
its sartorius, and suspend it by its pelvic end. Arrange two electrodes to
send a current transversely across this end. On closing the current the free
end moves towards the kathodic side of the muscle, on opening towards the
anodic.

Experiment 8. — Prepare a sartorius from a curarised frog that has been
previously kept at a low temperature for some hours. Place it on a pair of
unpolarisable electrodes. On closing the current the rmiscle passes into
tetanus, and if the muscle be carefully observed, it will be seen that the only
part in persistent contraction is that around the kathode. On opening, the
muscle also commonly passes into tetanus, but in this case the contraction
is limited to the region of the anode. This experiment is all the more
striking if Biedermann's plan, of striping the sartorius transversely with black
lines made by a bristle dipped in china ink, be adopted. The region in
contraction is then clearly marked by the approximation of the black lines.
The non-contracted part is seen to be stretched out, either by an actual
stretching due to the contracted part pulling on it, or due to an active
relaxation in that region. The latter is probably the main cause, as is well
exemplified by the following experiment.

Experiment 9. — A frog is pithed and its heart exposed. One electrode
is now placed on any part of the body, and the point of the other, which
should be a fine wire, is applied to the heart. If the electrode on the heart
be the anode it will be seen that at each contraction the part around the point
touched does not pale like the rest of the heart, i.e. that region does not con-
tract but is inhibited. Conversely, if it be the kathode, it is seen that that
spot remains pale during relaxation of the heart, which means that those
fibres immediately affected do not relax properly. This latter point is not so
easy to make out as the former.

Polarisation of Electrodes. — If a pair of clean platinum wires be
immersed in water, and a current sent through them for a time, it is
found that both of the platinum terminals become covered with bubbles
of gas. That one in connection with the negative pole of the battery-
is covered with hydrogen, and the other with oxygen. If now the
battery be removed and the two electrodes connected to the two
terminals of a galvanometer, it is found that a current is shown by
the galvanometer, which has a direction, in the galvanometer circuit,
from the electrode covered with oxygen to that covered with hydrogen.
It is, in other words, in the reverse direction to that of the initially
employed current. The production of this state at the electrodes is
spoken of as polarisation of electrodes. The same usually occurs,
though to different degrees, if solutions of salts be tested instead of
distilled water, and no matter what metal the electrodes are made of.
Eegnault discovered, however, that if the electrodes were made of

a



82 EXPERIMENTAL PHYSIOLOGY

pure zinc, and the solution in which they were immersed was a strong
solution of zinc sulphate, that no polarisation occurred ; and the same
was found to be the case with less purified zinc if its surfaces were
thoroughly amalgamated.

If instead of a solution a piece of fresh animal tissue connects a.
pair of wire electrodes the same polarisation occurs. Chemical changes
are set up where the wires touch the tissue which can act in the
reverse manner, and set up a small current if the battery be removed
and the electrodes connected by a conductor. This acts as a source
of fallacy in many experiments, and becomes of great importance
where we are dealing with a very excitable tissue, such as nerve.
The existence of this polarisation current may be proved, as in the
previous example, by sending it through a galvanometer ; but we are-
also able to show it by its effect in exciting a nerve, as in the following
arrangement : —

Experiment 10. — Arrange the apparatus as shown in fig. 72, open the kejr
K, and close the key k,. The current is thus sent through the nerve by the
electrodes e, which it will polarise. Note that there is no contraction during




Fig. 72. — Method of Arranging the Apparatus to Show-
Polarisation of Electrodes.

the time the current is passing. In about a minute open k ; and then rapidly
close and open K 2 , when contractions will occur which are due to the closure
and opening of the small current set up by the polarised electrodes. The-
contractions rapidly diminish in amount as the nerve becomes depolarised.

This experiment illustrates the necessity of avoiding this polarisa-
tion, if it be possible, when we are experimenting upon nerves or other
excitable tissues, and Du Bois-Eeymond utilised Eegnault's discovery
for making electrodes which would not show this defect. In his
original form (fig. 73, 1) each electrode consisted of a zinc trough on an
insulating vulcanite base, the outer surface being thoroughly varnished
and the inner well amalgamated. Into this a thick pad of filter
paper thoroughly soaked in a saturated solution of zinc sulphate is
fitted, and the part of the pad lying in the trough is then covered
with the saturated zinc sulphate solution. If, now, the piece of tissue
be placed between two such pads the zinc salt rapidly produces
corrosive effects, and the tissue is rapidly destroyed. To prevent
this, little masses of china clay worked up into a stiff paste with
normal saline solution are used, upon which to rest the tissue and



UNPOLARISABLE ELECTRODES



83



connect it with the electrodes. These do not cause any polarisation,
and at the same time are fairly good conductors. They are spoken
of as clay -guards. In many cases this form of electrode is too large,
and it becomes impossible to bring them into contact with any two




-Several Models of Unpolaeisable Electrodes

1 and 2, Dn Bois-Reymond's ; 3, Burdon-Sanderson's ; 4, von Pleischl's ; 5, d'Arsonval's.

In 1, 2, 3, and 4 the component parts are zinc, zinc sulphate, and saline clay ; in 5 a silver rod
coated with fused silver chloride dipping in normal saline contained in the tube from which a thread
projects. (Waller.)

desired points of the tissue to be experimented upon ; and to over-
come this many forms of electrode have been employed by different
workers. Some of these are shown in the accompanying figure (73).

Make a pair of unpolarisable electrodes in the following way. Take a piece
of glass tubing of fairly thick walls and with an internal diameter of about
\ inch. Rotate this with its centre in a blowpipe flame without draw-
ing it out until the central part becomes of a less diameter ; then draw it
out slightly and allow to cool. Cut it
through the centre of the constricted
part and round off these ends in a
flame. The bore at this end should
now be about ^ inch. Cut the tubes
so that they are 2^ inches long and
round off the upper ends in a flame.
This glass tube is shown at a, fig. 74.
Take a cork and bore half through it
with a cork-borer of such size that the
upper end of the glass tube is held firmly
in the hole bored. With a fine bradawl pierce the cork from tbe upper end, and
this will remove the lower bored piece of cork if that has not already come away

g2




Fig. 74. — Simple Form ok Unpolaris-
able Electrodes.



84 EXPERIMENTAL PHYSIOLOGY

inside the borer. Solder an insulated wire to one end of a straight piece of pnre
zinc wire of about three inches length, and then thoroughly amalgamate the
zinc wire by dipping it into the amalgamating fluid and rubbing it on a
clean duster. Now pass the zinc wire through the hole in the cork made by the
bradawl until it projects well on the lower side. Next take some powdered
kaolin and make it into a stiff clay with normal saline, and force a little of this
up the constricted end of the glass tube to act as a plug to close that orifice.
Then fill the tube with saturated zinc sulphate solution, and fit it into the
large hole in the cork, so that the zinc wire dips into the solution. All that is
now required to complete the electrode is to fix to the lower end a plug of
china clay whose apex may be moulded to any desired shape (see fig. 74).
In many cases it will be found convenient to fix in the centre of this china-
clay guard a coarse thread soaked in normal saline, which can then act as
the terminal part of the electrode. These electrodes can be kept, and all
that they will require at a future time is a fresh guard of clay, when they will
at once be ready for use.

Experiment 11. — Arrange the apparatus exactly as in experiment 10,
fig. 72, but employ these electrodes in the place of the wire ones of that experi-
ment. The nerve does not now become polarised.

Great as is the importance of using unpolarisable electrodes when
a current is to be sent through a nerve, their use is of still greater
necessity when we require to examine the currents produced by a
tissue, and for this purpose are making use of a sensitive galvano-
meter.



85



CHAPTEE VIII

SOME EXPEBIMENTS TO DETEEMINE THE FUNCTIONS OF NEKVES

In examining the functions of a nerve we have two main methods
which we may employ, section and stimulation.

By section we can observe the loss of function resulting from the
loss of impulses normally passing along it.

By stimulation we can observe the converse effect, i.e. the production
of some functional activity, such as a muscular movement, secretion
of a gland, &c, when impulses are sent along the nerve.

The two methods of experimentation illustrate the two great
physiological attributes of a nerve, viz. conductivity and excitability.
Section teaches us the function of a nerve by observation of the
results of interrupting its conductivity at some spot. Stimulation
makes use of its property of excitability to give us knowledge of the
results of impulses travelling along the particular nerve.

We have already made use of a nerve's excitability in our experi-
ments upon muscle, and in our study of the variations of function of
a nerve under different experimental conditions motor nerves are
generally employed, because the muscular response enables us to readily
determine small changes in the nerve's activity.

As conductors nerves carry impulses either to or from the central
nervous system, i.e. they are afferent or efferent in function. At their
entrance into the cord, the spinal nerves divide into two roots,
anterior and posterior, the former carrying only efferent, the latter only
afferent impulses. Show this upon the frog by the following
experiment : —

Experiment 1. — Decapitate a frog and cut away the upper third of the
vertebral column. With a fine pointed pair of scissors cut away the lamina;
of the remaining vertebrae, taking great care to avoid injuring the spinal cord.
This brings to view the cord in its lower part with the large roots of the
nerves which form the sciatic plexus. To see the anterior roots, the posterior,
which cover them up, must be displaced, (i.) Lift up the largest posterior
root with a seeker and pass a silk thread under it. Tie this close to the
cord and cut through the root between the ligature and the cord. Note that
on tying and on section irregular movements of the limbs are caused which



86 EXPERIMENTAL PHYSIOLOGY

vary, however, according to the degree of stimulation. Place the peripheral
end upon a fine pair of electrodes and stimulate. There is no muscular con-
traction.

(ii.) Place a second ligature round another posterior root, this time tying
as far from the cord as possible, and cut through the nerve peripherally to the
ligature. Stimulate the central end, i.e. the part still attached to the cord.
The limbs are thrown into convulsions, more or less marked and extensive
according to the strength of the stimulation. These two stimulations prove
that the posterior roots contain no efferent, but do contain afferent
fibres.

(iii.) Cut through all the posterior roots of that side. Any mechanical
stmralation to the skin no longer produces movements of the legs, though
stimulation of the skin of the opposite leg will produce movements in
both.

(iv.) The section of the posterior roots brings the anterior into view. By
placing ligatures round two of these and tying (a) near the cord and (&) near
the junction with the posterior root show that stimulation of the peripheral
end leads to contraction of muscles, but stimulation of the central end produces
no effect. The anterior root therefore contains efferent but no afferent
fibres.

(v.) Cut through all the anterior roots, and then show that no movement
of that leg can now be produced by stimulation of the skin of the leg of the
opposite side.

STIMULI which affect a nerve may be classified as follows : —

1. Thermal.

Experiment 2. — Make a nerve muscle preparation and touch the
end of the nerve with a copper wire which has been heated for a few
moments in a Bunsen flame. The muscle contracts. That heat should
act as a stimulus it is necessary that the temperature should be high.
If heat be gradually applied it will kill without stimulating.

2. Mechanical.

Experiment 3. — To produce mechanical stimulation cut through
the nerve used in the previous experiment just below the point to
which the hot wire was applied. The muscle contracts, thus showing
that the mechanical process of cutting has stimulated the nerve. In
the next place, pinch the upper end of the nerve and show that this
also acts mechanically as a stimulus. Show also that the nerve can
be stimulated by giving it a sharp tap with the edge of the handle of
a scalpel, thus pinching it between the scalpel and the solid support
upon which it rests.

3. Chemical.

Experiment 4. — Take a nerve-muscle preparation and lift up the


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Online LibraryThomas Gregor BrodieThe essentials of experimental physiology, for the use of students → online text (page 8 of 20)