The vertebrate heart in action
The purpose of this activity is:



to present for discussion a method previously used to investigate the function of
vertebrate hearts using frogs as experimental subjects
to study the results typically obtained from this method
to discover how heart beat is controlled in vertebrates
Procedure
SAFETY:
There are no health and safety issues for this experiment because it is presented for
discussion only – not as a protocol to be followed.
Preparation:
This is the procedure that would be followed to produce the results described
below. You are not being asked to carry out this practical.
a
b
c
d
Destroy the brain and spinal cord of an unconscious frog by pithing.
Pin out the frog on a cork dissecting mat; open its thorax and expose its beating
heart.
Cut away the thin membrane that completely surrounds the heart (the pericardium)
and attach a metal hook or ‘heart clip’ to the apex of the ventricle. Keep the
dissection moist throughout the investigation by pipetting frog Ringer’s solution over
the preparation.
Attach the heart clip by a thread to a mechanical lever system (see diagrams). This
lever will directly record the movements of the heart as it pumps (if attached to a
pen in contact with a kymograph) or record them indirectly (if a motion sensor is
attached).
Investigation 1: Effects of changing temperature
Notes:
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

a
b
c
Throughout the investigation, keep the dissection moist by pipetting frog Ringer’s
solution at room temperature over the preparation unless other conditions are
specified.
The traces shown were recorded from a number of different frogs, so you cannot
make meaningful comparisons between traces from two different investigations.
In the traces, contraction causes a downward movement of the heart lever.
The first trace below shows the heart beat at 20 °C (Trace 1).
A beaker of Ringer’s solution was cooled to 10 °C and then pipetted onto the heart.
Once the rate of beating had stabilised, this trace was recorded (Trace 2).
A beaker of Ringer’s solution was heated to 34 °C and then pipetted onto the heart.
After the beating stabilised, a third trace was recorded (Trace 3).
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The frog’s heart shows an interesting intermediate structure between that of the fish
and the mammal. Blood entering the heart does so from the lungs (into the left atrium)
and from the systemic circulation (into the right atrium via the sinus venosus). From the
two atria, the blood passes into a single ventricle. From the ventricle, the blood leaves
the heart along three pathways – a pulmo-cutaneous pathway to the lungs and skin, a
carotid pathway to the head and brain, and a systemic pathway to the rest of the body.
Trace 1 and Trace 3 show ‘bumps’ as different parts of the heart contract – first the
sinus venosus, then the atria, then the ventricle.
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Investigation 1 (continued)
d In the next traces, first a frog is bathed with Ringer’s solution at 17 °C to make a
continuous trace of its heart.
e Then a glass rod is warmed by immersing it in Ringer’s solution that has been
heated to 30 °C in a water bath.
f As the heart continues to beat, the warm glass rod is held in contact with the
ventricle, the atria and the sinus venosus in turn.
The diagram overleaf shows portions of traces produced from the steps above.
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QUESTIONS
1
What assumptions are made in investigating heart activity in frogs that are
treated in this way?
2
Calculate the cardiac frequency (number of beats per minute) for the heart at
each temperature.
Temperature
3
Cardiac frequency
What is the effect of changing temperature on cardiac frequency? What other
effects are caused by changing temperature?
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4
Calculate the cardiac frequency for each of the four treatments in the second set
of traces by measuring the mean distance between the major troughs (1 mm = 1
s).
Trace
5
Treatment
1
Heart at 17 °C
2
Ventricle touched with
glass rod at 30 °C
3
Atria touched with glass
rod at 30 °C
4
Sinus venosus touched
with glass rod at 30 °C
Cardiac frequency
What does this tell you about how increasing temperature increases the rate of
heart beat in the frog?
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Investigation 2: Control and coordination of the heart beat
a When a piece of cotton thread is tied tightly around the heart at the junction of the
sinus venosus and the right atrium, the sinus is isolated from the other chambers of
the heart. This is known as the Stannius ligature after the physiologist who first
performed the experiment. The picture below shows the position of the ligature.
As soon as the ligature is tied, the atrium and ventricle stop beating. After a short
time, they begin to contract again. The atrium beats slightly before the ventricle (as
in the untreated heart) but both beat more slowly than the sinus venosus. The sinus
venosus continues to beat at its normal frequency.
If a second ligature is tied around the top of the ventricle, where it joins the atria,
the ventricle stops contracting for a short time, but the atria continue to beat. After a
while, the ventricle begins to beat again, but at a slower rate than the atria. All the
chambers will now continue beating, each at its own rate.
b
There are two nerve branches that supply the heart. One of these (the vagus
nerve) is quite readily accessible in the upper thorax of the frog – as shown in the
diagram below. The vagus nerve originates in the frog’s brain.
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After Rowett, H G Q, Dissection guides No 1, The frog, John Murray 1971
c
d
e
The cardiac branch of the vagus nerve can be stimulated by positioning a pair of
electrodes under the nerve and applying a low voltage of repeated stimuli to the
nerve.
The trace below shows the traces produced by the beating heart when stimulated
in this way.
In another experiment, the heart was kept moist with Ringer’s solution at room
temperature and a trace made to record the heart beat. (Trace 1 below).
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f
g
A solution of adrenaline (1 part in 1000 parts Ringer’s solution) at room
temperature was pipetted onto the heart and the beat recorded (Trace 2 below). It
is thought that an accelerator nerve in the frog naturally releases noradrenaline
(which is very similar to adrenaline) at a spot in the muscular wall of the sinus
venosus known as the pacemaker.
The adrenaline was washed off with fresh Ringer’s solution, and once the beat had
returned to normal, a solution of acetylcholine (1 part in 1000 parts Ringer’s
solution) was pipetted onto the heart. Once again the beat was recorded (Trace 3
below). It is thought that the vagus nerve releases acetylcholine when it is
stimulated, also into the pacemaker in the sinus venosus.
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QUESTIONS
1
What kind of control system, do you think, might be affected by tying a ligature
around it? (Think about what happens if you sit awkwardly cross-legged or sleep
awkwardly on your arm.)
2
Having read the results of the Stannius ligature experiment, suggest a model for
how heart beat is controlled that is consistent with those results.
3
What is the effect on cardiac frequency of stimulating the vagus nerve with
electrical current at low voltage?
4
What is the effect on the amplitude of each contraction when the vagus nerve is
stimulated?
5
What is the effect of adrenaline on the frog’s heart?
6
What is the effect of acetylcholine on the frog’s heart?
7
Try to describe a model of control of the heart beat in a vertebrate such as the
frog, using all the information from these experiments.
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ANSWERS
Investigation of effects of changing temperature
1
The assumption is made that the surgical procedures on the frog have no effect
on the normal rate, rhythm, or contracting sequence of the frog’s heart. Also that
changing temperature, or otherwise stimulating the frog’s heart has the same
effect on a pithed frog as in a living animal.
2
3
4
The cardiac frequencies are…
Temperature (°C)
Cardiac frequency
(beats per minute)
10
19.2
20
40.8
34
63
It appears that increasing temperature (in this range) increases cardiac
frequency. It does not seem to cause any change in the amplitude of contraction.
At low temperatures there is a loss of definition between the phases of
contraction.
The cardiac frequencies are…
Trace
5
Treatment
Cardiac frequency
1
Heart at 17 °C
33
2
Ventricle touched with
glass rod at 30 °C
33
3
Atria touched with glass
rod at 30 °C
32.4
4
Sinus venosus touched
with glass rod at 30 °C
41.4
Warming the sinus venosus produces an increase in heart rate, but warming the
atria or ventricles does not increase the rate. Warming the sinus venosus causes
an increase of about 25% in rate. This suggests that something in the sinus
venosus is reacting to the increase in temperature and hence increasing the
heart rate in the frog.
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ANSWERS
Control and coordination
1
A control system based on excitable tissue in fibres – such as nerves or cardiac
muscle – could be disrupted by tying ligatures around it.
2
Tying a Stannius ligature disrupts co-ordination of the heart beat by physically
crushing the conducting bundles (of excitable tissue) within the heart muscle.
Waves of depolarization that emanate from the nodes cannot act upon the next
‘pacemaker’.
The ligature experiments suggest that the intrinsic rhythm of the whole heart
originates in the sinus venosus which acts as a pacemaker for the entire heart.
They suggest that the impulse spreads from the sinus venosus to the atria and
ventricle in turn. They also suggest that, in the absence of controlling impulses
from the sinus venosus, the atria themselves initiate a co-ordinating impulse
which can spread to the ventricle. It seems that extrinsic initiation of contraction is
not necessary for any of the chambers of the heart.
3
Before stimulation, the frequency is 0.64 beats per second (or 38.4 beats per
minute). After stimulation, the frequency is 0.45 beats per second (or 27 beats
per minute). This represents a 30% decrease in the rate of contraction.
The amplitude of contraction shows no significant change as a result of
stimulation of the vagus nerve.
4
5
The effect of adrenaline is to increase the heart rate by about 45%, to 57.6 beats
per minute from 39.6 beats per minute (or 0.96 beats per second from 0.66 beats
per second).
6
The effect of acetylcholine is to decrease the heart rate by about 20%, to 31.8
beats per minute from 39.6 beats per minute (or 0.53 beats per second from 0.66
beats per second).
7
Model for control: The frog’s heart rate changes as a result of stimulation of one
or other nerve, which then acts on the pacemaker in the sinus venosus, to speed
up or slow down the heart rate. The accelerator nerve produces adrenaline that
increases the heart rate and the vagus nerve produces acetylcholine that slows it.
This may be similar to what happens in our own bodies (as adrenaline makes our
hearts beat faster) but it is not always reliable to transfer information about one
type of animal to another. Further experiments with human heart muscle could
support or disprove this idea.
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