The Vertebrate Heart in Action

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.

aDestroy the brain and spinal cord of an unconscious frog by pithing.

bPin out the frog on a cork dissecting mat; open its thorax and expose its beating heart.

cCut 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.

dAttach 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:

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.

aThe first trace below shows the heart beat at 20 °C (Trace 1).

bA 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).

cA 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).

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.

Investigation 1 (continued)

dIn the next traces, first a frog is bathed with Ringer’s solution at 17 °C to make a continuous trace of its heart.

eThen a glass rod is warmed by immersing it in Ringer’s solution that has been heated to 30 °C in a water bath.

fAs 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.

QUESTIONS

1What assumptions are made in investigating heart activity in frogs that are treated in this way?

2Calculate the cardiac frequency (number of beats per minute) for the heart at each temperature.

Temperature / Cardiac frequency

3What is the effect of changing temperature on cardiac frequency? What other effects are caused by changing temperature?

4Calculate 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 / Treatment / Cardiac frequency
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

5What does this tell you about how increasing temperature increases the rate of heart beat in the frog?

Investigation 2: Control and coordination of the heart beat

aWhen 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.

bThere 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.

After Rowett, H G Q, Dissection guides No 1, The frog, John Murray 1971

cThe 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.

dThe trace below shows the traces produced by the beating heart when stimulated in this way.

eIn 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).

fA 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.

gThe 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.

QUESTIONS

1What 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.)

2Having read the results of the Stannius ligature experiment, suggest a model for how heart beat is controlled that is consistent with those results.

3What is the effect on cardiac frequency of stimulating the vagus nerve with electrical current at low voltage?

4What is the effect on the amplitude of each contraction when the vagus nerve is stimulated?

5What is the effect of adrenaline on the frog’s heart?

6What is the effect of acetylcholine on the frog’s heart?

7Try to describe a model of control of the heart beat in a vertebrate such as the frog, using all the information from these experiments.

ANSWERS

Investigation of effects of changing temperature

1The 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.

2The cardiac frequencies are…

Temperature (°C) / Cardiac frequency (beats per minute)
10 / 19.2
20 / 40.8
34 / 63

3It 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.

4The cardiac frequencies are…

Trace / 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

5Warming 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.

ANSWERS

Control and coordination

1A control system based on excitable tissue in fibres – such as nerves or cardiac muscle – could be disrupted by tying ligatures around it.

2Tying 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.

3Before 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.

4The amplitude of contraction shows no significant change as a result of stimulation of the vagus nerve.

5The 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).

6The 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).

7Model 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.