7.4.1 Myogenic cardiac muscle, the conducting system and ECGs Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 7

The 20 questions

1. Which region of the heart normally initiates each heartbeat and acts as its natural pacemaker?

   • Purkinje fibres, branching conducting fibres that spread impulses through the walls of the ventricles
   • Atrioventricular node, which delays impulses at the base of the atria before they reach the ventricles
   • Bundle of His, a bundle of conducting fibres that carries impulses down the interventricular septum
   • Sinoatrial node, a region of specialised cells in the wall of the right atrium that generates impulses

2. What does it mean to say that cardiac muscle is myogenic?

   • It contracts and relaxes rhythmically on its own, originating the beat within the muscle tissue itself
   • It can only contract while adrenaline is present in the blood, and stops once the hormone is cleared
   • It needs a nerve impulse from the brain for every single contraction, so it cannot beat without external signals
   • It contracts only when the skeletal muscles around the heart are also contracting during movement

3. Why is there a short delay of about 0.1 s between atrial contraction and ventricular contraction?

   • The SAN fires twice in quick succession, once for the left chamber and once for the right chamber of the heart
   • Purkinje fibres conduct impulses more slowly than the bundle of His, so the ventricles start after the atria
   • The AVN delays the impulse so the atria can empty fully into the ventricles before the ventricles contract
   • Atrial muscle has a much longer refractory period than ventricular muscle, so it relaxes later in the cycle

4. Which structure carries the wave of excitation from the AVN down the septum towards the ventricles?

   • Bundle of His, a bundle of specialised conducting fibres that carries the impulse down the interventricular septum
   • Vagus nerve, a parasympathetic nerve that slows the heart and carries impulses from the medulla oblongata
   • Pulmonary artery, which carries deoxygenated blood from the right ventricle to the lungs for gas exchange
   • Sinoatrial node, which sends the wave of excitation across the atrial walls to the atrioventricular junction

5. Which fibres spread the impulse through the walls of both ventricles so that they contract from the apex upwards?

   • Atrioventricular valves, which open and close to control the flow of blood between atria and ventricles
   • Purkinje fibres, which branch out from the bundle of His through the ventricle walls and start contraction at the apex
   • Bundle of His, which runs through the septum but does not branch into the ventricle walls
   • Sinoatrial node cells, which generate the initial impulse in the wall of the right atrium only

6. On an ECG trace, what does the P wave represent?

   • Ventricular repolarisation, which resets the ventricles ready for the next cycle of filling and emptying
   • Opening of the semilunar valves, which allows blood to leave the ventricles into the aorta and pulmonary artery
   • Atrial depolarisation spreading from the SAN across the atria, which causes atrial contraction
   • Ventricular depolarisation, which spreads through the ventricle walls and triggers ventricular contraction

7. On an ECG, the QRS complex corresponds to which event in the heart?

   • Ventricular relaxation in diastole, when the ventricles fill passively with blood from the atria
   • Atrial repolarisation, which resets the atria and is usually hidden within the larger QRS deflection
   • Atrial depolarisation, which spreads from the SAN across the atria and produces the first small deflection
   • Ventricular depolarisation, which spreads through the ventricle walls via the Purkinje fibres and triggers ventricular contraction

8. An ECG shows regular P waves, but some are not followed by a QRS complex. Which structure is most likely failing to conduct impulses to the ventricles?

   • Sinoatrial node, which would stop the atria beating and remove the P waves rather than the QRS complexes
   • Atrioventricular node, which passes impulses from the atria to the ventricles and can block them
   • Cardiovascular control centre in the medulla, which changes heart rate but does not conduct impulses through the heart
   • Purkinje fibres, which would only widen the QRS complex rather than remove it from the trace

9. On an ECG, which feature would suggest that the ventricles are depolarising more slowly than normal?

   • A regular R to R interval of 0.8 s, which shows a steady heart rate of 75 beats per minute
   • A P wave before every QRS complex, showing that the atria are depolarising normally in each cycle
   • A PR interval of 0.16 s, which falls within the normal range for a healthy adult heart
   • A widened QRS complex, showing that depolarisation takes longer to spread through the ventricle walls

10. Which property of the SAN allows it to generate impulses spontaneously without an external stimulus?

    • It receives a constant supply of acetylcholine from the vagus nerve, which keeps it firing continuously
    • It holds a fixed store of calcium ions that is released once per beat, triggering contraction
    • It is directly supplied by a branch of the pulmonary artery, which brings the stimulus needed to fire
    • Its cells slowly depolarise until they reach threshold, and then fire an impulse without any external trigger

11. A heart rate is 75 beats per minute. How long, in seconds, does one cardiac cycle last?

    • 1.25 s, found by dividing 75 beats per minute by the 60 seconds in one minute
    • 0.75 s, found by taking the heart rate value directly as the length of one cycle
    • 0.80 s, found by dividing 60 seconds by the heart rate of 75 beats per minute
    • 1.33 s, found by dividing 100 beats per minute by the 75 seconds in one cycle

12. The interval between two consecutive R waves on an ECG is 0.6 s. What is the heart rate in beats per minute?

    • 600 beats per minute, which is 60 seconds multiplied by ten beats for each tenth of a second
    • 360 beats per minute, which is 60 seconds multiplied by six beats for each second of the interval
    • 100 beats per minute, which is 60 seconds divided by the 0.6 s interval between consecutive R waves
    • 60 beats per minute, which is the number of seconds in one minute divided by the R to R interval in seconds

13. Which ECG measurement is used to indicate the delay at the AVN?

    • The QT interval, which measures the whole time from ventricular depolarisation to repolarisation
    • The PR interval, which runs from the start of the P wave to the start of the QRS complex
    • The height of the T wave, which reflects the size of the ventricular repolarisation signal
    • The QRS duration, which measures how long ventricular depolarisation takes to spread through the walls

14. Why must the atria and ventricles contract at different times in the cardiac cycle?

    • So the AVN can receive oxygenated blood from the coronary arteries, which only happens when the atria are relaxed
    • So the SAN can reset its rhythm between beats, which requires the ventricles to be quiet at that time
    • So blood can flow from the atria into the ventricles before the ventricles contract and pump it out
    • So the bundle of His can repolarise before the atria beat, which protects the conducting fibres from damage

15. A patient's ECG shows that atrial and ventricular rhythms are independent, with no fixed relationship between P waves and QRS complexes. What is the most likely explanation?

    • Complete heart block, in which impulses from the SAN are not conducted through the AVN, so ventricles are driven by a slower pacemaker
    • The bundle of His is generating impulses in the atria, which overrides the normal rhythm from the sinoatrial node
    • The Purkinje fibres have stopped conducting, causing the ventricles to stop completely until the next atrial beat
    • The SAN is firing twice as fast as the atria can contract, so the atria cannot follow the rhythm set by the node

16. What does a flat line with no P waves or QRS complexes on an ECG most likely indicate?

    • Electrical activity has stopped, so the heart is not generating impulses and is not pumping blood effectively
    • A pacemaker has been fitted correctly, which always produces a flat trace on a standard twelve-lead ECG
    • Normal sinus rhythm at a slow heart rate, in which every impulse is conducted through the AVN as expected
    • Only the ventricles are working normally, with the atria beating too quickly to produce visible P waves

17. Which statement correctly links the ECG to diagnosis of cardiovascular disease?

    • Abnormal waveform shapes or intervals can indicate conduction or rhythm problems that help doctors diagnose heart conditions
    • An ECG is used only to measure cardiac output in litres per minute, and is not used to find electrical faults
    • An ECG measures blood pressure, so abnormal readings always show that a coronary artery is completely blocked
    • An ECG measures the oxygen content of blood leaving the heart, so it can detect ischaemia directly in the muscle

18. A student says the heart is myogenic because nerves are not needed at all. Which statement best corrects this?

    • The heart needs no nervous input at all under any circumstances, since the beat is fully independent of the nervous system
    • The beat originates in the muscle, but nervous input from the medulla oblongata can speed up or slow down the rate
    • Nerves only affect the heart during strenuous exercise and have no role in controlling the rhythm at rest
    • Nerves control the rhythm completely, and the muscle only follows passively whatever signal the brain sends

19. Which pair correctly matches a conducting-system structure with its role?

    • Purkinje fibres: start the impulse; SAN: delays the impulse so the atria can empty fully into the ventricles
    • SAN: delays the impulse; AVN: starts the impulse before it reaches the ventricles through the bundle of His
    • Bundle of His: starts the impulse; Purkinje fibres: delay the impulse before it spreads to the ventricle walls
    • SAN: starts the impulse; AVN: delays the impulse before passing it to the ventricles along the bundle of His

20. An ECG shows a QRS complex that is narrow and occurs after every P wave, with a PR interval within the normal range. What does this indicate?

    • A normal sequence of atrial depolarisation followed by normal ventricular depolarisation through the conducting system
    • Complete heart block, in which the atria and ventricles beat independently of one another on separate rhythms
    • Ventricular fibrillation, with no coordinated contraction of the ventricles and no effective pumping of blood
    • Failure of the SAN, with the atria being driven by impulses that arrive from the ventricles instead of the node

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