Control of the heartbeat and ECGs

Myogenic muscle and the conduction pathway

Cardiac muscle is myogenic: it contracts without needing nerve impulses.

1. The sinoatrial node (SAN), the pacemaker in the wall of the right atrium, sends out regular waves of electrical excitation.

2. The wave spreads across both atria, which contract (atrial systole).

3. A layer of non-conducting tissue stops it passing straight to the ventricles. It reaches the atrioventricular node (AVN), which delays it briefly (about 0.1 s) so the atria finish emptying and the ventricles fill.

4. The AVN passes the wave down the bundle of His in the septum to the Purkyne fibres in the ventricle walls.

5. The ventricles contract from the apex (the bottom tip) upwards, squeezing blood up into the arteries.

The ECG

An ECG records electrical activity using electrodes on the skin.

• P wave: depolarisation of the atria (atrial systole follows)
• QRS complex: depolarisation of the ventricles (ventricular systole follows); larger because ventricles have more muscle
• T wave: repolarisation of the ventricles

Heart rate = 60 / time between R peaks (s).

Diagnosis with ECGs

Doctors compare a patient's ECG with a normal trace:

• Tachycardia: fast resting rate (over 100 beats per minute)
• Bradycardia: slow resting rate (under 60 beats per minute)
• Fibrillation: irregular, uncoordinated contraction; no clear P waves or QRS pattern
• Myocardial infarction (heart attack): changes such as an elevated ST segment show damaged heart muscle
• Ectopic heartbeat: an extra or early beat

Worked example

On an ECG, the R peaks are 0.8 s apart. Calculate the heart rate.

1. Heart rate = 60 / 0.8
2. = 75

Answer: 75 beats per minute.

Check yourself

1. The R-R interval on an ECG is 0.6 s. Calculate the heart rate in beats per minute.
   Show answer

   100 beats per minute. 60 / 0.6 = 100.

2. On ECG paper each large square is 0.2 s. R peaks are 4 large squares apart. What is the heart rate in beats per minute?
   Show answer

   75 beats per minute. Interval = 0.8 s; 60 / 0.8 = 75.

3. What does myogenic mean?
   • Contraction is started by the brain
   • The muscle cannot contract
   • It is controlled only by hormones
   • Contraction starts in the muscle itself, without nerve impulses
   Show answer

   Contraction starts in the muscle itself, without nerve impulses. The SAN generates its own impulses.

4. Why is there a short delay at the AVN?
   • So the atria can empty and the ventricles fill before they contract
   • To let the SAN recover
   • So the atria and ventricles contract at the same time
   • To slow blood in the aorta
   Show answer

   So the atria can empty and the ventricles fill before they contract. The delay keeps the chambers in the right order.

5. What does the P wave on an ECG represent?
   • Depolarisation of the ventricles
   • Repolarisation of the ventricles
   • Depolarisation of the atria
   • Closing of the semilunar valves
   Show answer

   Depolarisation of the atria. P = atrial depolarisation.

6. Why do the ventricles contract from the apex upwards?
   • To push blood up towards the arteries at the top of the heart
   • To push blood back into the atria
   • Because the SAN is at the apex
   • So the AV valves open
   Show answer

   To push blood up towards the arteries at the top of the heart. Purkyne fibres carry the impulse to the apex first.

7. An ECG shows an irregular trace with no clear QRS complexes. What does this suggest?
   • Normal sinus rhythm
   • Bradycardia only
   • Fibrillation
   • A healthy athlete
   Show answer

   Fibrillation. Uncoordinated activity gives a chaotic trace.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• Indiana University Pressbooks, Human Physiology: Conduction system of the heart and the ECG

Last updated 7 November 2026. Spotted a mistake? Tell us.