The Role of Muscle in the Cardiac Cycle (Edexcel International A Level Biology)

• The cells making up cardiac muscle are myogenic, which means they contract without any external stimulus
• This intrinsic rhythm means the heart beats at around 60 times per minute
• The sinoatrial node (SAN) is a group of cells in the wall of the right atrium
  • The SAN initiates a wave of depolarisation that causes the atria to contract
• There is a region of non-conducting tissue which prevents the depolarisation spreading straight to the ventricles
  • Instead, the depolarisation is carried to the atrioventricular node (AVN)
  • This is a region of conducting tissue between atria and ventricles
• After a slight delay, the AVN is stimulated and passes the stimulation along the bundle of His
  • This delay means that the ventricles contract after the atria
• The bundle of His is a collection of conducting tissue in the septum (middle) of the heart
  • The bundle of His divides into two conducting fibres which carry the impulse to the Purkyne fibres
    • Purkyne fibres are also known as Purkinje fibres
• Purkyne fibres spread around the ventricles and initiate the depolarisation of the ventricles from the apex (bottom) of the heart
• This makes the ventricles contract from the bottom upward and blood is forced out of the ventricles into the pulmonary artery and aorta

Stages in the Cardiac Cycle Table

The wave of depolarisation spreads across the heart in a co-ordinated manner

• Electrocardiography can be used to monitor and investigate the electrical activity of the heart
• Electrodes that are capable of detecting electric signals are placed on the skin
• These electrodes produce an electrocardiogram (ECG)
  • An ECG shows a number of distinctive electrical waves produced by the activity of the heart
• A healthy heart produces a distinctive shape in an ECG

The ECG of a healthy heart

• The P wave
  • Caused by the depolarisation of the atria, which results in atrial contraction (systole)
• The QRS complex
  • Caused by the depolarisation of the ventricles, which results in ventricular contraction (systole)
  • This is the largest wave because the ventricles have the largest muscle mass
• The T wave
  • Caused by the repolarisation of the ventricles, which results in ventricular relaxation (diastole)
• The U wave
  • Scientists are still uncertain of the cause of the U wave, some think it is caused by the repolarisation of the Purkyne fibres
• The bigger the wave, the greater the electrical activity passing through the heart, which results in a stronger contraction

Using ECGs to diagnose heart problems

• If someone has a suspected heart problem a doctor will often use an ECG as a diagnostic tool
• Some heart problems produce certain shapes or waves in an ECG which allow for a diagnosis
• Tachycardia
  • When the heart beats too fast it is tachycardic
  • An individual with a resting heart rate of over 100 bpm is said to have tachycardia
• Bradycardia
  • When the heart beats too slow it is bradycardic
  • An individual with a resting heart rate below 60 bpm is said to have bradycardia
  • A lot of fit individuals or athletes tend to have lower heart rates and it is usually not dangerous
• Ectopic heartbeat
  • This condition is caused by an early heartbeat followed by a pause
  • This could be due to an earlier contraction of either the atria or ventricles
  • It is common in the population and usually requires no treatment unless very severe
• Fibrillation
  • An irregular heartbeat will disrupt the rhythm of the heart
  • The atria or ventricles stop contracting properly
  • Severe cases of fibrillation can be very dangerous, even fatal

Each of these ECGs shows different faulty heartbeats. The speed or rhythm/regularity of the heartbeat is very important