nodes

• The auto-rhythmic cells of the sino-atrial node (SAN) or pacemaker, located in the wall of the right atrium, set the rate at which the heart contracts.
• The timing of cardiac muscle contraction is controlled by impulses from the SAN spreading through the atria causing atrial systole.
• Impulses from the SAN travel to the (atrioventricular node) AVN, located in the centre of the heart. Impulses from the AVN travel down fibres in the central wall of the heart and then up through the walls of the ventricles, causing ventricular systole.

  
• Impulses in the heart generate currents that can be detected by an electrocardiogram.
• P WAVE: Contraction of the atria (systole)

  QRS WAVE: Contraction of the ventricles (ventricular systole)

  T wave: Diastole

  
• The brain’s medulla regulates the rate of the sino-atrial node through the antagonistic action of the autonomic nervous system (ANS).
• • A sympathetic nerve releases noradrenaline which increases heart rate.
  • A parasympathetic nerve releases acetylcholine which decreases heart rate.