Control of heart rate

• The cardiac muscle is myogenic. What does this mean?

  It produces its own action potentials which cause its own contraction
• Describe how the cardiac muscle is myogenic

  1. The sinoatrial node (SAN) emits a wave of nerve impulses across the atrial muscle, causing its contraction
  2. A layer of non-conducting tissue prevents the impulses from causing immediate ventricular contraction
  3. The atrioventricular node (AVN) receives the impluses from the SAN. After a delay to allow ventricles to fill with blood, the AVN emits its own impulses
  4. These are then transmitted through the Purkinje fibres in the bundle of His, which cause ventricular contraction from the bottom upwards
• Describe the autonomic nervous system

  Controls the automatic process in the body

  Its control centre is in the medulla oblongata in the brain stem

  The sympathetic nerve and parasympathetic nerves transmit nerve impulses from the medulla to the organs
• Describe how heart rate is increased during exercise

  1. Chemoreceptors in the walls of aorta and carotid arteries detect a decrease in bllood pH due to higher CO2 concentration
  2. Chemoreceptors send more nerve impulses to the cardiac centre in the medulla oblongata
  3. The medulla sends more impulses down the sympathetic nerve, which synapses with the sinoatrial node (SAN), using noradrenaline as the neurotransmitter
  4. The SAN emits more impulses which increases the heart rate
• Describe how heart rate is decreased when blood pressure is high


  1. Baroreceptors in the wall of the aorta and carotid arteries detect an increase in blood pressure due to increased stretching of the elastic layer
  2. Baroreceptors send more impulses to the cardiac centre in the medulla oblongata
  3. The medulla sends more impulses down the parasympathetic nerve, which synapses with the sinoatrial node (SAN), using acetylcholine as the neurotransmitter
  4. The SAN emits fewer impulses, which decreases the heart rate