3.6.1.3 Control of heart rate

• The heart muscle is referred to as myogenic because of its ability to initiate its own contraction without needing external electrical stimulation.
• In the wall of the right atrium, there is a region of specialised fibres called the sinoatrial node (SAN) which is the pacemaker of the heart.
• The sinoatrial node (SAN) initiates a wave of electrical stimulation which causes the atria to contract.
• Describe the roles of the sinoatrial node (SAN) and atrioventricular node (AVN) in coordinating the heartbeat.

  • The SAN acts as the pacemaker, initiating the heartbeat by generating electrical impulses.
  • These impulses spread across the atria, causing them to contract (atrial systole).
  • The impulses reach the AVN, which introduces a slight delay.
  • This delay ensures the ventricles contract after the atria have fully emptied.
  • Impulses are then conducted through the Bundle of His and Purkyne fibres, resulting in coordinated ventricular contraction.
• The ventricles do not start contracting until the atria have finished contracting and emptied due to the presence of tissues at the atria which is unable to conduct the wave of excitation.
• The atrioventricular node (AVN) is located between the two atria and it passes the wave of excitation to ventricles, down the bundle of His to the apex of the heart.
• The bundle of His branches into Purkyne fibres which carry the electrical wave upwards. This causes the ventricles to contract, this empties them into the vessels.
• Autonomic Nervous System (ANS): Controls involuntary actions. It consists of two divisions:

  • Sympathetic Nervous System (SNS): Speeds up heart rate (fight-or-flight response).
  • Parasympathetic Nervous System (PNS): Slows down heart rate (rest-and-digest response).
• The medulla oblongata in the brainstem contains two key centres: the cardioacceleratory centre and the cardioinhibitory centre.
• The Cardioacceleratory Centre is linked to the heart via the sympathetic nervous system.

  • Increases the frequency of impulses sent to the sinoatrial node (SAN) to raise the heart rate.
• The Cardioinhibitory Center is linked to the heart via the parasympathetic nervous system (vagus nerve).

  • Decreases the frequency of impulses sent to the SAN to slow the heart rate.
• Changes in blood pressure and changes in pH are factors which increase the heart rate.
• Changes in pH caused by high carbon dioxide concentration, are detected by chemoreceptors located in carotid arteries, the aorta and the brain.

  • receptors send impulses to the medulla oblongata more frequently as a result via the sympathetic pathway
  • more frequent impulses are sent to the SAN, which increases heart rate
  • increases blood flow to the lungs where carbon dioxide can be expelled
• Changes in blood pressure are detected by baroreceptors in the sinus.

  • receptors send impulses to medulla oblongata more frequently via the parasympathetic pathway to the SAN
  • causes heart rate to decrease, lowering the blood pressure
• Cardiac output (CO) = heart rate (R) x stroke volume (SV)
• How does myogenic control coordinate the heartbeat?

  • The sinoatrial node (SAN) generates electrical impulses that spread across the atria.
  • The atrioventricular node (AVN) introduces a delay before transmitting impulses to the ventricles via the Bundle of His and Purkyne fibres.
• The Bundle of His conducts electrical impulses from the AVN to the ventricles, ensuring coordinated contraction.
• What is the role of the autonomic nervous system in heart rate control?

  • The sympathetic nervous system increases heart rate.
  • The parasympathetic nervous system decreases heart rate.