6.1.11 Myogenic Stimulation of the Heart

Myogenic Nature of the Heart

The heart is referred to as myogenic, meaning it can initiate its own contractions without external nervous input. This ability ensures that the heart beats continuously and rhythmically.

Control of Heartbeat

1. Sinoatrial Node (SAN):

• Located in the wall of the right atrium, the SAN acts as the pacemaker of the heart.
• It generates a wave of electrical excitation that causes the atria to contract simultaneously, pushing blood into the ventricles.

2. Atrioventricular Node (AVN):

• Located between the two atria.
• There is a delay at the AVN, ensuring the ventricles contract only after the atria have fully emptied.

3. Bundle of His:

• The electrical impulse is passed from the AVN to the Bundle of His, located in the septum of the heart.
• The Bundle of His conducts the impulse to the apex of the heart.

4. Purkyne Fibres:

• The impulse is transmitted from the apex upwards through Purkyne fibres, which spread throughout the walls of the ventricles.
• This causes the ventricles to contract from the apex upwards, efficiently emptying the heart by pushing blood into the aorta and pulmonary artery.

Order of Electrical Activity

1. SAN initiates the impulse → causes atrial contraction.
2. Impulse travels to the AVN → delay ensures atrial contraction is complete.
3. Impulse passes through the Bundle of His → moves to the apex.
4. Impulse spreads through Purkyne fibres → causes ventricular contraction from the apex upwards.

Key Points to Remember

• SAN sets the pace of the heart.
• AVN delay ensures coordinated contraction of atria and ventricles.
• The heart contracts in a wave-like motion, ensuring blood is effectively pumped out.