3.2.9 Heart Action: Initiation & Control

Question 1

Initiation and Control

Answer 1

Control of the basic heartbeat is myogenic, which means that the heart will beat 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
The annulus fibrosus is a region of non-conducting tissue which prevents the depolarisation spreading straight to the ventricle
- 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 2 conducting fibres, called Purkyne tissue, and carries the wav we of excitation along them
The Purkyne fibres spread around the ventricles and initiate the depolarization of the ventricles from the apex (bottom) of the heart
This makes the ventricles contract and blood is forced out of the pulmonary artery and aorta

Question 2

Stages in the Cardiac Cycle

Answer 2

1 - sinoatrial node sends out a wave of excitation
2 - atria contract
3 - atrioventricular node sends out a wave of excitation
4 - Purkyne tissue conducts the wave of excitation
5 - ventricles contract