explain the roles of the sinoatrial node, the atrioventricular node and the Purkyne tissue in the cardiac cycle (knowledge of nervous and hormonal control is not expected)
Cambridge A‑Level Biology 9700 – The Heart: Conduction System
The Heart – Electrical Conduction System
The heart contracts in a coordinated sequence because of specialised cardiac muscle cells that generate and transmit electrical impulses. The three key components are the sinoatrial (SA) node, the atrioventricular (AV) node and the Purkinje tissue. Understanding their roles explains the timing of atrial and ventricular contraction during the cardiac cycle.
Sinoatrial (SA) Node
The SA node is the primary pacemaker of the heart.
Location: Upper wall of the right atrium, near the opening of the superior vena cava.
Function: Generates spontaneous depolarisations at a rate of about 60–100 beats min⁻¹.
Effect: Initiates the electrical impulse that spreads across both atria, causing atrial contraction (atrial systole).
Control: Its intrinsic rate is modified by autonomic input, but this is not required for the objective.
Atrioventricular (AV) Node
The A \cdot node acts as a delay mechanism between atrial and ventricular contraction.
Location: Lower part of the interatrial septum, near the tricuspid valve.
Function: Receives the impulse from the SA node and slows its transmission by about 0.1 s.
Purpose of delay: Allows the atria to finish emptying blood into the ventricles before ventricular contraction begins.
After the delay, the impulse is conducted to the Bundle of His.
Purkinje Tissue
Purkinje fibres constitute the rapid conduction network within the ventricles.
Location: Subendocardial layer of the ventricular walls, branching from the Bundle of His.
Function: Transmit the impulse quickly throughout the ventricular myocardium.
Result: Synchronous contraction of the ventricles (ventricular systole) and efficient ejection of blood.
Sequence of Electrical Events in One Cardiac Cycle
SA node depolarises → impulse spreads across atria → atrial systole.
Impulse reaches A \cdot node → brief delay.
Impulse passes to Bundle of His → divides into right and left bundle branches.
Repolarisation of atrial and ventricular muscle follows, preparing for the next cycle.
Comparison of the Three Conduction Elements
Structure
Location
Primary Role
Timing in Cardiac Cycle
SA Node
Right atrial wall (near S \cdot C)
Initiates impulse; sets heart rate
Atrial depolarisation → atrial systole
A \cdot Node
Interatrial septum (near tricuspid valve)
Delays impulse to allow ventricular filling
Pause between atrial and ventricular systole (≈0.1 s)
Purkinje Tissue
Ventricular subendocardium
Rapidly distributes impulse to ventricles
Ventricular depolarisation → ventricular systole
Suggested diagram: Conduction pathway of the heart showing the SA node, A \cdot node, Bundle of His, right and left bundle branches, and Purkinje fibres.
Key Points to Remember
The SA node is the heart’s natural pacemaker.
The A \cdot node provides a crucial delay that synchronises atrial and ventricular contraction.
Purkinje fibres ensure that ventricular contraction is rapid and coordinated.
These three components together produce the characteristic “lub‑dub” sounds of the heartbeat (first sound = closure of A \cdot valves after ventricular systole; second sound = closure of semilunar valves after ventricular diastole).