Initiation and Coordination of Heart Action

Unlike skeletal muscle, cardiac muscle does not need a nerve impulse to make it contract — it is myogenic, meaning that the signal to contract originates within the muscle tissue itself. A single intrinsic pacemaker coordinates the activity of the whole heart and ensures that the four chambers contract in the correct order. This lesson examines the conducting system of the heart (SAN, AVN, Bundle of His and Purkyne tissue), how nervous and hormonal input modify the heart rate, and how all this activity can be monitored non-invasively using an electrocardiogram (ECG). Content matches OCR A-Level Biology A specification 3.1.2 (h)–(i).

Key Definitions:

• Myogenic — able to initiate contraction without any external (nervous) signal.
• Pacemaker — the tissue that sets the rhythm of heart contraction; in mammals this is the sinoatrial node (SAN).
• Electrocardiogram (ECG) — a recording of the electrical activity of the heart, made by electrodes placed on the skin.

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The Conducting System

A special group of modified cardiac muscle fibres carries electrical signals rapidly through the heart, ensuring coordinated contraction. The key components are:

1. Sinoatrial node (SAN)
2. Atrioventricular node (AVN)
3. Bundle of His
4. Purkyne tissue (Purkinje fibres)

flowchart TB
    SAN[SAN in right atrium wall] -->|Wave of depolarisation across atria| ATR[Atria contract]
    ATR --> AVN[AVN at base of atria]
    AVN -->|Delay about 0.1 s| BH[Bundle of His in septum]
    BH --> PK[Purkyne fibres in ventricle walls]
    PK --> VENT[Ventricles contract from apex upwards]

Step 1: The SAN

The sinoatrial node is a small patch of modified cardiac muscle in the wall of the right atrium, near the entry of the superior vena cava. It spontaneously generates a wave of depolarisation about 75 times per minute at rest. This is the intrinsic heart rate, and why the SAN is called the pacemaker.

Step 2: Atrial Contraction

The wave of depolarisation spreads across the walls of both atria, causing atrial systole. Because atrial muscle cells are electrically coupled by intercalated discs containing gap junctions, the wave travels rapidly through the atrial syncytium.

Step 3: The AVN

The wave cannot cross directly into the ventricles because the base of the atria is separated from the ventricles by a non-conducting fibrous ring (the annulus fibrosus). Instead, it is funnelled through the atrioventricular node (AVN), located at the base of the right atrium near the septum. The AVN introduces a delay of about 0.1 s, which is essential to allow the atria to finish contracting and to empty completely into the ventricles before the ventricles themselves start to contract.

Step 4: The Bundle of His

From the AVN, the signal passes down the Bundle of His, a strand of specialised conducting fibres in the interventricular septum. It divides into left and right bundle branches that run towards the apex (tip) of the heart.

Step 5: Purkyne Fibres and Ventricular Contraction

At the apex, the bundle branches spread out into a network of Purkyne fibres (Purkinje fibres) that penetrate the walls of the ventricles. These conduct the wave rapidly through the ventricular muscle, triggering contraction from the apex upwards — effectively squeezing blood towards the semilunar valves at the top, like squeezing toothpaste out of a tube. This is why ventricular contraction is so efficient at ejecting blood.

Exam Tip: Always emphasise the delay at the AVN and the apex-to-base direction of ventricular contraction. These two features ensure the correct sequence and efficient emptying, and are worth marks in exam answers.

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Myogenic Heart Muscle

"Myogenic" means generated by the muscle itself. If the heart is removed and kept in a suitable saline solution, it will continue to beat — because the SAN does not require any nerve impulse to trigger it. In contrast, skeletal muscle is neurogenic: it only contracts when stimulated by a motor neuron.