Edexcel A Level (IAL) Biology -7.12 The Role of Muscle in the Cardiac Cycle- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -7.12 The Role of Muscle in the Cardiac Cycle- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -7.12 The Role of Muscle in the Cardiac Cycle- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 7.12 (i) know the myogenic nature of cardiac muscle
(ii) understand how the normal electrical activity of the heart coordinates the heartbeat, including the roles of the sinoatrial node (SAN), the atrioventricular node (AVN), the bundle of His and the Purkyne fibres
(iii) understand how the use of electrocardiograms (ECGs) can aid in the diagnosis of abnormal heart rhythms
Cardiac Muscle and Heart Electrical Activity
🌟 Introduction
The heart beats rhythmically to pump blood. This is possible because cardiac muscle is myogenic (can contract on its own) and the heartbeat is coordinated by specialized electrical conduction systems. Understanding this helps explain how the heart functions normally and how we can detect problems using ECGs.
🧩 (i) Myogenic Nature of Cardiac Muscle
Definition: Cardiac muscle is myogenic, meaning it initiates its own contractions without nerve stimulation.
Key Features
- Found only in the heart.
- Striated like skeletal muscle but involuntary.
- Muscle fibres are branched and interconnected by intercalated discs.
- Contractions are rhythmic and coordinated.
- Nervous system can modulate rate and strength but does not initiate heartbeat.
Memory trick: “Heart beats itself – myogenic!”
⚡ (ii) Coordination of the Heartbeat
The heartbeat is controlled by the cardiac conduction system, ensuring atria contract first, then ventricles, producing effective blood flow.
Key Components and Roles
| Component | Location | Function |
|---|---|---|
| Sinoatrial Node (SAN) | Right atrium | Pacemaker of the heart. Generates action potentials that start atrial contraction. Sets heart rate. |
| Atrioventricular Node (AVN) | Between atria and ventricles | Receives impulses from SAN. Delays them slightly to allow atria to empty into ventricles. |
| Bundle of His | Ventricular septum | Transmits impulses from AVN to ventricles. |
| Purkyne fibres | Spread throughout ventricular walls | Conduct impulses rapidly to ventricular muscle, causing coordinated ventricular contraction. |
Sequence of Electrical Activity
- SAN fires: atria contract.
- Impulse reaches AVN: slight delay ensures ventricles fill.
- Bundle of His carries impulse down septum.
- Purkyne fibres distribute impulse across ventricles.
- Ventricles contract in a coordinated wave.
Key point: Coordination ensures efficient pumping of blood from atria to ventricles and then to lungs/body.
🧪 (iii) Electrocardiograms (ECGs)
Definition: ECGs are graphs showing the electrical activity of the heart over time.
Main Features of an ECG
- P wave: Atrial depolarisation (atria contract)
- QRS complex: Ventricular depolarisation (ventricles contract)
- T wave: Ventricular repolarisation (ventricles relax)
Clinical Importance
- ECGs detect abnormal heart rhythms (arrhythmias).
- Examples:
- Bradycardia: Heart beats too slowly.
- Tachycardia: Heart beats too fast.
- Atrial fibrillation: Irregular atrial contractions.
- Can indicate heart attack or conduction blockages.
Memory trick: “P-QRS-T tells heart rhythm story.”
📌 Summary Table
| Topic | Key Points |
|---|---|
| Myogenic cardiac muscle | Beats on its own, involuntary, striated, branched, intercalated discs |
| SAN | Pacemaker, initiates heartbeat, atria contract |
| AVN | Delays impulse for ventricular filling |
| Bundle of His | Carries impulse to ventricles |
| Purkyne fibres | Spread impulse through ventricles for coordinated contraction |
| ECG | P (atria), QRS (ventricles), T (ventricular relaxation); detects arrhythmias |
Heart is myogenic → beats without nerves.
SAN starts heartbeat → atria contract.
AVN delays → ventricles fill.
Bundle of His + Purkyne fibres → ventricles contract together.
ECG shows electrical activity: P-QRS-T.
Abnormalities in ECG indicate arrhythmias or heart problems.