Cardiac muscle contraction is initiated by a process called excitation-contraction coupling, which is unique to heart muscle. This process converts an electrical signal into a mechanical contraction.
Excitation-Contraction Coupling in Cardiac Muscle
The primary mechanism driving cardiac muscle contraction involves a process called calcium-induced calcium release.
Key Steps:
- Action Potential: The process begins with an action potential that spreads across the cardiac muscle cell membrane.
- Calcium Influx: This electrical signal causes calcium ions ($Ca^{2+}$) to enter the cell from the extracellular space via voltage-gated calcium channels.
- Calcium-Induced Calcium Release (CICR): The influx of $Ca^{2+}$ triggers the release of even more $Ca^{2+}$ from the sarcoplasmic reticulum (SR), an intracellular store of calcium. This is the key distinguishing feature of cardiac muscle contraction.
- Myofilament Interaction: The increased concentration of $Ca^{2+}$ in the cytoplasm then binds to troponin, a protein on the actin filaments, leading to a conformational change that exposes the myosin-binding sites on actin.
- Muscle Contraction: Myosin heads can now bind to actin, forming cross-bridges, and initiate the power stroke, which shortens the sarcomere and contracts the muscle.
- Relaxation: For relaxation, calcium pumps actively transport $Ca^{2+}$ back into the sarcoplasmic reticulum and out of the cell, causing the muscle to relax.
Comparison with Skeletal Muscle
While skeletal muscle contraction also relies on calcium influx, cardiac muscle uniquely employs calcium-induced calcium release for contraction. In skeletal muscle, the action potential directly causes the SR to release calcium.
| Feature | Cardiac Muscle | Skeletal Muscle |
|---|---|---|
| Calcium Source | Extracellular and SR via CICR | Primarily SR |
| Calcium Release Trigger | Influx of Ca2+ from extracellular space | Action potential directly on SR |
| CICR | Yes | No |
Practical Insight:
Understanding this detailed process is crucial for understanding how the heart functions, including its role in blood circulation. Irregularities in the process of excitation-contraction coupling can lead to various heart conditions.
