Excitation–contraction coupling

Excitation–contraction coupling is a physiological process of converting an electrical stimulus to a mechanical response. It is the link (coupling) between the action potential generated in the sarcolemma and the start of a muscle contraction. The process is fundamental to muscle physiology, influencing physical properties from the cellular level to overall muscle behavior.

Process

The process of excitation-contraction coupling occurs when an action potential is initiated by the sinoatrial node (or pacemaker cells) and conducted along the sarcolemma. This electrical signal triggers the release of calcium ions from the sarcoplasmic reticulum, a specialized form of endoplasmic reticulum. The increase in calcium ion concentration initiates the interaction of actin and myosin, the contractile proteins of the muscle, leading to muscle contraction.

Mechanism

The mechanism of excitation-contraction coupling involves several key steps:

1. The action potential travels along the sarcolemma and down the T-tubule.
2. This triggers the release of calcium ions from the sarcoplasmic reticulum.
3. Calcium ions bind to troponin, a regulatory protein within the muscle cell.
4. This binding causes a conformational change in tropomyosin, another regulatory protein, which exposes the binding sites on actin for myosin.
5. Myosin heads bind to these exposed sites on actin, forming cross-bridges.
6. The myosin heads pivot, pulling the actin filaments towards the center of the sarcomere. This is the power stroke that causes muscle contraction.
7. ATP binds to the myosin heads, causing them to release actin and return to their original position.
8. The process repeats as long as calcium ions and ATP are available.

Role in Muscle Contraction

Excitation-contraction coupling is essential for muscle contraction. It is the process that translates the electrical signal of the action potential into the mechanical action of muscle contraction. Without this process, the electrical signals generated by the nervous system would not lead to muscle movement.

See Also

• Muscle contraction
• Sarcomere
• Calcium in biology
• Action potential