Excitation-Contraction Coupling: From Action Potential to Muscle Contraction

Excitation-contraction (E-C) coupling is the process linking sarcolemmal depolarization to myofibrillar contraction in skeletal muscle.

Steps

1. Action potential arrival at neuromuscular junction → ACh release → binds nicotinic receptors → end-plate potential
2. Sarcolemma depolarization propagates along surface and down T-tubules
3. DHPR (L-type Ca²⁺ channel) conformational change in T-tubule membrane
4. Mechanical coupling to RyR1 on sarcoplasmic reticulum (SR) → Ca²⁺ release
5. Ca²⁺ binds troponin C → tropomyosin shift → exposes myosin-binding sites on actin
6. Cross-bridge cycling → filament sliding → contraction

Electromechanical Delay (EMD)

The brief interval (~2–10 ms) between electrical stimulation (or action potential arrival) and onset of measurable force development. EMD reflects the time required for E-C coupling steps (Ca²⁺ release, diffusion, binding, cross-bridge formation).

EMD is shorter in fast-twitch fibers, longer in fatigue. It is exploited in electromechanical dynamometry and EMG-force modeling.