MSK – Coupling

Master answer

Excitation Contraction Coupling in SM

1. Motor neuron depolarisation
2. ACh vesicular release
3. Multiple MEPs → End plate potential
4. Muscle AP propagates via T-tubles
5. Opens L-type Ca channels
6. Influx of EC Ca → ↑ IC [Ca]
   • Variable amount of Sarcoplamic reticula in SM
   • Can have some ↑ IC [Ca] via Ca induced Ca release.
7. Ca binds Calmodulin → Ca-Calmodulin complex
8. Ca-Calmod → ↑ MLCK activity
   • → Phosphorylation of Myosin Light Chain (MLC)
   • → Activation of Myosin ATPase
9. Cross-bridge cycling.

Cross Bridge Cycling

1. “Flexed” Myosin without ATP binds to Actin.
2. Binding of ATP to Myosin causes release of Actin/Myosin complex and extension of the head
3. In the presence of Ca-Calmod complex
   • Actin binding sites are open and binding of myosin/Actin/ATP complex is formed
   • Hydrolysis of ATP to ADP + P in myosin head by Myosin ATPase
     • → conformational change “initial flexing” the myosin head and releasing phosphate
4. Release of ADP further flexes Myosin head
5. In presence of Ca return to step 2.

Relaxation

1. Ca removed from cell
   • Ca ATPase
   • Ca2+/Na+ antiport
2. Myosin Light Chain Phosphatase
   • dephosphorylates MLC → inhibition of myosin ATPase

Gladwin 2016

Answer by Gladwin