Muscle Contraction


  1. Once an action potential (AP) is generated at the motor end plate it will spread like an electrical current along the sarcolemma of the muscle fiber 
  2. The AP will also spread into the T-tubules, exciting the terminal cisternae of the sarcoplasmic reticula 
  3.  This will cause Calcium (Ca+2 ) gates in the SR to open, allowing Ca+2 to diffuse into the sarcoplasm 
  4. Calcium will bind to troponin (on the thin myofilament), causing it to change its shape. This then pulls tropomyosin away from the active sites of actin molecules. 
  5. The exposure of the active sites allow the sliding of the filaments 
  6. Calcium (Ca+2 ) gates in the SR open, allowing Ca+2 to diffuse into the sarcoplasm  Myosin heads are “energized” by the presence of ADP + PO4 3- at the ATP binding site (energy is released as phosphate bond of ATP breaks) 
  7. Once the active sites are exposed, the energized myosin heads hook into actin molecules forming cross-bridges 
  8. Using the stored energy, the attached myosin heads pivot toward the center of the sarcomere 
  9.  The ADP & phosphate group are released from the myosin head 4 
  10. A new molecule of ATP binds to the myosin head, causing the cross bridge to detach from the actin strand 
  11. The myosin head will get re-energized as the ATP 
  12.  ADP+P 
  13.  As long as the active sites are still exposed, the myosin head can bind again to the next active site 
  14.  If there are no longer APs generated on the motor neuron, no more Ach will be released 
  15. AchE will remove Ach from the motor end plate, and AP transmission on the muscle fiber will end 
  16.  Ca+2 gates in the SR will close & Ca+2 will be actively transported back into the SR 
  17. With Ca+2 removed from the sarcoplasm (& from troponin), tropomyosin will re-cover the active sites of actin 
  18. No more cross-bridge interactions can form 
  19. Thin myofilaments slide back to their resting state


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