MUSCULAR CONTRACTION

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The process of muscular contraction occurs over a number of key steps, including:
  • Depolarisation and calcium ion release
  • Actin and myosin cross-bridge formation 
  • Sliding mechanism of actin and myosin filaments
  • Sarcomere shortening (muscle contraction)



1.  Depolarisation and Calcium Ion Release
  • An action potential from a motor neuron triggers the release of acetylcholine into the motor end plate
  • Acetylcholine initiates depolarisation within the sarcolemma, which is spread through the muscle fibre via T tubules 
  • Depolarisation causes the sarcoplasmic reticulum to release stores of calcium ions (Ca2+)
  • Calcium ions play a pivotal role in initiating muscular contractions
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2.  Actin and Myosin Cross-Bridge Formation
  • On actin, the binding sites for the myosin heads are covered by a blocking complex (troponin and tropomyosin)
  • Calcium ions bind to troponin and reconfigure the complex, exposing the binding sites for the myosin heads
  • The myosin heads then form a cross-bridge with the actin filaments
Resultado de imagen de Actin and Myosin Cross-Bridge Formation

3.  Sliding Mechanism of Actin and Myosin
  • ATP binds to the myosin head, breaking the cross-bridge between actin and myosin
  • ATP hydrolysis causes the myosin heads to change position and swivel, moving them towards the next actin binding site
  • The myosin heads bind to the new actin sites and return to their original conformation
  • This reorientation drags the actin along the myosin in a sliding mechanism
  • The myosin heads move the actin filaments in a similar fashion to the way in which an oar propels a row boat

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4.  Sarcomere Shortening
  • The repeated reorientation of the myosin heads drags the actin filaments along the length of the myosin
  • As actin filaments are anchored to Z lines, the dragging of actin pulls the Z lines closer together, shortening the sarcomere
  • As the individual sarcomeres become shorter in length, the muscle fibres as a whole contracts
Resultado de imagen de process of sarcomere shortening

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