Muscle Contraction

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  • A skeletal muscle, or muscle fibre, contains many nuclei and is several centimetres long.
  • The cytoplasm contains blocks of parellel structures called myofibrils, which are bundles of thin actin and thick myosin filaments.
  • One block is called a sarcomere.
  • The cell surface membrane, or sarcolemma , has inturnings called transverse tubules , which surround the myofibrils.
  • The endoplasmic reticulum is specialised as a sarcoplasmic reticulum into which calcium ions are pumped, using ATP, when the muscle is relaxed.

Nerve Impulse at Neuromuscular Junction

  • The sarcolemma of the muscle fibre is depolarised, and an action potential spreads across it, inclusing the membranes of transverse tubules.
  • This depolarises the membrane of the sarcoplasmic reticulum, which becomes permeable to its enclosed calcium ions.
  • Calcium ions flood into the cytoplasm and bind to a protein associated with the actin filaments.
  • This makes it possible for myosin to bind to actin,


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