Muscle Contraction

• 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,…