• Striated muscle?contracts?when it receives an?impulse?from a?motor neurone?via the?neuromuscular junction
• When an impulse travelling along the axon of a motor neurone arrives at the?presynaptic membrane, the action potential causes?calcium ions to diffuse?into the neurone
• This stimulates?vesicles?containing the neurotransmitter?acetylcholine?(ACh) to?fuse?with the presynaptic membrane
• The ACh that is released diffuses across the neuromuscular junction and?binds to receptor proteins?on the?sarcolemma?(surface membrane of the muscle fibre cell)
• This stimulates?ion channels?in the sarcolemma to?open, allowing?sodium ions?to?diffuse in
• This?depolarises the sarcolemma,?generating an?action potential?that passes down the?T-tubules?towards the centre of the muscle fibre
• These action potentials cause?voltage-gated calcium ion channel proteins?in the membranes of the?sarcoplasmic reticulum?(which lie very close to the T-tubules) to?open
• Calcium ions?diffuse?out of the sarcoplasmic reticulum?(SR) and?into the sarcoplasm?surrounding the myofibrils
• Calcium ions?bind to troponin molecules, stimulating them to?change shape
• This causes the?troponin?and?tropomyosin?proteins to?change position?on the thin (actin) filaments
• The?myosin-binding sites are exposed?on the actin molecules
• The process of muscle contraction (known as the sliding filament model) can now begin

How the myofibrils within muscle fibres are stimulated to contract