15.1.10 Stimulating Contraction in Striated Muscle

• 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