Stimulating Contraction in Striated Muscle (Cambridge (CIE) A Level Biology): Revision Note

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

• Inward diffusion of Ca²⁺ ions 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