A2 OCR Biology: Muscle Contraction Sequence

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Emily Summers
1. An action potential arrives at the muscle cell from the motor neurone, sending a
wave of depolarisation into the sarcolemma, that travels down T tubules which are
folded regions of the sarcolemma into the sarcoplasmic reticulum.
2. This causes Ca2+ ions that are stored in the sarcoplasmic reticulum to be released,
they then bind to troponin molecules in the actin filament so they change shape. This
pulls attached tropomyosin with it, so it unblocks the actinmyosin binding site.
3. This allows the globular myosin head to bind to the actinmyosin binding site, forming
a cross bridge.
4. The Ca2+ ions trigger the enzyme ATPase to break down ATP into ADP and Pi, the
energy released is used for muscle contraction
5. The myosin head moves, pulling the acting filament in a rowing action.
6. When the muscle is no longer stimulated, Ca2+ ions travel back to the sarcoplasmic
reticulum via active transport
7. The troponin molecules return to their original shape, pulling tropomyosin with it to
reblock the actinmyosin binding site
8. The myosin head cannot bind to the binding site, so no cross bridge can occur and no
muscle contraction happens
9. The sarcomere is relaxed and lengthens

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