The Muscle System - Lancaster High School

Muscle cells (aka myofibers or muscle fibers) are made up of bundles of filaments called myofibrils. The filaments contained within these fibrils are made of protein. There are 2 main protein filaments you need to know. Actin is the protein that composes the thin filaments while myosin is the protein that composes the thick filaments. ................
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