CLAUS LUNAU / SCIENCE PHOTO LIBRARY CLAUS LUNAU / SCIENCE PHOTO LIBRARY
Human muscle fibres. Diagram showing the structure of human muscles from the cellular to the macroscopic level. At lower right, muscle cells contain many myofibrils, organelles that are composed of units (sarcomeres) of actin and myosin bound together end to end. It is the contraction of these units (red arrows, bottom right) that occurs when muscles contract. The myofibrils are secured at either end to the cell membrane. Many cells are then bound together to form a muscle fibre (middle) and many fibres are in turn bound to form muscle tissue (top). Layers of muscle are shown on the human figure at upper left.
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