Sarcomere
A sarcomere is the basic component of a striated muscle's myofibril. Sarcomeres are composed of myosin and actin. A muscle cell, from a bicep, may contain 100,000 sacromeres. The myofibril of smooth muscle is not arranged into sarcomeres.
The sarcomeres are what give skeletal muscles their striated appearance, reflecting the alternating regions of isotropic and anisotropic material inside muscle cells. The Isotropic bands (or I-bands) contain actin filaments, whereas the anisotropic bands (or A-bands) contain myosin filaments and actin filaments in the overlap region. Upon muscle shortening, the A-bands maintain their length (1.6 micrometer in mammalian skeletal muscle) whereas the I-bands shorten.
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