Muscle Cell Function

Striated or skeletal muscle is attached to bone and makes movement possible. A muscle’s structural pattern is a series of increasingly smaller parallel units. The muscle is composed of fascicles. Each fascicle consists of several fibers. Each fiber is an elongated cell with many nuclei. Within each fiber are myofibrils composed of thick and thin filaments made of protein. The regular arrangement of these filaments gives striated muscle its striped appearance. The basic functional unit of a muscle is the sarcomere, a section of a myofibril. One is shown here, bordered by the crooked blue lines. A sarcomere is composed of thick filaments of myosin, in red, and thin filaments of actin, in blue. Muscle contraction is the result of these thick and thin filaments sliding past one another.
At the start of contraction, stored calcium ions are released into the cytoplasm.
Calcium ions expose binding sites on the actin molecules.
Globular heads on the myosin can bind to these sites, forming cross bridges.
Repeated binding and release moves the filaments relative to one another and as this occurs simultaneously in many sarcomeres, the entire muscle shortens or contracts.
This process requires energy in the form of ATP.

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