ANATOMY

Detailed explanation-1: -Individual myofibrils (approximately 1 m in diameter) comprise approximately 80% of the volume of a whole muscle. The number of myofibrils ranges from 50 per myocyte in the muscles of a fetus to approximately 2000 per myocyte in the muscles of an untrained adult.

Detailed explanation-2: -Just as each muscle fiber contains many myofibrils, each myofibril is in turn composed of many filaments. These filaments come in two main varieties: the thin filament and the thick filament. The major constituent of the thin filament is actin; the main component of the thick filament is myosin.

Detailed explanation-3: -Myofibrils run the entire length of the muscle fiber, and because they are only approximately 1.2 µm in diameter, hundreds to thousands can be found inside one muscle fiber. They attach to the sarcolemma at their ends, so that as myofibrils shorten, the entire muscle cell contracts (Figure).

Detailed explanation-4: -Within a muscle fiber, proteins are organized into organelles called myofibrils that run the length of the cell and contain sarcomeres connected in series. Because myofibrils are only approximately 1.2 m in diameter, hundreds to thousands (each with thousands of sarcomeres) can be found inside one muscle fiber.