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A myocyte (also known as a muscle cell or muscle fiber) is the type of cell found in muscle tissue. They are long, tubular cells that arise developmentally from myoblasts to form muscles. There are various specialized forms of myocytes: cardiac, skeletal, and smooth muscle cells, with various properties. Cardiac myocytes are responsible for generating the electrical impulses that control the heart rate, among other things.
The unusual microstructure of muscle cells has led cell biologists to create specialized terminology. However, each term specific to muscle cells has a counterpart that is used in the terminology applied to other types of cells:
|Muscle cell||Other organismal cells|
|sarcoplasmic reticulum||smooth endoplasmic reticulum (SER)|
|sarcolemma||cell membrane or plasma membrane|
Each myocyte contains myofibrils, which are very long chains of sarcomeres, the contractile units of the cell. A cell from the biceps brachii muscle may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres. The sarcomeres are composed of thin and thick filaments. Thin filaments are actin filaments, whereas thick filaments consist of an arrangement of myosin proteins. The sarcomere does not contain organelles or a nucleus. Individual muscle fibrils are surrounded by endomysium.
Within the muscle cell, the myofibrils are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle tissue, which is enclosed in a sheath of epimysium. Muscle spindles are distributed throughout the muscles and provide sensory feedback information to the central nervous system.
Skeletal muscle fibers are made when myoblasts fuse together; muscle fibers therefore have multiple nuclei (each nucleus originating from a single myoblast). The fusion of myoblasts is specific to skeletal muscle (e.g., biceps brachii) and not cardiac muscle or smooth muscle.
Myoblasts that do not form muscle fibers dedifferentiate back into satellite cells. These satellite cells remain adjacent to a muscle fiber, situated between the sarcolemma and the endomysium (the connective tissue investment that divides the muscle fascicles into individual fibers).