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A myocyte (also known as a muscle cell or muscle fiber) is the type of cell found in muscle tissue. Myocytes are long, tubular cells that develop from myoblasts to form muscles in a process known as myogenesis. 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 muscle fiber is surrounded by a layer of connective tissue called the endomysium.
The sarcoplasm is the cytoplasm of a muscle fiber. Most of the sarcoplasm is filled with myofibrils, which are long protein cords composed of myofilaments. The sarcoplasm is also composed of glycogen, which provides energy to the cell with heightened exercise, and myoglobin, the red pigment that stores oxygen until needed for muscular activity.
There are three types of myofilaments:
Thick filaments are composed of protein molecules called myosin. In striations of muscle bands, these are the dark filaments that make up the A band.
Thin filaments are composed of protein molecules called actin. In striations of muscle bands, these are the light filaments that make up the I band.
Elastic filaments are composed of titin, a large springy protein; these filaments anchor the thick filaments to the Z disc.
The sarcoplasmic reticulum(SR), also known as the smooth endoplasmic reticulum, forms a network around each myofibril of the muscle fiber. This network is composed of groupings of two dilated end-sacs called terminal cisternae, and a single transverse tubule, or T tubule, which bore through the cell and emerge on the other side; together these three components form the triads that exist within the network of the sarcoplasmic reticulum, in which each T tubule has two terminal cisternae on each side of it. The SR serves as reservoir for calcium ions, in which the T tubule signals the SR to release calcium from the gated membrane channels to stimulate a muscle contraction. 
The sarcolemma is the plasma membrane of a muscle fiber. Within the muscle fiber pressed against the sarcolemma are multiple flattened nuclei; this multinuclear condition results from multiple myoblasts fusing to produce each muscle fiber where each myoblast contributes one nucleus. 
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 myocytes are surrounded by endomysium.
Myocytes 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).