muscle fiber, any of the cells of skeletal or cardiac muscle tissue. Skeletal muscle fibers are cylindrical polynuclear cells containing contracting myofibrils, across which run transverse striations, enclosed in a sarcolemma. Cardiac muscle fibers contain one or sometimes two nuclei and myofibrils and are separated from one another by an intercalated disk; although striated, cardiac muscle fibers branch to form an interlacing network.
