myofibril

myofibril /-fī″bril/ [Gk, mys + L, fibrilla, small fiber] , a slender striated strand within skeletal and cardiac muscle fibers and composed of bundles of myofilaments. Myofibrils occur in groups of branching threads running parallel to the cellular long axis of the fiber.

Molecular structure of a myofibril (Patton and Thibodeau, 2016)