Muscle fiber is constructed of bundles small long organelles called what?

The story behind the answer

Myofibrils are the long, rod-like organelles that pack the interior of a muscle fiber. They run lengthwise and are built from repeating units called sarcomeres, which contain the contractile proteins actin and myosin. The precise alignment of sarcomeres across myofibrils produces the characteristic striations of skeletal muscle and enables contraction via the sliding filament mechanism.

By contrast, myocardium is the heart muscle tissue as a whole, epimysium is the outer connective tissue sheath around an entire muscle, and myofilaments are the thin and thick filaments (actin and myosin) inside each myofibril. Memory tip: “Myo-” means muscle; “-fibril” suggests a small fiber. So a myofibril is a tiny fiber