noun, plural: myofibrils
(1) Long, cylindrical organelle in striated muscle cells, composed mainly of actin and myosin filaments that run from one end of the cell to the other.
(2) The contractile unit of a striated muscle.
The actin and myosin filaments are organized into repeated subunits along the length of the myofibril. These subunits are called sarcomeres.
Word origin: myo- (muscle) + fibrila.