Final answer:
The fibers coming off the M line are myosin fibers, while the fibers extending off the Z line are actin fibers.
Step-by-step explanation:
The fibers that come off the M line are myosin fibers. Myosin is a protein that is responsible for muscle contraction. It forms thick filaments within the muscle fibers.
The fibers that extend off of the Z line are actin fibers. Actin is another protein that forms thin filaments within the muscle fibers. These actin fibers interact with the myosin fibers during muscle contraction.