(1) Inside the fibre are the myofibrils, which constitute the contractile apparatus, and a system for controlling the myofibrils through changes in calcium concentration.(2) Individual muscle cells are extremely elongated and enclose many-stranded cables woven from threads called myofibrils .(3) In cross section, skeletal muscle fibers are seen to be composed of numerous small aggregates of contractile substance, the myofibrils .(4) The number of myofibrils within the enlarged muscle fiber can increase more than 10-fold.(5) Briefly, the cardiac myofibrils are composed of the major contractile proteins, actin and myosin.
