The myofibrils have alternate light and dark bands, which contain protein filaments responsible for the muscle's contractile ability and give the muscle its typical striped appearance under the microscope. The filaments of myofibrils, myofilaments, consist of two types, thick and thin.
Thin filaments consist primarily of the protein actin, coiled with nebulin filaments. Thick filaments consist primarily of the protein myosin, held in place by titin filaments. The protein complex composed of actin and myosin is sometimes referred to as actomyosin.