Final answer:
Myofibrils contain myofilaments which are composed of three types of proteins: myosin in thick filaments, actin in thin filaments, and regulatory proteins troponin and tropomyosin that control muscle contraction.
Step-by-step explanation:
Myofibrils are composed of smaller structures called myofilaments, which are essential for muscle contraction. There are two main types of myofilaments: thick filaments and thin filaments. The thick filaments are primarily made up of the protein myosin, which is a contractile protein that interacts with actin to facilitate muscle contraction. The thin filaments are predominantly composed of the protein actin. In addition to these two major proteins, there are also regulatory proteins involved in muscle contraction: troponin and tropomyosin. These regulatory proteins are crucial in controlling the interaction between actin and myosin during the process of muscle contraction.