Final answer:
Myofibrils are composed of smaller structures called myofilaments, which include thick filaments made of myosin and thin filaments made of actin, playing a crucial role in muscle contraction.
Step-by-step explanation:
Each myofibril contains smaller subunits called myofilaments that are made up of actin and myosin. Myofilaments can be further categorized into thick filaments and thin filaments. Thick filaments are primarily composed of the protein myosin, which interacts with actin during muscle contraction. Thin filaments mainly consist of the protein actin, which is anchored on structures called Z lines. These structures come together to form a sarcomere, which is the smallest contractile portion of a muscle fiber.
The functional organization of these myofilaments within the myofibrils allows muscles to contract. As the muscles contract, structural changes occur within the myofibril, such as the shortening of the H zone, though the A band remains the same length. The intricate arrangement of actin and myosin, alongside regulatory proteins like troponin and tropomyosin, facilitates the process of muscle contraction which is vital for movement and stability in the human body.