Final answer:
Myofibrils consist of myofilaments, specifically thick filaments made of myosin and thin filaments made of actin, which are necessary for muscle contraction. Regulatory proteins like troponin and tropomyosin are also present, but the primary components remain actin and myosin.
Step-by-step explanation:
Myofibrils are primarily composed of two types of myofilaments. The thick filaments are made of the protein myosin, while the thin filaments are largely composed of the protein actin. Myosin and actin are crucial for muscle contraction, interacting to allow muscles to shorten and generate force. Additionally, myofibrils contain regulatory proteins such as troponin and tropomyosin, which play a role in controlling muscle contraction.
The structural organization of these filaments is seen in the sarcomere, the smallest contractile unit within a myofibril. Myosin and actin are arranged in such a way that they create a striated appearance due to the alignment of their repeating units. Intracellular structures such as the sarcoplasmic reticulum help coordinate the contraction process by releasing calcium ions, and proteins like troponin sensitize the muscle to these calcium ions. Hence, given the options provided, myofibrils are composed primarily of actin and myosin (A).