What do myofilaments consist of?

Myofilaments are fundamental components of striated muscle tissue, consisting primarily of two types of proteins: actin and myosin. These proteins work together to facilitate muscle contraction, which is a critical function in both voluntary and involuntary movements.

Actin is a thin filament that forms the backbone of the muscle's structure, providing sites for myosin heads to attach during contraction. Myosin, on the other hand, is a thick filament that has globular heads capable of binding to actin and pulling the filaments past one another. This interaction, often referred to as the sliding filament theory, is what allows muscles to contract and produce force.

The other choices mention proteins and fibers that are important in other biological contexts—such as collagen and elastin in connective tissues or elastic and reticular fibers in various tissue types—but they do not participate directly in the muscle contraction mechanism like actin and myosin do. Tropomyosin and troponin, while crucial for regulating contractions in muscle fibers, are part of the regulatory complex on the actin filament and not myofilaments per se. Thus, the answer that identifies actin and myosin as the constituents of myofilaments accurately reflects their essential role in muscle physiology.