Which two filaments are contained within a sarcomere?

• A. Actin and Myosin
• B. Titin and Nebulin
• C. Actin and Tropomyosin
• D. Myosin and Titin

Answer: (A)

The contractile machinery of a sarcomere is built from thin and thick filament systems. The thin filaments are primarily actin, extending from each Z-disc toward the center of the sarcomere. The thick filaments are myosin, packed in the center where their heads reach outward to bind actin and generate force. When contraction is triggered, calcium alters regulatory proteins on actin to expose binding sites, allowing myosin heads to attach and pull, shortening the sarcomere. Other proteins like titin and nebulin provide structural support and help set filament length, while tropomyosin and troponin regulate access to actin’s binding sites. But the two filament types involved in the contraction process themselves are actin and myosin.