What is a cross bridge?

• A. The gap between Z-lines
• B. The diffusion channel in sarcoplasmic reticulum
• C. The connection where the myosin heads grab onto actin
• D. The boundary between sarcomeres

Answer: (C)

During muscle contraction, a cross-bridge is the connection formed when a myosin head binds to an actin filament, creating a link between the thick and thin filaments. The myosin head, part of the thick filament, attaches to exposed binding sites on actin when calcium has moved troponin-tropomyosin away from those sites. This attachment enables the power stroke, where the myosin head pivots and pulls the actin filament toward the center of the sarcomere, shortening the muscle.

ATP binding allows the myosin head to detach from actin, and the hydrolysis of ATP re-cocks the head for another cycle. The other descriptions refer to different structures: the gap between Z-lines marks the sarcomere boundaries, the diffusion channel in the sarcoplasmic reticulum isn’t the cross-bridge, and the boundary between sarcomeres is defined by those Z-lines.