ANATOMY

Detailed explanation-1: -Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin.

Detailed explanation-2: -– The A-band contains both thick and thin filaments and is the center of the sarcomere that spans the H zone. During contraction, the H-zone, I-band, the distance between Z-lines, and the distance between M-lines all become smaller. However, the A band’s size remains constant during contraction.

Detailed explanation-3: -During contraction, the myosin heads extend from the thick filament surface and attach to actin in the thin filaments, causing the filaments to slide past each other in a cyclic, ATP-driven process.

Detailed explanation-4: -The myosin and actin filaments overlap in peripheral regions of the A band, whereas a middle region (called the H zone) contains only myosin. The actin filaments are attached at their plus ends to the Z disc, which includes the crosslinking protein -actinin.