Actin and Myosin in Muscle Contraction

A sarcomere is the smallest contractile unit of skeletal muscle, bounded by Z discs that mark sarcomere borders. It comprises two main filament types: thick filaments made of myosin II bundles and thin filaments containing actin, tropomyosin, and troponin. The overlapping arrangement of these filaments enables muscle contraction through coordinated protein interactions.

Calcium ions bind to troponin subunits attached to tropomyosin on thin filaments. This binding causes tropomyosin to shift position, exposing myosin-binding sites on actin. Once exposed, myosin heads can attach to actin and initiate the cross-bridge cycle that pulls filaments together, shortening the sarcomere.

ATP serves two critical functions in muscle contraction. First, ATP hydrolysis in the myosin head provides energy for the power stroke that pulls actin filaments toward the sarcomere center. Second, fresh ATP binding to the myosin head stops sliding and detaches myosin from actin, allowing the cycle to reset for the next contraction.

Tropomyosin strands block myosin-binding sites on actin filaments, preventing actin-myosin interactions when muscles are relaxed. Troponin, with three globular subunits binding to tropomyosin, actin, and calcium respectively, maintains this blocking position until calcium arrives. This regulatory mechanism ensures muscles contract only when signaled.

The myosin head contains a binding site for actin and an ATPase site for ATP hydrolysis. After phosphate release from the previous ATP cycle, the myosin head binds actin's globular protein, forming a cross-bridge. As ADP dissociates, the energy released pulls the actin filament inward, generating the power stroke that shortens the sarcomere.

Actin is a globular contractile protein with myosin-binding sites that enable cross-bridge formation. Myosin II has a tail region connecting to other myosin molecules to form thick filament bundles, and globular heads positioned on either side to interact with overlapping thin filaments. This complementary architecture allows coordinated sliding and force generation.

Skeletal muscle appears striated under a microscope because actin and myosin are arranged in repeating units along myofibrils. Dark A bands contain thick filaments and overlapping thin filaments, while light I bands contain only thin filaments. The Z discs separating sarcomeres create the characteristic banded pattern visible in muscle tissue.