The Sarcomere and Sliding Filament Theory

Sarcomere

The functional unit of a skeletal muscle fiber; the region from one Z-line to the next Z-line; contains actin and myosin

Action (Thin Filaments)

Contractile protein with a binding site for myosin; the binding site is normally blocked by tropomyosin

Myosin (Thick Filament)

Contractile protein with head regions that bind actin to form cross-bridges during contraction; hydrolyzes ATP to generate the power stroke

Tropomyosin

Regulatory protein that blocks the myosin-binding site on actin at rest

Troponin

Regulatory protein that binds calcium ions; when Ca²⁺ binds, troponin undergoes a conformational change that moves tropomyosin, exposing the myosin-binding site on actin

Cross-Bridge

The bond formed between a myosin head and an actin binding site during muscle contraction; enables the power stroke

Power Stroke

When myosin pulls actin toward the center of the sarcomere after phosphate (Pi) is released; causes sarcomere shortening

Sliding Filament Model

Mechanism of muscle contraction where the Z-lines move closer together and the I band shortens as actin slides past myosin; the A band width remains constant

Titin, Nebulin, Dystrophin

Structural proteins of the sarcomere that maintain structure but do not participate in contraction