Actin filaments are sliding past myosin filaments. To view an animation showing myofilament contraction, click here.
Excitation of skeletal muscle occurs as an action potential emitted by a motor neuron moves along the sarcolemma (plasma membrane of a muscle cell) and down into invaginations of the membrane called T-tubules.
As an action potential sweeps through the T-tubules, calcium channels on the sarcoplasmic reticulum are triggered to open within the cell, releasing calcium. Increased intracellular calcium is then free to bind to the regulatory protein, troponin (blocks the myosin binding sites on the actin filament), causing a conformational change (change in shape).
Troponin is associated with another regulatory protein, tropomyosin, which wraps helically around the actin filament, and covers the myosin binding sites on actin (double security mechanism).
Contraction of skeletal muscle proteins occurs when the binding sites on actin are uncovered, allowing the myosin proteins to bind to the actin, and undergo a powerstroke.
During a powerstroke, myosin heads use the energy from ATP to bind to and pull the actin filaments together across the H-zone, resulting in the actin filaments sliding past the myosin filament.
During this contraction, the H-zone decreases and the sarcomere shortens.
To learn more, refer to this ATP Muscle Lab from Moodle.wpcp.org.