Overview
This lecture covers the process of muscle contraction, focusing on the roles of ATP, actin, myosin, and the sliding filament theory within the sarcomere.
ATP and Muscle Contraction
- ATP provides energy for myosin to produce movement during muscle contraction.
- ATP hydrolysis breaks a phosphate bond, energizing the myosin head.
- The energized myosin head cocks back, preparing to bind to actin.
Cross-Bridge Cycle
- Myosin head binds to actin at the myosin binding site, forming a cross-bridge.
- ADP and phosphate remain attached to the myosin head initially.
- Power stroke occurs when ADP and phosphate are released, causing myosin head to rotate and pull actin.
- The power stroke shortens the sarcomere by drawing actin toward the midline.
- Myosin detaches from actin only when a new ATP molecule binds to it.
- The contraction cycle repeats as long as ATP and calcium are present.
Sliding Filament Theory
- Sarcomere shortens during contraction, but actin and myosin filaments do not change length.
- Overlap between actin and myosin increases, reducing the distance from Z disc to Z disc.
- The A band appears larger as the degree of filament overlap increases.
- The sliding of filaments causes the entire muscle to contract as multiple sarcomeres shorten.
Key Terms & Definitions
- ATP (Adenosine Triphosphate) — energy-carrying molecule that powers myosin head movement.
- Hydrolysis — chemical breakdown involving water, used here to split ATP for energy.
- Myosin Head — motor part of the myosin protein that binds and moves actin.
- Cross-Bridge — connection between myosin head and actin during contraction.
- Power Stroke — myosin head movement pulling actin, resulting in sarcomere shortening.
- Sarcomere — basic contractile unit of muscle, from one Z disc to the next.
- Sliding Filament Theory — theory describing muscle contraction as sliding of actin and myosin past one another.
Action Items / Next Steps
- Prepare drawing materials or an iPad for the next video’s draw-along on excitation-contraction coupling.