Overview
This lecture explains the sequence of events that lead to skeletal muscle contraction, focusing on the molecular interactions within the sarcomere after a muscle fiber is stimulated.
Sarcomere Structure and Resting State
- Thick filaments are primarily composed of myosin, and thin filaments consist of actin covered by tropomyosin.
- Troponin holds tropomyosin in place, blocking the active sites on actin during rest.
- Myosin heads cannot bind to actin when active sites are covered.
Muscle Fiber Stimulation and Calcium Release
- An action potential travels down the motor neuron, releasing acetylcholine at the neuromuscular junction.
- Acetylcholine triggers sodium influx at the sarcolemma, generating a muscle action potential.
- The action potential moves along the sarcolemma and into the cell via transverse (T) tubules.
- T tubules interact with the sarcoplasmic reticulum at triads, where calcium is stored.
- Action potential causes calcium release from the terminal cisternae of the sarcoplasmic reticulum.
Molecular Events: Cross-Bridge Cycle
- Calcium binds to troponin, causing a shape change that moves tropomyosin and exposes actin’s active sites.
- Myosin heads bind to exposed active sites, forming cross-bridges.
- Myosin heads pivot (power stroke), pulling actin filaments toward the sarcomere center.
- Myosin releases ADP and phosphate during the power stroke and remains attached to actin.
- New ATP binds to myosin, causing it to release from actin.
- Myosin hydrolyzes ATP, returning to the cocked (ready) position for another cycle.
- The cycle repeats as long as calcium and ATP are present, resulting in sarcomere shortening and muscle contraction.
Key Terms & Definitions
- Sarcomere — The functional contractile unit of muscle fibers, defined by Z lines.
- Myosin — Motor protein making up thick filaments, interacts with actin for contraction.
- Actin — Protein forming thin filaments with binding sites for myosin.
- Tropomyosin — Protein that blocks actin’s active sites in resting muscle.
- Troponin — Protein that binds calcium and moves tropomyosin off actin’s active sites.
- Sarcoplasmic Reticulum — Organelle storing and releasing calcium for muscle contraction.
- ATP (Adenosine Triphosphate) — High-energy molecule required for myosin function during contraction.
Action Items / Next Steps
- Review muscle contraction diagrams, focusing on calcium and ATP roles.
- Study the sequence of the cross-bridge cycle for better understanding.