Overview
This lecture explains the microscopic structure of skeletal muscle and the sequence of molecular events that lead to muscle contraction, focusing on action potentials and excitation-contraction coupling.
Structure of Skeletal Muscle
- Skeletal muscle is composed of fascicles, which contain multinucleated muscle fibers (cells).
- Muscle fibers contain myofibrils, which consist of myofilaments organized into sarcomeres.
- Sarcomeres are the contractile units defined from one Z disc to the next.
- Thick filaments (myosin) are found in A bands; thin filaments (actin) are in I bands and partially in A bands.
- Titin is an elastic filament running from the Z disc through the thick filament.
Sarcomere Anatomy
- A bands are dark regions; I bands are lighter and both contribute to muscle striation.
- The H zone is the lighter area in the center of the A band, bisected by the M line (made of myomesin).
- Myosin heads contain binding sites for ATP and actin, enabling cross-bridge formation.
- Tropomyosin blocks actin’s myosin-binding sites in relaxed muscle; troponin regulates tropomyosin’s position.
Muscle Activation & Action Potential
- The neuromuscular junction is where a motor neuron meets a muscle fiber.
- The axon terminal releases acetylcholine into the synaptic cleft, which binds to receptors on the muscle cell membrane (sarcolemma).
- Acetylcholine binding opens ion channels, letting in sodium and letting out potassium, causing depolarization.
- If depolarization reaches a threshold, an action potential is generated and travels across the sarcolemma and down T tubules.
Excitation-Contraction Coupling
- The action potential causes the sarcoplasmic reticulum to release calcium ions into the cytosol.
- Calcium binds to troponin, causing it to move tropomyosin away from actin’s binding sites.
- Myosin heads bind actin, perform power strokes using ATP, and pull actin filaments, shortening the muscle.
- When calcium levels drop, troponin and tropomyosin return to their original positions, and the muscle relaxes.
Key Terms & Definitions
- Sarcomere — The functional contractile unit of muscle between two Z discs.
- Myosin — Protein forming thick filaments with heads that bind actin and ATP.
- Actin — Protein forming thin filaments with sites for myosin binding.
- Tropomyosin — Protein blocking myosin binding sites on actin when the muscle is relaxed.
- Troponin — Calcium-binding protein complex controlling tropomyosin’s position on actin.
- Sarcoplasmic Reticulum — Organelle storing and releasing calcium ions in muscle fibers.
- Action Potential — Rapid depolarization and repolarization of the muscle cell membrane triggering contraction.
- Neuromuscular Junction — Synapse between a motor neuron’s axon terminal and a muscle fiber.
Action Items / Next Steps
- Review the differences between skeletal and smooth muscle contraction.
- Prepare for discussion on the muscular system as a whole in the next class.