Overview
This lecture explains how skeletal muscle contracts at the molecular level, detailing the muscle structure, sarcomere organization, the role of ions, and the action potential that triggers contraction.
Muscle Structure and Organization
- Skeletal muscles are made of fascicles, composed of multinucleated muscle fibers (muscle cells).
- Muscle fibers contain myofibrils, which consist of myofilaments arranged into contractile units called sarcomeres.
- Sarcomeres are bordered by Z discs, with A bands (dark), I bands (light), H zones (in A band center), and M lines (center of H zone).
- Thick filaments contain myosin, thin filaments contain actin, and elastic filaments consist of titin.
Sarcomere and Myofilament Details
- Myosin heads have ATP and actin binding sites; cross bridges form during contraction.
- Actin filaments have binding sites blocked by tropomyosin during relaxation.
- Troponin complex binds actin, tropomyosin, and calcium, controlling access to binding sites.
Supporting Structures
- The sarcoplasmic reticulum (SR) surrounds myofibrils and regulates calcium storage and release.
- T tubules encircle sarcomeres at A band–I band junctions, ensuring signal transmission throughout the muscle fiber.
Sliding Filament Model of Contraction
- Nerve stimulation causes myosin heads to pull actin filaments toward the sarcomere's center, shortening the muscle.
- The I band shortens, H zone disappears, and A bands move closer together, resulting in contraction.
Neuromuscular Junction and Action Potential
- The neuromuscular junction is the interface between a neuron and muscle fiber.
- Acetylcholine (ACh) released from axon terminals binds receptors on the muscle sarcolemma, opening ion channels.
- Sodium ions enter the cell, causing depolarization and initiating an action potential.
Depolarization, Repolarization, and Muscle Contraction
- Depolarization opens more sodium channels, spreading the action potential along the sarcolemma and T tubules.
- Acetylcholinesterase breaks down ACh, closing ion channels.
- Potassium exits the cell during repolarization, restoring the negative membrane potential.
- Action potential triggers calcium release, calcium binds troponin, tropomyosin moves, and myosin-actin cross bridging occurs.
Relaxation and Review
- When calcium decreases, troponin returns to its original shape, tropomyosin blocks actin, and the muscle relaxes.
- Key steps: nerve signal → ACh release → ion movement → action potential → calcium release → contraction.
Key Terms & Definitions
- Sarcomere — functional contractile unit of muscle, bordered by Z discs.
- Myosin — thick filament protein with heads for ATP and actin binding.
- Actin — thin filament protein with myosin binding sites.
- Tropomyosin — protein blocking actin’s binding sites during relaxation.
- Troponin — calcium-binding protein regulating tropomyosin on actin.
- Sarcoplasmic Reticulum (SR) — tubular network storing and releasing calcium.
- T Tubule — membrane channel transmitting action potentials into the muscle fiber.
- Neuromuscular Junction — synapse between neuron and muscle fiber.
- Acetylcholine (ACh) — neurotransmitter triggering muscle contraction.
- Depolarization — reduction in membrane potential making the inside less negative.
- Repolarization — return of membrane potential to resting state.
Action Items / Next Steps
- Review the lecture basics before moving on to muscular system overviews.
- Prepare for upcoming discussions on time frames of contraction and differences in muscle types.