Overview
This lecture covers the structure and function of muscle tissue, focusing on the sliding filament theory of muscle contraction, the roles of various proteins and ions, and the differences among skeletal, cardiac, and smooth muscles.
Muscle Contraction: Sliding Filament Theory
- Muscle contraction involves the sliding of actin (thin filament) and myosin (thick filament) past each other.
- Four phases: excitation, excitation-contraction coupling, contraction, and relaxation.
- Excitation begins with a nerve impulse that triggers the release of acetylcholine (ACh) at the neuromuscular junction.
- ACh binds to the sarcolemma, opening chemically gated ion channels, allowing sodium in and potassium out, creating an end plate potential (localized depolarization).
- Nearby voltage-gated channels open, producing an action potential that moves along the sarcolemma and into T-tubules.
- Action potential triggers calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin, causing tropomyosin to shift and expose actin’s binding sites.
- ATPase on myosin hydrolyzes ATP, cocking the myosin head, leading to cross-bridge formation with actin.
- Power stroke occurs as myosin pulls actin, causing muscle contraction; numerous myosin heads act asynchronously.
- Muscle relaxation involves acetylcholinesterase breaking down ACh and ATP-driven reuptake of calcium into the sarcoplasmic reticulum, allowing tropomyosin to cover actin’s binding sites.
Muscle Types and Characteristics
- Skeletal muscle: striated, voluntary, contracts quickly, fatigues easily, requires nerve stimulation.
- Cardiac muscle: striated, involuntary, can contract without nerve input, has intercalated discs.
- Smooth muscle: non-striated, involuntary, can contract without nerve input, found in organs like the stomach and bladder.
- Muscle tissue characteristics: excitability (responds to stimuli), contractility (shortens), extensibility (stretches), elasticity (returns to original length).
- Functions include movement, posture, joint stabilization, heat generation, and organ protection.
Muscle Cell Structure
- Muscle fibers (cells) are long and multinucleated with striations.
- Bundles of muscle fibers form fascicles, surrounded by perimysium; fascicles bundle into muscles, surrounded by epimysium.
- Each fiber is wrapped in endomysium.
- The sarcomere (between two Z lines) is the contractile unit with overlapping actin and myosin.
- Sarcolemma: muscle cell membrane; sarcoplasm: muscle cell cytoplasm; sarcoplasmic reticulum: stores calcium; T-tubules: transmit action potential.
Neuromuscular Junction and Ionic Events
- The neuromuscular junction is where a nerve terminal meets a muscle fiber.
- At rest, muscle cells are polarized (negative inside).
- Depolarization via sodium influx triggers contraction; repolarization is needed for relaxation.
- Fluctuations in calcium and ATP are crucial for contraction-relaxation cycles.
Clinical Relevance and Examples
- Acetylcholinesterase inhibitors (e.g., in pesticides) cause continuous contraction (spasms and paralysis).
- Tetanus toxin causes overstimulation and continuous contraction (can be fatal).
- Curare blocks ACh receptors, causing muscle relaxation (used as a muscle relaxant).
Key Terms & Definitions
- Actin — thin protein filament involved in contraction.
- Myosin — thick protein filament with heads forming cross-bridges.
- Sarcolemma — muscle cell membrane.
- Sarcomere — contractile unit of muscle cell.
- T-tubule — invagination of sarcolemma conducting action potentials.
- Sarcoplasmic Reticulum — muscle cell organelle storing calcium.
- Acetylcholine (ACh) — neurotransmitter triggering muscle contraction.
- Acetylcholinesterase — enzyme breaking down ACh for relaxation.
- Troponin/Tropomyosin — regulatory proteins controlling actin’s binding sites.
Action Items / Next Steps
- Watch the posted muscle contraction videos.
- Study the steps in the sliding filament theory.
- Review muscle cell and tissue structure diagrams.
- Prepare for Exam 3 by mastering muscle contraction mechanisms.