Overview
This lecture covers the structure, arrangement, and function of skeletal muscle fibers, including key components from gross anatomy to microscopic details, and highlights the clinical relevance of Duchenne muscular dystrophy.
Skeletal Muscle Organization
- Skeletal muscle fibers (cells) are long, multinucleated, voluntary, and attached to bones by tendons.
- Groups of muscle fibers are bundled together and surrounded by endomysium (extracellular matrix).
- Bundles of muscle fibers form a fascicle, surrounded by perimysium.
- Multiple fascicles are grouped to create a whole muscle, surrounded by epimysium (outermost layer).
Muscle Fiber Microanatomy
- The sarcolemma is the plasma membrane of the muscle cell; sarcoplasm is its cytoplasm.
- Inside a muscle fiber are many myofibrils, each made up of repeating sarcomeres end to end.
- Myofibrils are surrounded by sarcoplasmic reticulum (SR), which stores and releases calcium.
- T-tubules are extensions of the sarcolemma that interact with the SR to transmit depolarization.
Sarcomere Structure and Function
- Sarcomeres are bordered by Z-lines and contain thick filaments (myosin) and thin filaments (actin).
- Myosin heads pull actin filaments toward the sarcomere midline, shortening the muscle.
- Regulatory proteins include troponin (binds calcium) and tropomyosin (blocks myosin binding sites on actin).
- Titin proteins act as elastic "springs," maintaining sarcomere elasticity and alignment.
Clinical Connection: Duchenne Muscular Dystrophy
- Duchenne muscular dystrophy is caused by a genetic defect in dystrophin, a protein connecting the sarcolemma to myofibrils.
- Without dystrophin, muscle fibers degenerate, are replaced by non-contractile connective tissue, and lead to progressive muscle weakness.
- Both skeletal and cardiac muscles are affected, leading to life-threatening complications.
Key Terms & Definitions
- Skeletal Muscle Fiber (Cell) — Long, multinucleated, voluntary muscle cell.
- Endomysium — Connective tissue surrounding each muscle fiber.
- Fascicle — Bundle of muscle fibers.
- Perimysium — Connective tissue wrapping each fascicle.
- Epimysium — Outer connective tissue surrounding the whole muscle.
- Sarcolemma — Plasma membrane of a muscle cell.
- Sarcoplasm — Cytoplasm of a muscle cell.
- Myofibril — Rod-like organelle composed of repeating sarcomeres.
- Sarcoplasmic Reticulum (SR) — Specialized endoplasmic reticulum that stores/releases calcium.
- T-tubule — Tubular invaginations of the sarcolemma; transmit action potentials.
- Sarcomere — Functional contractile unit of myofibril.
- Myosin — Thick filament protein responsible for contraction.
- Actin — Thin filament protein pulled by myosin.
- Troponin — Regulatory protein that binds calcium.
- Tropomyosin — Regulatory protein that blocks myosin binding sites on actin.
- Titin — Elastic protein maintaining sarcomere structure.
- Dystrophin — Protein anchoring sarcolemma to myofibrils; defective in Duchenne muscular dystrophy.
Action Items / Next Steps
- Review Chapter 10 for detailed muscle anatomy.
- Prepare for the next lecture on muscle cell excitability and the neuromuscular junction.