Overview
This lecture focuses on the detailed anatomy and function of skeletal muscle, highlighting its structure, connective tissues, cellular components, and the mechanism of muscle contraction.
Types of Muscle Tissue
- Skeletal muscle is voluntary, striated, and moves bones.
- Cardiac muscle is striated but involuntary and found only in the heart.
- Smooth muscle is involuntary and non-striated.
Structure of Skeletal Muscle
- Muscles attach to bones via tendons (rope-like) or aponeuroses (sheet-like).
- Each muscle has an origin (fixed attachment) and insertion (movable attachment).
- Muscles are covered by fascia and connective tissue layers: epimysium (entire muscle), perimysium (fascicles), and endomysium (individual muscle fibers).
- Fascicles are bundles of muscle fibers (cells); each fiber is multinucleated.
Muscle Fiber Organization
- Muscle fibers contain myofibrils, which are composed of repeating units called sarcomeres.
- Sarcomeres are the functional units of contraction, bordered by Z lines.
- Thick filaments (myosin) are anchored at the M line; thin filaments (actin) are anchored at the Z line.
- Myosin heads pull actin filaments toward the M line, shortening the sarcomere (sliding filament theory).
Sarcomere Bands and Proteins
- A band: dark, contains thick filaments (myosin), some overlap with actin.
- I band: light, contains thin filaments (actin) only.
- H band: zone within A band with only myosin.
- Titan is an elastic protein that maintains sarcomere structure and prevents overstretching.
Muscle Cell Terminology
- Sarcolemma: muscle cell membrane.
- Sarcoplasm: muscle cell cytoplasm.
- Sarcoplasmic reticulum: specialized endoplasmic reticulum storing calcium ions.
- T-tubules: invaginations of sarcolemma that transmit action potentials into the cell.
Electrical Properties and Contraction Initiation
- Resting membrane potential of muscle cells is about -90 mV.
- Sarcolemma and motor neurons are excitable membranes that can conduct action potentials.
- Action potentials are necessary for muscle contraction.
Neuromuscular Junction
- Motor neurons signal muscle fibers at the neuromuscular junction.
- The motor endplate is the specialized region of sarcolemma receiving the signal.
Detailed Sarcomere Anatomy
- Myosin filaments have projecting heads that interact with actin during contraction.
- Actin filaments are composed of F-actin (fibrous) and G-actin (globular) subunits with active sites.
- Tropomyosin covers actin's active sites (acting as a gate).
- Troponin holds tropomyosin in place (acting as a lock).
- Calcium binds troponin, causing tropomyosin to reveal active sites, allowing contraction.
Key Terms & Definitions
- Tendon — Rope-like dense connective tissue attaching muscle to bone.
- Aponeurosis — Sheet-like connective tissue attachment for muscles.
- Origin — Fixed muscle attachment point.
- Insertion — Movable muscle attachment point.
- Fascicle — Bundle of muscle fibers.
- Sarcomere — Functional contractile unit of muscle fiber.
- Epimysium — Outer layer covering the whole muscle.
- Perimysium — Connective tissue surrounding fascicles.
- Endomysium — Connective tissue surrounding each muscle fiber.
- Sarcolemma — Muscle cell membrane.
- Sarcoplasm — Cytoplasm of a muscle cell.
- Sarcoplasmic reticulum — Organelle storing calcium in muscle cells.
- T-tubules — Transverse tubules transmitting signals into muscle cells.
- Myofibril — Bundle of myofilaments (actin and myosin) inside muscle fibers.
- Troponin — Protein that binds calcium and regulates muscle contraction.
- Tropomyosin — Protein that blocks actin's active sites at rest.
- Motor endplate — Specialized sarcolemma region at the neuromuscular junction.
Action Items / Next Steps
- Review diagrams showing the structure of muscle tissue and sarcomeres.
- Study the sliding filament theory and the role of calcium in contraction.
- Prepare for upcoming sections on the physiological mechanisms of muscle contraction.