Overview
This lecture covers the types of muscle tissue, their features, and a detailed explanation of how skeletal muscle contraction works at the cellular level.
Types of Muscle Tissue
- Three muscle tissue types: cardiac, smooth, and skeletal.
- Cardiac muscle is found in the heart, is striated, branched, sometimes has two nuclei, and contains intercalated discs; control is involuntary.
- Smooth muscle is non-striated, spindle-shaped, single nucleus, found in organs like the digestive system and blood vessels; control is involuntary.
- Skeletal muscle is striated, multinucleated, long cylinders, attaches to bone or skin, and is under voluntary control.
Muscle Tissue Characteristics
- All muscle tissues are extensible (can stretch), elastic (return to original shape), excitable (respond to stimuli), and contractile (can shorten to produce force).
Structure of Skeletal Muscle
- Skeletal muscles are made of muscle fibers (cells), which contain many myofibrils.
- Myofibrils are composed of repeating units called sarcomeres, which create the striated appearance.
- Muscles attach to bones at two points: origin (fixed) and insertion (moves).
- Agonists (prime movers) produce movement; antagonists oppose action.
The Sarcomere and Contraction
- Sarcomeres contain thin (actin) and thick (myosin) filaments.
- Thin filaments (actin) attach to Z lines; thick filaments (myosin) are held by M lines.
- Muscle contraction occurs when sarcomeres shorten due to sliding of thin and thick filaments past each other.
- Myosin has heads that bind to actin, forming cross bridges.
The Sliding Filament Model
- Myosin heads hydrolyze ATP to bind to actin, perform a power stroke to pull actin, and release ADP and phosphate.
- A new ATP binds to myosin, allowing it to detach from actin.
- ATP is essential for both contraction and relaxation; lack of ATP causes rigor mortis.
Regulation of Contraction
- Tropomyosin blocks myosin binding sites on actin, preventing contraction.
- Troponin complex, upon binding calcium ions, moves tropomyosin to expose binding sites.
- Neuronal stimulation releases calcium ions, triggering contraction.
Key Terms & Definitions
- Striated — having a striped appearance due to sarcomere arrangement.
- Extensibility — tissue can be stretched.
- Elasticity — tissue returns to original length.
- Excitability — ability to respond to electrical stimuli.
- Contractility — ability to shorten and produce force.
- Sarcomere — the basic contractile unit in muscle fibers.
- Actin — thin filament protein in sarcomeres.
- Myosin — thick filament protein in sarcomeres.
- Cross bridge — connection formed between myosin head and actin.
- Tropomyosin — protein blocking binding sites on actin.
- Troponin — regulatory protein complex that binds calcium and moves tropomyosin.
- ATP (Adenosine Triphosphate) — energy molecule needed for muscle contraction and relaxation.
Action Items / Next Steps
- Review diagrams of skeletal muscle to learn names and locations.
- Study the sliding filament model and the role of regulatory proteins in contraction.
- Check root word definitions for common muscle names for easier identification.