Overview
This lecture covers the main types of muscle tissue, their characteristics, and focuses on the molecular mechanism of skeletal muscle contraction, including the sliding-filament model.
Types of Muscle Tissue
- There are three types of muscle tissue: cardiac, smooth, and skeletal.
- Cardiac muscle is found in the heart, is striated, branched, usually has one or two nuclei, and contracts involuntarily.
- Cardiac muscle fibers have intercalated discs that help coordinate wave-like contractions.
- Smooth muscle is found in organs like the digestive tract, blood vessels, bladder, and eyes; it is non-striated, spindle-shaped, has one nucleus, and contracts involuntarily.
- Skeletal muscle attaches to bones or skin, is striated, long, cylindrical, multinucleated, and contracts voluntarily.
Muscle Tissue Properties
- Extensibility: muscle can stretch.
- Elasticity: muscle can return to original length after stretching.
- Excitability: muscle can respond to electrical stimuli (action potentials).
- Contractility: muscle can shorten to produce movement.
Skeletal Muscle Structure & Naming
- Skeletal muscles are often named by their location or shape, frequently using Latin or Greek roots.
- The origin is where the muscle attaches to a stationary bone; insertion is where it attaches to the moving bone.
- The agonist is the main muscle causing movement; the antagonist performs the opposite action.
Sarcomere and Muscle Contraction
- Muscle fibers contain myofibrils, which have repeating units called sarcomeres.
- Sarcomeres give skeletal muscle its striated appearance.
- Sarcomeres contain thin filaments (actin) and thick filaments (myosin).
Sliding-Filament Model of Contraction
- Muscle contraction occurs when sarcomeres shorten, not by filament shortening, but by actin and myosin sliding past each other.
- Myosin heads hydrolyze ATP, bind to actin (cross bridge), perform a power stroke, and then detach when new ATP binds.
- ATP is essential for muscle contraction and relaxation; lack of ATP causes rigor mortis.
Regulation of Contraction
- Tropomyosin and troponin regulate muscle contraction by covering or exposing myosin binding sites on actin.
- When Ca2+ ions bind troponin, tropomyosin shifts, exposing binding sites, allowing contraction.
Key Terms & Definitions
- Cardiac muscle — striated heart muscle with intercalated discs; contracts involuntarily.
- Smooth muscle — non-striated, spindle-shaped muscle found in organs; contracts involuntarily.
- Skeletal muscle — striated, voluntary muscle attached to bones.
- Sarcomere — repeating structural unit in myofibrils responsible for muscle contraction.
- Actin — protein forming thin filaments in sarcomeres.
- Myosin — protein forming thick filaments in sarcomeres; has heads that bind to actin.
- Sliding-filament model — explains muscle contraction by actin and myosin sliding past each other.
- Tropomyosin — regulatory protein that blocks myosin-binding sites on actin.
- Troponin — regulatory protein complex that binds calcium and controls tropomyosin movement.
Action Items / Next Steps
- Review diagrams of muscle types and sarcomere structure.
- Study root words for muscle names to aid identification.
- Prepare for questions on the sliding-filament model and regulatory proteins.