Overview
This lecture covers the structure and types of muscle tissue, focusing on skeletal muscles and the mechanism of muscle contraction via the sliding-filament model.
Types of Muscle Tissue
- Muscle tissue is made of muscle fibers (cells) specialized for contraction.
- There are three types: cardiac, smooth, and skeletal muscle tissue.
- Cardiac muscle is found in the heart, is striated and branched, has 1–2 nuclei per fiber, contains intercalated discs, and is involuntary.
- Smooth muscle lacks striations, has spindle-shaped fibers with one nucleus, is found in organs like the digestive tract and blood vessels, and is involuntary.
- Skeletal muscle is striated, multinucleated, cylindrical, attaches to bone or skin, and is under voluntary control.
General Properties of Muscle Tissue
- Extensibility: can stretch or extend.
- Elasticity: can return to original length after stretching.
- Excitability: can respond to stimulation by generating electrical impulses.
- Contractility: can contract to produce movement.
Skeletal Muscle Structure & Naming
- Skeletal muscles often named by location, shape, or Latin/Greek roots (e.g., rectus abdominis, deltoid).
- Attachment point moved by contraction is the insertion; the fixed point is the origin.
- The agonist (prime mover) creates movement; antagonists oppose movement.
Microscopic Anatomy of Skeletal Muscle
- Muscle fibers contain myofibrils, which consist of repeating sarcomeres.
- Sarcomeres give skeletal muscles their striated appearance.
- Sarcomeres are composed of thin filaments (actin) and thick filaments (myosin).
Muscle Contraction: Sliding Filament Model
- Muscle contraction occurs when sarcomeres shorten, not by filament shortening but by sliding past each other.
- Z lines are drawn closer as thin filaments slide inward, powered by myosin heads.
- Myosin heads hydrolyze ATP, bind actin (cross bridge), perform a power stroke, and detach with new ATP.
Regulation of Muscle Contraction
- Tropomyosin blocks myosin-binding sites on actin; troponin complex controls this blockade.
- Calcium ions released upon nerve stimulation bind troponin, moving tropomyosin and allowing contraction.
Key Terms & Definitions
- Muscle fiber — A single muscle cell specialized for contraction.
- Striated — Showing a striped pattern, as seen in skeletal and cardiac muscle.
- Sarcomere — The structural and functional unit of a myofibril.
- Actin — Protein forming thin filaments in muscle fibers.
- Myosin — Protein forming thick filaments in muscle fibers.
- Cross bridge — Connection formed when myosin head binds to actin.
- Power stroke — Action of myosin pulling actin during contraction.
- Tropomyosin — Protein blocking myosin-binding sites on actin at rest.
- Troponin — Protein complex regulating tropomyosin’s position on actin.
- Agonist — Muscle responsible for a specific movement.
- Antagonist — Muscle opposing the agonist’s action.
Action Items / Next Steps
- Review muscle diagrams and naming conventions.
- Study the sliding filament model steps and associated proteins.
- Check the root words of muscle names for easier identification.