Overview
This lecture covers the structure and function of skeletal muscle, focusing on muscle anatomy, sarcomere organization, the role of actin and myosin, accessory proteins, and the mechanism of muscle contraction.
Types of Muscle Cells
- Four main classes: skeletal muscle fibers, cardiac muscle cells, smooth muscle cells, and myoepithelial cells.
- This lecture focuses on skeletal muscle fibers.
Skeletal Muscle Structure
- A whole muscle is made of fasciculi (bundles), which contain muscle fibers.
- Each muscle fiber is multinucleated and contains many myofibrils.
- Myofibrils are composed of repeating units called sarcomeres.
Sarcomere and Myofilaments
- Sarcomeres are bordered by Z discs and are the functional units of contraction.
- Sarcomeres contain thick (myosin) and thin (actin) filaments.
- Myosin forms thick filaments with heads (cross-bridges) that interact with actin.
- Actin filaments are made of polymerized G-actin subunits (F-actin).
Organization and Accessory Proteins
- Actin plays roles beyond muscle, including maintaining cell shape and movement.
- Accessory proteins like alpha-actinin, tropomyosin, cap Z, and nebulin regulate actin and sarcomere stability.
- Alpha-actinin keeps actin filaments properly spaced; tropomyosin stabilizes actin and blocks binding sites.
- Nebulin acts as a molecular ruler to determine actin filament length.
- Titan acts as a molecular spring, maintaining sarcomere structure during stretching.
Muscle Contraction Mechanism
- Muscle contraction occurs by sliding of myosin filaments past actin filaments, shortening the sarcomere.
- The length of actin and myosin filaments does not change; they simply slide past each other.
- Myosin heads (cross-bridges) attach to actin, perform a power stroke, then detach and repeat.
Cross-Bridge Cycling (Walk Along Theory)
- ATP binds to myosin head, causing it to release from actin.
- ATP hydrolysis resets the myosin head to its starting position.
- Release of phosphate allows weak binding to a new site on actin.
- Release of phosphate triggers the power stroke and ADP release.
Regulation by Troponin and Calcium
- The troponin complex regulates actin-myosin interaction: Troponin I inhibits, Troponin T binds tropomyosin, Troponin C binds calcium.
- Calcium binding to Troponin C moves tropomyosin, exposing binding sites for myosin and allowing contraction.
Key Terms & Definitions
- Sarcomere — functional contractile unit between two Z discs in muscle.
- Myosin (thick filament) — motor protein capable of cross-bridge cycling.
- Actin (thin filament) — protein forming filaments for myosin binding.
- Cross-bridge — myosin head binding to actin, enabling contraction.
- Alpha-actinin — accessory protein spacing actin filaments.
- Tropomyosin — protein stabilizing actin, blocking myosin binding sites.
- Nebulin — molecular ruler for actin length.
- Titan — spring-like protein maintaining sarcomere structure.
- Troponin complex — regulates contraction via calcium binding.
Action Items / Next Steps
- Watch the recommended videos on the course page for visual explanations of sarcomere structure and cross-bridge cycling.