Overview
This lecture explains the structure and function of sarcomeres, the basic units of muscle contraction, and describes how muscle contraction occurs at the molecular level.
Sarcomere Structure
- A sarcomere is the basic contractile unit of muscle, repeating along myofibrils.
- Myofibrils are bundles within muscle cells, perfectly aligned to create striations (alternating dark and light bands).
- Dark bands (A bands) contain thick filaments and appear darker due to overlapping proteins.
- Light bands (I bands) contain only thin filaments.
- Z-disks (or Z-lines) are found in the middle of I bands and define the boundaries of a sarcomere.
- Thousands of sarcomeres line up end-to-end in a muscle fiber, enabling contraction.
Myofilaments Within the Sarcomere
- Thick myofilaments are made of the protein myosin.
- Thin myofilaments are made of the protein actin.
- The A band contains both myosin and actin filaments.
- Regions with only myosin in the A band form the H zone.
- The M line is the middle of the sarcomere, anchoring myosin filaments.
Muscle Contraction Mechanism
- Muscle contraction occurs when myosin heads bind to actin's active sites and pull, shortening the sarcomere—a process called the sliding filament theory.
- Myosin molecules have long tails, hinge regions, and two globular heads that attach to actin.
- Actin consists of two strands of subunits twisted together.
Regulatory Proteins and Role of Calcium
- Tropomyosin blocks the active sites on actin, preventing myosin binding at rest.
- Troponin acts as a clamp, holding tropomyosin in place.
- Calcium binds to troponin, causing tropomyosin to shift and expose actin's active sites.
- When active sites are exposed, myosin heads bind actin, enabling contraction if ATP is available.
- Muscle relaxation occurs when calcium is removed and tropomyosin blocks the active sites again.
Key Terms & Definitions
- Sarcomere — the fundamental unit of muscle contraction between two Z-disks.
- Myofilaments — protein filaments (myosin and actin) within the sarcomere.
- A band — dark band containing thick (myosin) and thin (actin) filaments.
- I band — light band containing only thin (actin) filaments.
- Z-disk — boundary of a sarcomere anchoring thin filaments.
- H band — region of A band with only myosin.
- M line — center of sarcomere anchoring myosin filaments.
- Tropomyosin — protein blocking actin's active sites.
- Troponin — protein that binds calcium and shifts tropomyosin.
- Sliding filament theory — explains muscle contraction via myosin sliding along actin.
Action Items / Next Steps
- Review diagrams of sarcomere structure and the sliding filament theory.
- Memorize the roles of myosin, actin, troponin, and tropomyosin.
- Understand the sequence of events in muscle contraction and relaxation.