Overview
This lecture explains the structure of thin filaments in muscle cells, focusing on key proteins and how they regulate muscle contraction.
Structure of Thin Filaments
- Thin filaments are primarily made up of actin protein molecules.
- Actin forms two strands that wind around each other in a double helix shape.
- Each actin molecule has a myosin binding site for muscle contraction.
Regulatory Proteins in Thin Filaments
- Tropomyosin is a protein that covers and blocks the myosin binding sites on actin.
- When tropomyosin blocks the binding sites, muscle contraction cannot occur.
- Troponin is another protein that holds tropomyosin in place over the binding sites.
- Troponin has a binding site specifically for calcium ions.
Role of Calcium in Muscle Contraction
- Calcium is stored in the sarcoplasmic reticulum inside the muscle cell.
- When calcium binds to troponin, troponin changes shape.
- The shape change in troponin moves tropomyosin away from the actin binding sites.
- Once the binding sites are exposed, myosin can attach to actin and contraction can happen.
Organization of Myofilaments
- Thick and thin filaments are not randomly arranged but are highly organized.
- This organization creates visible stripes or striations in muscle under a microscope.
- The repeating unit of organization in myofibrils is called the sarcomere.
Key Terms & Definitions
- Actin — Protein forming the main component of thin filaments with myosin binding sites.
- Myosin — Protein in thick filaments that binds to actin to cause contraction.
- Tropomyosin — Protein that blocks myosin binding sites on actin to prevent contraction.
- Troponin — Protein that holds tropomyosin in place; has a binding site for calcium ions.
- Sarcoplasmic Reticulum — Organelle in muscle cells that stores calcium ions.
- Sarcomere — The basic contractile unit of a myofibril, responsible for muscle striations.
Action Items / Next Steps
- Review how sarcomeres are structured and how they contribute to muscle contraction.