Overview
This lecture explains the structure of skeletal muscle and details the sliding filament theory describing how muscle contraction occurs at the cellular level.
Structure of Skeletal Muscle
- Muscles are composed of bundles of muscle fibers.
- Each bundle contains 10–100 individual muscle fibers.
- Muscle fibers contain cylindrical organelles called myofibrils.
- Myofibrils are made up of protein filaments: actin and myosin.
- The sarcoplasmic reticulum surrounds myofibrils and stores calcium.
Sarcomere and Protein Filaments
- Myofibrils are divided into repeating units called sarcomeres.
- Sarcomeres contain actin (thin filament) and myosin (thick filament) arranged in a specific pattern.
Sliding Filament Theory Mechanism
- A nerve impulse releases acetylcholine at the muscle.
- Acetylcholine causes muscle depolarization, triggering calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin, altering its shape and moving tropomyosin away from actin's active sites.
- Myosin attaches to exposed actin forming a cross-bridge.
- ATP breakdown provides energy for myosin to pull actin inward, contracting the muscle.
- Binding of new ATP to myosin causes detachment from actin, breaking the cross-bridge.
- Myosin can reattach further along actin, repeating the contraction cycle (ratchet mechanism).
- Contraction continues as long as ATP and calcium are available.
Muscle Relaxation Process
- When nerve impulses stop, calcium is pumped back into the sarcoplasmic reticulum.
- Actin returns to its resting position, causing the muscle to relax and lengthen.
Key Terms & Definitions
- Myofibril — A cylindrical organelle within muscle fibers composed of actin and myosin.
- Sarcomere — The basic functional and repeating unit of a myofibril responsible for muscle contraction.
- Actin — Thin protein filament involved in muscle contraction.
- Myosin — Thick protein filament with heads that bind to actin to cause contraction.
- Sarcoplasmic Reticulum — Organelle that stores and releases calcium ions in muscle fibers.
- Acetylcholine — Neurotransmitter that initiates muscle contraction.
- ATP (Adenosine Triphosphate) — Molecule that provides energy for muscle contraction.
- Troponin — Protein that binds calcium and regulates muscle contraction.
- Tropomyosin — Protein that blocks myosin-binding sites on actin until moved by troponin.
- Cross-Bridge — Connection formed between actin and myosin during muscle contraction.
- Ratchet Mechanism — Repeated action of myosin heads pulling actin filaments for contraction.
Action Items / Next Steps
- Review the structure of skeletal muscle and key protein components.
- Study each step of the sliding filament theory.
- Memorize key terms and their functions.