Overview
This lecture explains the structure of skeletal muscle and details the sliding filament theory, which describes how muscles contract at the cellular level.
Structure of Skeletal Muscle
- Skeletal muscle consists of bundles of muscle fibers containing 10 to 100 individual fibers each.
- Each muscle fiber contains myofibrils, which are made up of actin and myosin protein filaments.
- Myofibrils are surrounded by the sarcoplasmic reticulum, which stores calcium ions.
- Myofibrils are divided into repeating units called sarcomeres, the functional unit of contraction.
Sliding Filament Theory
- Sarcomeres contain actin (thin filaments) and myosin (thick filaments).
- Muscle contraction begins when a nerve impulse releases acetylcholine at the muscle.
- Acetylcholine causes depolarization, leading to calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin, changing its shape and moving tropomyosin, exposing actin’s active sites.
- Myosin heads attach to actin, forming cross bridges.
- ATP breakdown provides energy for myosin to pull actin inward, shortening the sarcomere (contraction).
- A new ATP molecule binds to myosin, causing it to detach from actin and break the cross bridge.
- This cycle repeats as long as ATP and calcium are available, known as the ratchet mechanism.
- When the nerve impulse stops, calcium is pumped back into the sarcoplasmic reticulum, and the muscle relaxes.
Key Terms & Definitions
- Myofibril — cylindrical organelle in muscle fibers, made of actin and myosin.
- Sarcomere — the repeating functional unit within myofibrils responsible for contraction.
- Actin — thin protein filament involved in muscle contraction.
- Myosin — thick protein filament that pulls on actin during contraction.
- Sarcoplasmic reticulum — network storing calcium ions in muscle cells.
- Acetylcholine — neurotransmitter that initiates muscle contraction.
- Troponin — protein that binds calcium, facilitating access to actin binding sites.
- Tropomyosin — protein blocking actin’s active sites until moved by troponin.
- Cross bridge — attachment between myosin head and actin filament.
- ATP (Adenosine triphosphate) — molecule providing energy for muscle contraction.
Action Items / Next Steps
- Review the structure and function of sarcomeres, actin, and myosin.
- Study diagrams illustrating the sliding filament theory process.
- Prepare for questions on the roles of calcium, ATP, and neurotransmitters in muscle contraction.