Overview
This lecture explains how muscle contraction works, focusing on the roles of the proteins actin and myosin in the sliding filament model, and provides an overview of skeletal muscle structure and function.
Muscle Tissue Types and Functions
- There are three types of muscle tissue: smooth, cardiac, and skeletal.
- Smooth muscle is found in hollow organs and works involuntarily to move materials.
- Cardiac muscle is found only in the heart, is striated, and contracts involuntarily to pump blood.
- Skeletal muscles are striated, mostly voluntary, attach to bones, and create movement by contracting.
Structure of Skeletal Muscles
- A skeletal muscle consists of bundles within bundles: myofibrils make up muscle fibers, which form fascicles, grouped into the muscle organ.
- Muscle fibers are muscle cells with multiple nuclei, mitochondria, and a sarcolemma (cell membrane).
- Connective tissue sheaths provide support and protection for muscles.
- Each muscle is supplied with its own nerve, artery, and vein for energy and control.
The Sliding Filament Model of Contraction
- Myofibrils are divided into sarcomeres, the functional units containing actin (thin) and myosin (thick) filaments.
- Sarcomeres are bordered by Z lines; contraction shortens sarcomeres by bringing Z lines closer.
- Muscle contraction is powered by the repeated binding and unbinding of myosin and actin, using ATP and calcium.
Molecular Steps in Muscle Contraction
- At rest, actin and myosin do not bind due to tropomyosin and troponin blocking binding sites.
- An action potential triggers the release of calcium from the sarcoplasmic reticulum.
- Calcium binds to troponin, which shifts tropomyosin, exposing actin’s binding sites.
- Myosin heads, loaded with ADP and phosphate, bind to actin, pull it (power stroke), and contract the muscle.
- New ATP binds to myosin, causing it to release actin and reset for another cycle.
- Calcium is pumped back into the sarcoplasmic reticulum, ending contraction and starting relaxation.
Key Terms & Definitions
- Actin — Thin protein filament in muscle cells involved in contraction.
- Myosin — Thick protein filament that binds to actin to cause muscle contraction.
- Sarcomere — Basic contractile unit in muscle, bordered by Z lines.
- Sarcoplasmic Reticulum — Specialized endoplasmic reticulum in muscle cells that stores calcium.
- Tropomyosin — Protein that blocks actin’s binding sites when the muscle is at rest.
- Troponin — Protein that binds calcium, causing tropomyosin to move.
- ATP (Adenosine Triphosphate) — Energy molecule required for muscle contraction.
Action Items / Next Steps
- Review detailed steps of the sliding filament model.
- Memorize structural hierarchy of skeletal muscle.
- Study the functions of muscle tissue types and associated key terms.