Overview
This lecture explains how muscle contraction occurs through the interaction of actin and myosin proteins, focusing on the structure and function of different muscle tissues and the sliding filament model.
Types of Muscle Tissue
- There are three types of muscle tissue: smooth, cardiac, and skeletal.
- Smooth muscle is found in walls of hollow organs and functions involuntarily.
- Cardiac muscle makes up the heart, is striated, and contracts involuntarily.
- Skeletal muscle is striated, mostly voluntary, and allows movement by pulling on bones.
Skeletal Muscle Structure
- Skeletal muscles are organs made of muscle tissue, connective tissue, blood vessels, and nerves.
- Muscle fibers are muscle cells containing mitochondria, nuclei, and a sarcolemma (cell membrane).
- Myofibrils are bundles inside muscle fibers, and fascicles are larger bundles of fibers.
- Supportive connective tissue sheaths reinforce muscle structure.
The Sliding Filament Model of Contraction
- Myofibrils are divided into sarcomeres, which have thin (actin) and thick (myosin) filaments.
- Sarcomeres contract when Z lines are pulled closer together, shortening the muscle.
- At rest, actin and myosin do not bind because tropomyosin and troponin block the sites.
- Calcium ions and ATP remove these blocks, allowing myosin and actin to bind.
- Myosin uses ATP to change shape, pull actin, and contract the sarcomere.
- ATP binds to myosin, causing it to release actin, and the cycle repeats as long as calcium and ATP are present.
Mechanism of Muscle Contraction
- A motor neuron sends an action potential to the muscle cell, releasing acetylcholine.
- This triggers an action potential in the sarcolemma, which travels via T-tubules.
- The sarcoplasmic reticulum releases calcium ions in response.
- Calcium binds to troponin, moving tropomyosin, and exposing binding sites on actin.
- Myosin heads bind actin, pull, and release in cycles powered by ATP hydrolysis.
Key Terms & Definitions
- Sarcomere — the contractile unit of a muscle fiber, bordered by Z lines.
- Actin — thin protein filament involved in contraction.
- Myosin — thick protein filament with “heads” that pull actin.
- Sliding Filament Model — theory explaining how muscles contract via actin and myosin sliding past each other.
- Sarcoplasmic Reticulum — specialized endoplasmic reticulum in muscle, stores and releases calcium ions.
- Troponin/Tropomyosin — regulatory proteins that block or expose actin binding sites.
- ATP (Adenosine Triphosphate) — molecule providing energy for muscle contraction.
Action Items / Next Steps
- Review and memorize the steps of the sliding filament model.
- Learn the key structural components of skeletal muscle (fiber, myofibril, sarcomere).
- Study the differences between smooth, cardiac, and skeletal muscle tissues.