Overview
This lecture explains how muscle contraction occurs at the cellular level, focusing on the structure and function of muscle types, the role of actin and myosin, and the sliding filament model.
Types and Structure of Muscle Tissue
- There are three types of muscle tissue: smooth, cardiac, and skeletal.
- Smooth muscle is found in hollow organs and acts involuntarily to move fluids.
- Cardiac muscle, found only in the heart, is striated and moves blood involuntarily.
- Skeletal muscle is striated, mostly voluntary, attached to bones, and responsible for body movement.
- Each skeletal muscle is an organ made of muscle tissue, connective tissue, blood vessels, and nerves.
Anatomy of Skeletal Muscle
- Muscle tissue is organized into bundles: myofibrils → muscle fibers (cells) → fascicles → entire muscle.
- Muscle fibers have mitochondria, multiple nuclei, and a membrane called the sarcolemma.
- Protective connective tissue sheaths reinforce muscles.
The Sliding Filament Model of Contraction
- Myofibrils are divided into sarcomeres, the basic contractile units of muscle.
- Sarcomeres contain thin filaments (actin) and thick filaments (myosin).
- Actin and myosin interact to cause muscle contraction by sliding past each other, shortening the sarcomere.
Molecular Mechanism of Contraction
- At rest, actin and myosin are prevented from binding by troponin and tropomyosin.
- An action potential triggers calcium release from the sarcoplasmic reticulum into the cell.
- Calcium binds to troponin, causing tropomyosin to move and expose binding sites on actin.
- Myosin heads, charged by ATP, bind to actin, release their stored energy, and pull the actin (power stroke).
- After the power stroke, a new ATP molecule causes myosin to release actin and reset.
Role of ATP and Calcium
- ATP provides energy for myosin head movement and detachment from actin.
- Calcium ions control access to actin's binding sites by affecting troponin and tropomyosin.
- Calcium is actively pumped back into the sarcoplasmic reticulum for muscle relaxation.
Key Terms & Definitions
- Sarcomere — The smallest contractile unit of a muscle fiber, bordered by Z lines.
- Myofibril — Bundles of protein filaments within muscle fibers.
- Actin — Thin filament protein involved in muscle contraction.
- Myosin — Thick filament protein with heads that bind to actin.
- Tropomyosin — Protein blocking actin’s binding sites.
- Troponin — Protein that binds calcium and moves tropomyosin.
- Sarcoplasmic reticulum — Specialized endoplasmic reticulum storing and releasing calcium.
- Sliding filament model — Theory describing how muscles contract by actin and myosin sliding past each other.
Action Items / Next Steps
- Review the sliding filament steps and rehearse the sequence for recall.
- Study diagrams of sarcomere structure for visual understanding.
- Prepare for questions on how ATP and calcium regulate contraction and relaxation.