Overview
This lecture explains the structural organization of skeletal muscle from the whole organ down to the protein level, focusing on key components involved in muscle contraction.
Muscle Structure Hierarchy
- Muscles attach to bones via tendons.
- A whole muscle (organ level) is made up of bundles called fascicles (tissue level).
- Each fascicle consists of multiple muscle fibers (cellular level).
- Muscle fibers are individual muscle cells, also known as muscle fibers.
- Muscle fibers contain several specific organelles important for contraction.
Specialized Muscle Cell Organelles
- The sarcolemma is the muscle cell's membrane.
- Transverse tubules (T tubules) are membrane folds that conduct action potentials into the cell.
- Sarcoplasmic reticulum stores calcium necessary for muscle contraction.
- Myofibrils are organelles within muscle fibers, made up of myofilaments (protein filaments).
Myofibrils and Myofilaments
- Myofibrils consist of repeating units called sarcomeres.
- Myofilaments are protein filaments within myofibrils responsible for contraction.
- Two main myofilaments: actin (thin filament) and myosin (thick filament).
- Myosin has "heads" that grab and pull on actin during contraction.
The Sarcomere and Muscle Contraction
- The sarcomere is the functional unit of a myofibril, spanning from one Z line to the next.
- During contraction, myosin heads pull actin filaments, shortening the sarcomere (distance between Z lines decreases).
- Muscle relaxation occurs when myosin heads detach and sarcomere lengthens.
Levels of Organization Summary
- Muscle (organ) > Fascicle (tissue) > Muscle fiber (cell) > Myofibril (organelle) > Myofilament (molecule/protein).
Key Terms & Definitions
- Tendon — connects muscle to bone.
- Fascicle — bundle of muscle fibers.
- Muscle fiber — muscle cell.
- Sarcolemma — muscle cell membrane.
- Transverse tubule (T tubule) — conduct signals into muscle fibers.
- Sarcoplasmic reticulum — stores calcium for contraction.
- Myofibril — muscle fiber organelle made of myofilaments.
- Myofilament — protein filaments (actin and myosin) enabling contraction.
- Sarcomere — basic contractile unit in muscle cells.
- Actin — thin protein filament in muscle.
- Myosin — thick protein filament with heads for pulling actin.
Action Items / Next Steps
- Practice identifying the five levels of muscle organization from largest (muscle) to smallest (myofilament).
- Review diagrams showing structural hierarchy of muscle components.