Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 50.5 The Physical Interaction of Protein Filaments Is Required for Muscle Function
Muscle contraction results from the interaction of actin (thin filaments) and myosin (thick filaments) in muscle cells. These interactions are controlled by calcium ions and regulated through complex mechanisms. Vertebrates possess skeletal, cardiac, and smooth muscle types, each specialized for different functions.
1. Structure of Skeletal Muscle
- Skeletal muscle is made of muscle fibers, each a multinucleated cell
- Fibers contain myofibrils composed of repeating sarcomeres
- Sarcomeres have striations from thick (myosin) and thin (actin) filaments
- Thin filaments attach to Z lines; thick filaments anchor at the M line
2. The Sliding-Filament Model
- Filaments slide past each other to shorten the sarcomere
- Myosin heads bind actin and form cross-bridges
- ATP powers the stroke that pulls thin filaments inward
- ATP binding releases myosin for a new cycle
- Each thick filament has hundreds of rapidly cycling myosin heads
3. The Role of Calcium and Regulatory Proteins
- Tropomyosin blocks binding sites on actin at rest
- Troponin binds Ca²⁺, shifting tropomyosin to expose sites
- Ca²⁺ triggers contraction; its removal stops it
- Ca²⁺ is stored and released by the sarcoplasmic reticulum (SR)
4. Excitation-Contraction Coupling
- Motor neurons release acetylcholine (ACh)
- ACh initiates an action potential that spreads via T tubules
- Ca²⁺ released from SR into cytosol
- Ca²⁺ binds troponin → contraction begins
- After stimulation, Ca²⁺ returns to SR → muscle relaxes
5. Energy Sources for Contraction
- ATP is needed for each cycle
- ATP regenerated by:
- Creatine phosphate (short-term, ~15 sec)
- Aerobic respiration of glycogen (longer duration)
- Lactic acid fermentation (short-term anaerobic)
6. Nervous System Control and Graded Contractions
- One motor neuron + its muscle fibers = motor unit
- More motor units = stronger contraction
- Summation: repeated stimulation increases tension
- Tetanus: sustained high-frequency stimulation
7. Muscle Fiber Types
- Oxidative fibers: high mitochondria, myoglobin; use aerobic respiration
- Glycolytic fibers: rely on glycolysis; fatigue faster
- Slow-twitch fibers: sustained contractions, slower Ca²⁺ removal
- Fast-twitch fibers: rapid, powerful; oxidative or glycolytic
- Training can convert fast glycolytic → fast oxidative
8. Other Muscle Types
- Cardiac muscle: striated, contracts without nerve input, has intercalated disks
- Smooth muscle: non-striated, in organs, uses calmodulin instead of troponin
- Contraction starts with Ca²⁺ entry from outside the cell
9. Muscle-Skeleton Interaction
- Muscles connect to bones via tendons
- Movement uses antagonistic muscle pairs (e.g., biceps and triceps)
- Contraction pulls the skeleton; relaxation returns it
- Skeleton supports body and protects organs
In a Nutshell
Muscle contraction depends on ATP-driven interactions between actin and myosin filaments within sarcomeres. Regulated by calcium and the nervous system, this mechanism powers all movement. Skeletal, cardiac, and smooth muscles differ in structure and function, but all rely on the same core sliding-filament process to generate force and enable locomotion.