The Sliding Filament Theory of Muscle Contraction
Basic Structure Review
Before diving into how muscles contract, let's quickly review the key players:
- Myofibrils contain sarcomeres (the basic functional unit of muscle contraction)
- Sarcomeres contain two types of protein filaments:
- Thick filaments (made of myosin)
- Thin filaments (made of actin)
The arrangement of these filaments gives skeletal muscle its characteristic striped (striated) appearance under a microscope.
The Sliding Filament Theory Explained
1. The Resting State
In a relaxed muscle:
- Myosin heads are bound to ATP
- Tropomyosin covers the binding sites on actin
- No cross-bridges are formed
- Calcium ions are stored in the sarcoplasmic reticulum
2. The Contraction Process
Step 1: Neural Stimulation
- A nerve impulse arrives at the neuromuscular junction
- Acetylcholine is released
- This triggers an action potential in the muscle fiber
Step 2: Calcium Release
- The action potential travels along the sarcolemma
- T-tubules carry the signal deep into the muscle fiber
- Calcium ions are released from the sarcoplasmic reticulum
Think of calcium as the "key" that unlocks the contraction process!
Step 3: Cross-bridge Formation
- Calcium binds to troponin
- Troponin changes shape
