💪Muscle Physiology
Muscle contraction is a remarkable example of molecular machinery converting chemical energy into mechanical work. Understanding the sliding filament theory, excitation-contraction coupling, and the differences between muscle types is fundamental to physiology.
🔬 Three Types of Muscle
🦴 Skeletal
- • Voluntary control
- • Striated appearance
- • Multinucleated fibers
- • Fast, powerful contractions
- • Fatigable
❤️ Cardiac
- • Involuntary (autonomic)
- • Striated appearance
- • Single nucleus, branched
- • Intercalated discs
- • Resistant to fatigue
🫀 Smooth
- • Involuntary
- • Non-striated
- • Single nucleus, spindle-shaped
- • Slow, sustained contractions
- • Found in organs, vessels
🔄Sliding Filament Theory
Muscle contraction occurs when thick filaments (myosin)pull on thin filaments (actin), causing them to slide past each other. The sarcomere shortens without the filaments themselves changing length.
Cross-Bridge Cycle
- 1.Attachment: Myosin binds actin (rigor state)
- 2.Release: ATP binds → myosin releases actin
- 3.Cocking: ATP → ADP + Pᵢ (head cocks)
- 4.Power stroke: Pᵢ release, filament sliding
Regulatory Proteins
- •Tropomyosin: Blocks myosin binding sites at rest
- •Troponin C: Binds Ca²⁺ to initiate contraction
- •Troponin I: Inhibitory subunit
- •Troponin T: Tropomyosin-binding subunit
💪Excitation-Contraction Coupling Simulator
1.5 µm (compressed)2.2 µm (optimal)3.6 µm (stretched)
Parameters
Length-Tension Factor:
100%
Twitch Duration:
50 ms
Max Tension:
100%
Skeletal: Direct mechanical coupling between DHPR and RyR. Fast, powerful contractions.
📏Length-Tension Relationship
Active Tension
100%
Key Lengths:
- 1.27 µm: Complete overlap, no tension
- 1.67 µm: Thin filaments meet in center
- 2.0-2.2 µm: Optimal overlap (100%)
- 3.6 µm: No overlap, zero tension
Frank-Starling Law: The heart's ability to increase force when stretched (preload) directly follows from this length-tension relationship.
🏃Skeletal Muscle Fiber Types
| Property | Type I (Slow) | Type IIa (Fast Oxidative) | Type IIx (Fast Glycolytic) |
|---|---|---|---|
| Myosin ATPase | Slow | Fast | Fastest |
| Metabolism | Oxidative | Oxidative + Glycolytic | Glycolytic |
| Mitochondria | Many | Many | Few |
| Myoglobin | High (red) | High (red) | Low (white) |
| Fatigue Resistance | High | Medium | Low |
| Example Use | Posture, marathon | Middle-distance | Sprinting, jumping |
🏥Clinical Correlations
Duchenne Muscular Dystrophy
Dystrophin gene mutation → membrane instability → progressive muscle weakness
Myasthenia Gravis
Autoantibodies against nAChR → impaired neuromuscular transmission
Malignant Hyperthermia
RyR1 mutation → uncontrolled Ca²⁺ release → life-threatening hypermetabolism
Heart Failure
Impaired Ca²⁺ handling → reduced contractility → altered length-tension