Muscle Physiology: Skeletal Muscle Contraction covers skeletal muscle contraction for INI CET (AIIMS PG).
Muscle Architecture:
- Sarcomere: Basic contractile unit — Z-disc to Z-disc
- A-band: Myosin (thick) filaments — length constant during contraction
- I-band: Actin (thin) filaments only — shortens during contraction
- H-zone: Myosin only (center of A-band) — shortens during contraction
- Z-disc: Anchors actin filaments; moves closer during contraction
- M-line: Center of sarcomere; holds myosin in register
- Sarcoplasmic Reticulum (SR): Ca²⁺ storage; wraps around myofibrils; terminal cisternae (coupled to T-tubules at triad)
Molecular Mechanism (Sliding Filament Theory):
- AP in muscle fiber: T-tubule → DHPR (dihydropyridine receptor — L-type Ca²⁺ channel) → activates ryanodine receptor (RYR1) on SR → Ca²⁺ release
- Cross-bridge cycle:
- Ca²⁺ binds troponin C → tropomyosin shifts → myosin-binding sites on actin exposed
- Myosin head binds actin (strong binding in ADP + Pi state)
- Power stroke: Pi and ADP released → myosin head pivots → actin filament slides toward M-line (7-9 nm stroke)
- New ATP binds myosin → myosin dissociates from actin
- ATP hydrolysis → myosin head returns to cocked position
- Relaxation: Ca²⁺-ATPase (SERCA) pumps Ca²⁺ back into SR → Ca²⁺ dissociates from troponin → tropomyosin blocks myosin-binding sites
Excitation-Contraction Coupling:
- Motor neuron AP → ACh release at NMJ → muscle fiber depolarization
- Depolarization spreads across sarcolemma → down T-tubules
- Triad: T-tubule (with DHPR) + junctional SR (with RYR) + terminal cisternae
- DHPR mechanically opens RYR1 → Ca²⁺ release (footplate model)
Mechanics:
- Twitch: Single stimulus → single contraction-relaxation cycle
- Latent period: 5–10 ms (excitation-contraction coupling)
- Contraction time: 20–80 ms
- Relaxation time: 20–100 ms
- Isotonic vs Isometric: Isotonic (load constant, muscle changes length); Isometric (length constant, tension develops)
- Length-tension relationship: Optimal overlap at sarcomere length ~2.0–2.2 μm; either stretched or shortened → reduced tension
- Force-velocity relationship: As load decreases, velocity of shortening increases (Hill’s equation)
Muscle Fiber Types:
| Type | Color | Metabolism | twitch | Example |
|---|---|---|---|---|
| Type I (slow oxidative) | Red | Aerobic, many mitochondria | Slow, fatigue-resistant | Postural muscles |
| Type IIa (fast oxidative) | Red | Aerobic + anaerobic | Fast, fatigue-resistant | Postural + phasic |
| Type IIb/IIx (fast glycolytic) | White | Anaerobic (glycolysis) | Fast, fatigable | Rapid movement |
NMJ Physiology:
- Motor end plate = specialized postsynaptic region
- Miniature end plate potentials (MEPPs): Spontaneous quantal release of ACh (one vesicle)
- EPP (End Plate Potential): Sum of ~200 MEPPs → reaches threshold → muscle AP
- Safety factor: EPP always > threshold; ensures reliable transmission
- Denervation: Supersensitivity (upregulation of ACh receptors — spreads beyond motor end plate)
⚡ Exam Tip for INI CET (AIIMS PG): Myasthenia gravis = autoimmune antibodies against ACh receptors (postsynaptic) → fatigable weakness (worse with repeated use) → treat with AChE inhibitors (pyridostigmine). Lambert-Eaton myasthenic syndrome = antibodies against presynaptic voltage-gated Ca²⁺ channels → weakness + autonomic dysfunction; reflexes improve after exercise (facilitation).