BP104T • Unit 2Topic 3 of 3⏱ 5 Min Read

3. Sarcomere Ultrastructure, Neuromuscular Junction & Sliding Filament Theory

💡 Core Concept & Clinical Context:

Skeletal muscle fibers are multinucleated cylindrical cells packed with myofibrils. The Sarcomere is the functional contractile unit of a muscle fiber, bounded by two adjacent Z-discs. It contains Thin Filaments (composed of Actin, Tropomyosin, and the Troponin complex: TnT, TnI, TnC) and Thick Filaments (composed of hundreds of Myosin motor molecules with flexible heads containing ATP-binding and actin-binding sites). The Sliding Filament Theory (Huxley, 1954) states that during muscle contraction, thin actin filaments slide past thick myosin filaments toward the central M-line, narrowing the I-band and H-zone while the A-band length remains constant.

Clinical Scenarios & Pharmacopoeial Examples:

1Excitation-Contraction Coupling & Cross-Bridge Cycle Sequence

From somatic nerve impulse to force generation.

Step-by-Step Breakdown:

•1. Neuromuscular Junction Transmission:

•- Action potential arrives at motor neuron axon terminal -> Voltage-gated Ca2+ channels open -> Ca2+ influx triggers exocytosis of Acetylcholine (ACh) into synaptic cleft.

•- ACh binds to Nicotinic receptors (Nm) on motor end-plate -> Influx of Na+ generates End-Plate Potential (EPP) -> Fires sarcolemma action potential.

•2. Calcium Release:

•- Action potential propagates deep into muscle fiber via Transverse Tubules (T-tubules).

•- Depolarization of Dihydropyridine (DHP) voltage sensors triggers Ryanodine Receptors (RyR1) on Sarcoplasmic Reticulum (SR) terminal cisternae -> Massive efflux of Ca2+ into sarcoplasm.

•3. Cross-Bridge Cycle:

•- Ca2+ binds to Troponin-C (TnC) -> Induces conformational change that shifts Tropomyosin away from Myosin-binding sites on actin.

•- Myosin head (charged with ADP + Pi) binds to exposed actin site (Cross-bridge formation).

•- Power Stroke: Release of Pi and ADP triggers 45-degree pivoting of myosin head, pulling actin filament toward M-line.

•- Detachment: A new ATP molecule binds to myosin head, causing detachment from actin.

•- Cocking: ATP is hydrolyzed by Myosin ATPase (ATP -> ADP + Pi), resetting head to high-energy 90-degree state.

•4. Relaxation:

•- Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA) pumps Ca2+ back into SR against gradient; Tropomyosin re-blocks actin binding sites.

📋Key Takeaways & Exam Highlights

A-band: Length of thick myosin filaments (remains constant during contraction).
I-band: Region containing only thin actin filaments (shortens during contraction).
H-zone: Central region containing only thick filaments (narrows/disappears during contraction).
Rigor Mortis: After death, cellular ATP synthesis ceases -> $Ca^{2+}$ leaks from SR, binding TnC and forming actin-myosin cross-bridges. Because ATP is required for myosin head detachment, muscles remain locked in a rigid state of rigor mortis for 24-48 hours until autolytic lysosomal enzymes degrade the myofilaments.

🇮🇳 हिन्दी व्याख्या (3. Sarcomere Ultrastructure, Neuromuscular Junction & Sliding Filament Theory)

मांसपेशी सिकुड़ने का आधार सार्कोमियर (Sarcomere) है। नसों से एसिटिलकोलीन (ACh) निकलते ही कैल्शियम (Ca2+) सार्कोप्लाज्मिक रेटिकुलम से बाहर आता है और एक्टिन तथा मायोसिन के बीच क्रॉस-ब्रिज बनकर स्लाइडिंग होती है। मृत्यु के बाद ATP खत्म होने से मांसपेशियां अकड़ जाती हैं जिसे Rigor Mortis कहते हैं।