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Neuromuscular Transmission

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1. Definition

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2. Structure of the Neuromuscular Junction

Components:

• The axon loses its myelin sheath near the muscle and divides into several terminal boutons
• Each terminal bouton contains:
  • Many small clear synaptic vesicles (each containing ~10,000 molecules of acetylcholine / ACh)
  • Mitochondria (for energy supply - ATP for ACh synthesis)
  • Voltage-gated Ca²+ channels at active zones
• The terminal fits into junctional depressions of the motor endplate

• A gap of ~20-30 nm between the nerve terminal and muscle membrane
• Contains acetylcholinesterase (AChE) in high concentration - rapidly degrades ACh

• The thickened portion of the muscle fiber membrane beneath the nerve terminal
• Has characteristic junctional folds (subneural clefts) that increase surface area
• Nicotinic cholinergic receptors (N_M type) are concentrated at the tops of the junctional folds
• AChE is also located in the depths of the folds

Key Point for RGUHS: Each skeletal muscle fiber is innervated by a single motor nerve fiber (1:1 relationship at the endplate). One motor neuron can, however, innervate multiple muscle fibers (motor unit).

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3. Sequence of Events in Neuromuscular Transmission

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4. Endplate Potential (EPP)

• A graded, non-propagated depolarizing potential at the motor endplate
• Amplitude: approximately -15 to -20 mV (brings membrane from -90 mV toward threshold)
• Unlike an action potential, the EPP is not all-or-none; it is proportional to the amount of ACh released
• Normally, EPP is suprathreshold - it always generates a muscle action potential (high safety factor)
• The EPP exceeds the threshold by about 3-4 times (the "safety margin" of neuromuscular transmission)

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5. Quantal Release of Acetylcholine

• ACh is released in discrete packets called quanta (each quantum = contents of one synaptic vesicle = ~10,000 ACh molecules)
• At rest, single quanta are released spontaneously and randomly → produce Miniature Endplate Potentials (MEPPs)
• MEPP characteristics:
  • Amplitude: ~0.5 mV (well below threshold - does NOT cause muscle contraction)
  • Spontaneous, random
  • Reflects single-quantum (single vesicle) release
• During a nerve impulse: ~60 quanta are released simultaneously → large EPP → muscle AP

Regulation of quantal release:

• Ca²+: Quantal size varies directly with Ca²+ concentration at the endplate - more Ca²+ = more vesicle fusion
• Mg²+: Varies inversely with Mg²+ concentration - Mg²+ competes with Ca²+ at the active zone

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6. Role of Acetylcholine

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7. Neuromuscular Fatigue

1. Progressive depletion of readily-releasable ACh vesicles
2. Accumulation of choline and metabolites
3. Decrease in Ca²+ influx efficacy with very high frequency stimulation

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8. Denervation Hypersensitivity (Supersensitivity)

• When a motor nerve is cut and degenerates, the denervated muscle becomes hypersensitive to ACh
• Normally, nicotinic receptors are confined to the motor endplate region only
• After denervation: new receptors appear across the entire sarcolemma (not just at the endplate)
• This is called denervation supersensitivity
• The muscle shows fibrillations (spontaneous contractions) on EMG

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9. Pharmacological Agents Acting at the NMJ

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10. Diseases of Neuromuscular Transmission (RGUHS Clinical Correlate)

A. Myasthenia Gravis (MG)

• Definition: Autoimmune disease causing weakness and easy fatigability of skeletal muscles
• Incidence: 25-125 per million; bimodal - peaks in 20s (women) and 60s (men)
• Pathogenesis:
  • Circulating IgG antibodies against N_M (nicotinic) ACh receptors at the motor endplate
  • Antibodies destroy some receptors and cross-link others, triggering endocytosis (internalization and degradation)
  • 70-90% reduction in receptor number per endplate
  • Junctional folds become sparse, shallow, and widened/absent
  • With repetitive stimulation, normal decline in quantal ACh release goes uncorrected → transmission fails
• Features:
  • Muscle fatigue with sustained or repeated activity (hallmark)
  • Two forms: (1) Ocular - extraocular muscles only; (2) Generalized - all skeletal muscles including diaphragm
  • Associated with thymus hyperplasia (85%) and thymoma (10-15%)
  • Associations: rheumatoid arthritis, SLE, polymyositis
• Treatment:
  • AChE inhibitors: neostigmine, pyridostigmine (symptomatic)
  • Immunosuppression: prednisone, azathioprine, cyclosporine
  • Thymectomy - induces remission in 35%, improves symptoms in 45%
  • Plasmapheresis, IVIg in crisis

B. Lambert-Eaton Myasthenic Syndrome (LEMS)

• Pathogenesis: Autoimmune attack against voltage-gated Ca²+ channels in nerve endings at the NMJ
• Decreased Ca²+ influx → reduced ACh release
• Often paraneoplastic (associated with small cell carcinoma of lung)
• Feature: Proximal muscle weakness (lower limbs > upper), improves with repeated activity (unlike MG)
  • Repeated stimulation allows Ca²+ accumulation in terminal → more ACh released
• Distinguished from MG by: incremental response on repetitive nerve stimulation (vs. decremental in MG)

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11. Comparison: MG vs. LEMS

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12. Summary Diagram - Steps of NMT (Memory Aid)

Motor AP → Ca²+ entry → ACh exocytosis → ACh + N_M receptor
→ Na+ influx → EPP → Muscle AP → Muscle contraction
                    ↑
           AChE hydrolyzes ACh (terminates)
           Choline recycled back to terminal

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13. Important Points for RGUHS Examinations

• "Safety factor" of NMT: EPP is normally 3-4x the threshold - ensures reliable 1:1 transmission
• MEPPs prove quantal release and are used to assess presynaptic vs. postsynaptic defects in NMJ diseases
• In MG: MEPP amplitude is normal (presynaptic release intact), but EPP is small (receptors reduced)
• In LEMS: MEPP amplitude is reduced (presynaptic release reduced)
• Curare and competitive blockers can be reversed by AChE inhibitors; depolarizing blockers (succinylcholine) cannot
• The entire NMT takes approximately 0.5-1 ms

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Neuromuscular Transmission: Normal vs. Disorders

Part 1 - Mechanism Comparison

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Part 2 - Clinical Features Comparison

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Part 3 - Quick Memory Summary

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All NMT disorders are purely motor (no sensory loss), since sensory nerves do not use the NMJ. This is a useful clinical differentiator from spinal cord or peripheral nerve lesions.

1. Cover page - with both NMJ structure and events diagrams (from Ganong's) side by side
2. Normal NMT step-by-step table - all 9 steps with ions and results
3. Quantal release & MEPP - rest vs. impulse comparison
4. Mechanism comparison table - Normal vs. 6 disorders across 12 parameters
5. Clinical features table - same 7 columns covering muscle groups, autonomic features, reflexes, diagnosis, treatment
6. MG vs. LEMS detailed comparison - 18-row deep dive with key differentiators highlighted
7. Pharmacology at the NMJ - 12 drugs with mechanism, effect, and clinical use
8. RGUHS exam key points - 10 high-yield points formatted for quick revision