Q.104 The nerve impulse at the neuromuscular junction results in discharge of acetylcholine (Ach) from
its vesicles into the synaptic cleft. Ach gets degraded by acetylcholine esterase and is present in
which one of the following locations?
(A) Post synaptic membrane (B) Both pre and post – synaptic clefts
(C) Presynaptic membrane (D) Synaptic cleft
Acetylcholine (ACh) is released from presynaptic vesicles into the synaptic cleft at the neuromuscular junction, where it diffuses across to bind postsynaptic receptors before being rapidly hydrolyzed by acetylcholinesterase (AChE). AChE is precisely localized within the synaptic cleft, anchored to the basal lamina, ensuring quick degradation to terminate the signal and prevent overstimulation. The correct answer is (D) Synaptic cleft.
Correct Answer
(D) Synaptic cleft
Neuromuscular Junction AChE Localization
AChE is a tetrameric enzyme concentrated in the ~50 nm synaptic cleft, primarily associated with the synaptic basal lamina (not membranes). Quantitative EM studies show >95% of AChE sites in the primary cleft and postjunctional folds, positioned closer to the postsynaptic membrane to hydrolyze ACh bouncing off nicotinic receptors into choline + acetate within milliseconds. This prevents desensitization and enables high-frequency transmission.
Option Analysis
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(A) Post synaptic membrane: Wrong. AChE is not membrane-bound on the postsynaptic side; it’s extracellular in the cleft/basal lamina. Postsynaptic density hosts receptors, not AChE.
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(B) Both pre and post – synaptic clefts: Incorrect. No “pre-synaptic cleft” exists; the cleft is singular between pre- and postsynaptic elements. AChE isn’t duplicated across nonexistent divisions.
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(C) Presynaptic membrane: Wrong. Presynaptic terminal contains vesicles/synapsin but AChE is postsynaptic-biased in the cleft for efficient capture.
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(D) Synaptic cleft: Correct. High-resolution studies confirm AChE in primary cleft and folds, on basal lamina for optimal ACh hydrolysis geometry.
Introduction to Acetylcholine Esterase Location
Acetylcholine esterase synaptic cleft neuromuscular junction is a high-yield GATE Life Sciences topic, testing AChE’s precise role in Q.104. After vesicular release into the synaptic cleft, AChE hydrolyzes it there to terminate contraction signals at skeletal muscle NMJs.
NMJ Transmission Cycle
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Nerve impulse → Ca2+ influx → ACh vesicle fusion into synaptic cleft.
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ACh binds postsynaptic nAChR → Na+ influx → EPP → muscle AP.
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AChE in synaptic cleft → ACh → choline + acetate (recycled).
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Choline reuptaken presynaptically for resynthesis.
Q.104 Step-by-Step Solution
AChE is not on membranes but in the basal lamina of the synaptic cleft (primary + folds). EM-gold labeling proves >95% localization there. Answer: (D)—options A/B/C misplace the enzyme.
Why Synaptic Cleft for AChE?
| Option | Location Claim | Reality |
|---|---|---|
| (A) Postsynaptic membrane | Membrane-bound | Basal lamina in cleft |
| (B) Both clefts | Dual clefts | Single cleft only |
| (C) Presynaptic membrane | Terminal side | Postsynaptic bias |
| (D) Synaptic cleft | Extracellular space | Correct: 95%+ sites |
Optimal positioning catches “rebound” ACh, preventing toxicity.
GATE Neurophysiology Tips
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Mnemonic: “Cleft Clears Choline” (AChE in synaptic cleft).
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Compare: Botulinum blocks release; neostigmine inhibits AChE.
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Key fact: AChE tetramers on collagen IV in basal lamina.
Master acetylcholine esterase synaptic cleft neuromuscular junction for flawless neuro exam performance.
