On electron microscopy, the neuromuscular junction shows synaptic vesicles containing acetylcholine in the motor nerve terminal. The active zones of ACh release align precisely with which postsynaptic structure that contains ACh receptors?

• A The depths (troughs) of the primary junctional folds
• B The T-tubule openings of the muscle fiber
• C The basal lamina within the primary synaptic cleft
• D The crests of the secondary junctional folds of the postsynaptic membrane (motor end plate) ✓

Explanation

At the neuromuscular junction, the nerve terminal active zones are precisely aligned with the crests of the secondary junctional (postsynaptic) folds of the motor end plate, where nicotinic ACh receptors are densely concentrated. Acetylcholinesterase is located in the primary synaptic cleft and at the depths of the folds. The troughs of the folds contain voltage-gated sodium channels for action potential propagation. This precise ultrastructural alignment ensures efficient neurotransmission. In myasthenia gravis, autoantibodies reduce AChR density at the crests; in Lambert-Eaton, voltage-gated calcium channels in the presynaptic active zones are targeted.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.