On electron microscopy of a motor nerve terminal at the neuromuscular junction, which ultrastructural feature is responsible for storing acetylcholine and aligning it precisely over the postsynaptic junctional folds?

• A Mitochondria clustered in the presynaptic terminal
• B Active zones (dense bars) in the presynaptic membrane, aligned directly opposite the secondary (junctional) folds ✓
• C Schwann cell processes extending into the synaptic cleft
• D T-tubules entering the presynaptic terminal

Explanation

The neuromuscular junction has specific ultrastructural specializations: the presynaptic terminal contains active zones — electron-dense regions ('dense bars' or 'active zone material') where voltage-gated Ca2+ channels are located and synaptic vesicles dock for release. Active zones are precisely aligned opposite the crests of the junctional (secondary) folds in the postsynaptic membrane, where ACh receptors (nAChR) are concentrated. This ensures efficient synaptic transmission. Mitochondria are abundant but are not the release sites. Schwann cells cap the terminal but do not enter the cleft.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.