Three weeks after a complete radial nerve transection, electromyography of the denervated extensor muscles shows spontaneous fibrillation potentials and positive sharp waves at rest. The mechanism generating these spontaneous potentials is:

• A Persistent spontaneous firing of the transected motor axon stump
• B Upregulation of extrajunctional nicotinic receptors making the entire sarcolemma hypersensitive to circulating acetylcholine ✓
• C Denervation-induced proliferation of gap junctions between muscle fibers
• D Autoimmune destruction of the motor end plates

Explanation

After denervation, nicotinic acetylcholine receptors, normally confined to the junctional folds, are synthesized along the whole sarcolemma (denervation supersensitivity). Small amounts of circulating acetylcholine then produce random, tiny, unstable end plate-like potentials that occasionally reach threshold, triggering spontaneous single-fiber action potentials recorded as fibrillations and positive sharp waves. Fibrillation potentials indicate active denervation, distinguishing them from the positive sharp waves seen in some myopathies, and they disappear with reinnervation or fibrosis.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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