Two weeks after complete section of a peripheral nerve, needle EMG of the denervated muscle shows spontaneous fibrillation potentials. The receptor-level change responsible for these spontaneous discharges is:

• A Conversion of the muscle fibre to a pacemaker-like automaticity driven by sodium channels
• B Loss of acetylcholinesterase from the synaptic cleft
• C Spontaneous firing of the denervated alpha motor neuron stump
• D Upregulation of nicotinic receptors across the entire sarcolemma instead of only the junctional region ✓

Explanation

Denervation removes the trophic influence of the nerve, so extrajunctional fetal-type acetylcholine receptors appear over the whole sarcolemma. The fibre becomes exquisitely sensitive to circulating acetylcholine and fires spontaneously in small groups, recorded as fibrillation potentials. The nerve stump distal to the lesion is degenerate and cannot fire, and acetylcholinesterase loss alone would not create spontaneous activity without widespread receptors.

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

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