Neuromuscular transmission in single motor units in myasthenia gravis

Forty-eight single motor units (MUs) were recorded in the first dorsal interosseous muscle in 10 patients with generalized myasthenia gravis (MG) using the technique of intramuscular microstimulation. The data were compared statistically with 126 MUs recorded from 19 controls. Motor units were characterized as fast-twitch fatigable (FF), fast-twitch fatique resistant (FR), and slow-twitch (S). There was a preponderance of fast-twitch MUs in the MG sample. Decremental action potentials greater than 10% were recorded in both fast- and slow-twitch units in MG, but the mean percent decrement was greater in F than in S units. Mean twitch contraction and half-relaxation times were faster in MG MUs than in controls. No significant change in mean twitch tension was recorded in either F or S units. The study shows that the defect in neuromuscular transmission in MG is not selective for one MU type.