Neuromuscular transmission in myasthenic single motor units.

In patients with myasthenia gravis neuromuscular transmission has been tested in individual hypothenar and thenar motor units using trains of near threshold electrical stimuli delivered to the motor nerve. The most important observations included: (1) the proportion of motor units with pathological decrements varied from 0 to 90%, (2) the decrements in surface voltage were frequently much more normal in individual motor units than in the corresponding maximum compound potentials evoked by supramaximal nerve stimulation, and (3) the most abnormal decrements were observed in motor units at the low end of the surface voltage range. These observations can be interpreted to suggest important neurogenic factors in the pathogenesis of myasthenia gravis. An attractive alternative would be to suggest that the small motor units have the most abnormal neuromuscular transmission because they normally have a lower margin for safe neuromuscular transmission and, as a consequence, fail first in diseases of neuromuscular transmission.