Histometric study of neurornuscular junction ultrastructure: I. myasthenia gravis

External intercostal motor end-plate fine structure was quantitatively analyzed in control subjects without weakness (32 end plates) and in patients with myasthenia gravis (68 end plates). In myasthenia gravis the mean nerve terminal area and the postsynaptic membrane profile concentration were decreased, while the mean synaptic vesicle diameter and vesicle count per unit nerve terminal area were not significantly affected. Growing axon tips, immature-appearing end plates, and postsynaptic regions denuded of their nerve terminal were encountered. No relationship could be found between the measurable abnormalities and the duration of the disease, the dura tion of treatment, or the clinical classification of the illness. The same abnormalities were found in an untreated case of myasthenia gravis as in treated cases. The origin in time of the morphologic alterations was uncertain. If synaptic vesicles store single quanta of acetylcholine, then either the size of the quanta released from the nerve terminal is independent of vesicle diameter in myasthenia gravis or the small miniature end-plate potential in myasthenia gravis is not caused by the small size of the quanta released from the nerve terminal.