Histometric study of neuromuscular junction ultrastructure: II. myasthenic syndrome

The fine structure of 42 motor end plates from external intercostal muscles was quantitatively analyzed in the myasthenic syndrome, which often has associated with it a bronchogenic carcinoma. Thirty-two external intercostal end plates of patients without weakness served as controls. In the myasthenic syndrome the mean area of postsynaptic junctional folds and secondary clefts per nerve terminal and the ratio of the postsynaptic to the presynaptic membrane length are markedly increased. The mean nerve terminal area and the synaptic vesicle count per unit area are not significantly affected. There was no consistent relationship between the measurable abnormalities and the duration of the disease, previous therapy, or the severity of symptoms. The origin in time of the morphologic alterations is uncertain. The ultrastructural changes at the neuromuscular junction in the myasthenic syndrome are different from those encountered in myasthenia gravis. The low quantum content of the end-plate potential in the myasthenic syndrome may be related to an interference with the mechanism by which quanta of acetylcholine are released from the nerve terminal by nerve impulse.