A study of individual neuro-muscular junctions in myotonia

Abstract In this study the electrophysiological and anatomical characteristics of human myotonic muscle fibers have been compared with normal controls. The main abnormalities in the myotonic fibers appear to be partial depolarization and spontaneous oscillations in trans-membrane potential. Lacking evidence of the dramatic changes in potassium concentration gradient which would be required to give the observed membrane hyperactivity it has been proposed that a sodium “leak” exists in the diseased fibers studied, causing in most cases a reduction in membrane potential which would favor oscillation under a number of conditions. It is suggested that the fundamental disorder is either inadequate sodium extrusion or an increase in its specific permeability. Evidence is presented to show that pre-synaptic mechanisms, end-plate potentials, sensitivity to transmitter, content of acetylcholinesterase, and excitation-contraction coupling are not abnormal. Histologic examination has disclosed no pre-synaptic morphological changes which might account for the physiological disorder.