PSEUDOFACILITATION - A MISLEADING TERM

The possible causes of the transient enlargement of muscle compound ac tion potentials during repetitive stimulation (''pseudofacilitation'') are considered. The phenomenon cannot be due to mechanical artefact, while hypersynchronization of the muscle fiber action potentials, the usual explanation, can only make a minor contribution. A more convinci ng explanation, for which there is now experimental evidence, is that the muscle fibers undergo hyperpolarization, due to the intramuscular release of norepinephrine and consequent stimulation of the electrogen ic Na+,K+-pump. Defective phosphorylation of the Na+,K+-pump is a poss ible cause of the transient weakness and myotonia in myotonic dystroph y. (C) 1994 John Wiley & Sons, Inc.