Repetitive CMAPs: mechanisms of neural and synaptic genesis.
Repetitive compound muscle action potentials (R-CMAPs) occur when a single nerve shock excites muscle fibers repeatedly. "Double discharges" are due to intramuscular nerve reexcitation. "Synaptic" R-CMAPs, due to excess acetylcholine in the neuromuscular synapse, can occur in congenital myasthenia, the slow-channel syndrome, and acetylcholinesterase inhibition. Secondary nerve excitation can reexcite muscle fibers. Synaptic R-CMAPs in a patient consisted of two discharges. The second diminished during repetitive stimulation and began 3.5-4.0 ms after the first, which is slightly longer than the synapse-muscle refractory period. Neural R-CMAPs, due to ectopic nerve activity, occur in neuromyotonia (NMT). R-CMAPs in a patient consisted of about 20 discharges at 200-300 Hz. Studies in healthy subjects showed that such trains represent added single CMAPs. Impulse frequency in the patient lied close to the threshold of refractoriness. Refractoriness of the synapse-muscle cell assembly determines the characteristics of R-CMAPs regardless of the primary cause.
van Dijk
J G
JG
Department of Neurology, Leiden University Hospital, The Netherlands. vdijk@rullf2.leidenuniv.nl
Lammers
G J
GJ
Wintzen
A R
AR
Molenaar
P C
PC
D000200:Action Potentials; D000328:Adult; D001327:Autoimmune Diseases / Q000503:physiopathology; D004558:Electric Stimulation; D005260:Female; D006801:Humans; D009132:Muscles / Q000503:physiopathology*; D009157:Myasthenia Gravis / Q000503:physiopathology; D009222:Myotonia / Q000503:physiopathology; D009420:Nervous System / Q000503:physiopathology*; D009468:Neuromuscular Diseases / Q000503:physiopathology; D013569:Synapses / Q000502:physiology*