title: F-wave of single firing motor units: correct or misleading criterion of motoneuron excitability in humans?
abstract: Motoneuron excitability is a critical property for information processing during motor control. F-wave (a motoneuronal recurrent discharge evoked by a motor antidromic volley) is often used as a criterion of motoneuron pool excitability in normal and neuromuscular diseases. However, such using of F-wave calls in question. The present study was designed to explore excitability of single low-threshold motoneurons during their natural firing in healthy humans and to ascertain whether F-wave is a correct measure of motoneuronal excitability. Single motor units (MUs) were activated by gentle voluntary muscle contractions. MU peri-stimulus time histograms and motoneuron excitability changes within a target interspike interval were analysed during testing by motor antidromic and Ia-afferent volleys. It was found that F-waves could be occasionally recorded in some low-threshold MUs. However, during evoking F-wave, in contrast with the H-reflex, peri-stimulus time histograms revealed no statistically significant increase in MU discharge probability. Moreover, surprisingly, motoneurons appeared commonly incapable to fire a recurrent discharge within the most excitable part of a target interval. Thus, the F-wave, unlike the H-reflex, is the incorrect criterion of motoneuron excitability resulting in misleading conclusions. However, it does not exclude the validity of the F-wave as a clinical tool for other aims. It was concluded that the F-wave was first explored in low-threshold MUs during their natural firing. The findings may be useful at interpretations of changes in the motoneuron pool excitability in neuromuscular diseases.
authors.affiliation: Institute for Information Transmission Problems (Kharkevich Institute), Russian Academy of Sciences, 19, Bol'shoi Karetnyi pereulok, GSP-4, Moscow, 127994, Russia. kudina@iitp.ru.
authors.forename: Lydia P
authors.initials: LP
authors.lastname: Kudina
authors.affiliation: Institute for Information Transmission Problems (Kharkevich Institute), Russian Academy of Sciences, 19, Bol'shoi Karetnyi pereulok, GSP-4, Moscow, 127994, Russia.
authors.forename: Regina E
authors.initials: RE
authors.lastname: Andreeva
mesh_terms: D000200:Action Potentials / Q000502:physiology; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology
keywords: F-wave; Firing motoneuron excitability; H-reflex; Single motor units