title: Electromyographic and contractile properties of rabbit masseter motor units during fatiguing stimulation.
abstract: Trigeminal motoneurons were electrically stimulated in order to investigate the electromyographic (EMG) behavior in relation to the contractile properties of motor units of the masseter muscle. A total of 80 motor units were studied in situ in male New Zealand White rabbits ( n=46). The motor units were separated into two groups, each exposed to a specific fatiguing stimulation regimen. Motor unit action potential (MUAP) features, which comprised the amplitude (AMP) and inter-peak time (IPT), and the tetanic force were measured. All motor units were classified as fast (F) units. Forty-one motor units underwent a prolonged standard fatigue regimen of 40-Hz trains at 1 Hz for 20 min. While the MUAP showed an immediate decrease of mean AMP at the beginning of the stimulation, the mean force and IPT increased. After 2 min, the force declined, while the IPT continued to increase until 20 min. Only after 3 min of stimulation, did the degree of force decrease parallel the decline in MUAP AMP. After 20 min of stimulation, the majority of motor units ( n=34) still generated a force larger than 50% of the initial value, but only 17 motor units showed MUAP AMP of less than 50% of the initial EMG response. A more intensive fatigue regimen (40-Hz trains at 1.5 Hz) was applied to another group of 39 motor units. A rapid decline of force and MUAP amplitude to almost 50% was observed within the first 5 min of stimulation. After 20 min, only four motor units were still able to produce a tetanic force of more than 50% of the initial. Most strikingly, motor units with twitch contraction times faster than 22 ms exhibited a decrease in force more than in MUAP AMP, whereas the reverse was seen for units slower than 22 ms; motor units with a twitch contraction time of 22 ms showed equal decrease in AMP and force. This finding is suggestive of a division of fast masseter motor units into two classes, those which fatigue more rapidly mechanically and those which fatigue more readily electrically.
authors.affiliation: Department of Functional Anatomy, Academic Center for Dentistry Amsterdam (ACTA), Meibergdreef 15, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: S H S
authors.initials: SH
authors.lastname: Kwa
authors.forename: W A
authors.initials: WA
authors.lastname: Weijs
authors.forename: T M G J
authors.initials: TM
authors.lastname: Van Eijden
mesh_terms: D000200:Action Potentials; D000818:Animals; D004558:Electric Stimulation; D004576:Electromyography; D008297:Male; D008406:Masseter Muscle / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D011817:Rabbits