title: Dynamic changes of twitchlike responses to successive stimuli studied by decomposition of motor unit tetanic contractions in rat medial gastrocnemius.
abstract: Unfused tetanic contractions evoked by trains of stimuli at variable interpulse intervals (IPIs) were recorded for 10 fast fatigable (FF), 10 fast resistant (FR), and 10 slow (S) motor units (MUs) and subsequently decomposed with a mathematical algorithm into trains of twitch-shape responses to successive stimuli. The mean stimulation frequencies were matched for each MU to evoke tetani of similar fusion degrees, whereas the variability range of IPIs was in each case 50-150% of the mean IPI. Force and time parameters of decomposed twitches were analyzed and related to the first response. Considerable variability of the analyzed twitch parameters was observed in each MU, although the largest range of variability occurred in slow MUs. In general, the decomposed twitch responses had longer duration and higher force than single-twitch contractions, although for nine FF and six FR MUs some of the decomposed responses were slightly weaker (but not faster) than the first twitches of these MUs. Comparison of the strongest decomposed twitch to the first decomposed twitch revealed ratios of forces up to 2.35, 3.33, and 6.89 for FF, FR, and S MUs and ratios of force-time areas up to 3.54, 4.67, and 14.26 for FF, FR, and S MUs, whereas for the contraction times the ratios of the longest decomposed twitch to the first twitch amounted to 2.46, 2.07, and 3.52 for FF, FR, and S MUs, respectively. The results indicate that contractile responses to successive action potentials are considerably variable, especially for slow MUs.
authors.affiliation: Department of Neurobiology, University School of Physical Education, Poznań, Poland; and celichowski@awf.poznan.pl.
authors.forename: Jan
authors.initials: J
authors.lastname: Celichowski
authors.affiliation: Department of Neurobiology, University School of Physical Education, Poznań, Poland; and Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria.
authors.forename: Rositsa
authors.initials: R
authors.lastname: Raikova
authors.affiliation: Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria.
authors.forename: Hristo
authors.initials: H
authors.lastname: Aladjov
authors.affiliation: Department of Neurobiology, University School of Physical Education, Poznań, Poland; and.
authors.forename: Piotr
authors.initials: P
authors.lastname: Krutki
mesh_terms: D000200:Action Potentials; D000465:Algorithms; D000818:Animals; D003627:Data Interpretation, Statistical; D004558:Electric Stimulation / Q000379:methods; D008297:Male; D055595:Mechanical Phenomena; D008959:Models, Neurological; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D051381:Rats
keywords: force development; motor unit; random stimulation pattern; twitch