Dynamic changes of twitchlike responses to successive stimuli studied by decomposition of motor unit tetanic contractions in rat medial gastrocnemius

Abstract

Celichowski J, Raikova R, Aladjov H, Krutki P. Dynamic changes of twitchlike responses to successive stimuli studied by decomposition of motor unit tetanic contractions in rat medial gastrocnemius. J Neurophysiol 112: 3116 -3124, 2014. First published September 24, 2014 doi:10.1152/jn.00895.2013.-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. motor unit; force development; random stimulation pattern; twitch THE FORCE OF A SKELETAL MUSCLE is a result of summation of forces generated by numerous coactive motor units (MUs) and mechanical activity of each MU evoked by trains of action potentials generated by a motoneuron at variable time intervals (Adam et al. 1998;Boe et al. 2005;Masuda and De Luca 1991;Moritz et al. 2005;Westergaard and De Luca 2011). In general, successive MU action potentials recorded in a muscle are quite stable in amplitude and shape (Mambrito and De Luca 1983;Stålberg 2003), although small changes in duration and amplitude of MU action potentials, related to a decrease in muscle fiber conduction velocity, were also reported during 30 s of voluntary activity (Farina and Falla 2007). Contrary to this, the twitchlike responses to successive motoneuronal action potentials within a tetanic contraction are highly variable, as reported in several studies on cat Parmiggiani 1979, 1981) and rat (MacIntosh et al. 2007) muscles. These studies have shown that the amplitude of a single muscle twitch may be significantly smaller than a twitchlike response to one of the stimuli within a train. When the same method was used for tetanic contrac...

Decomposition of motor unit tetanic contractions of rat soleus muscle: Differences between males and females

“…These contractions were evoked by stimulation with variable, random IPIs to resemble discharge patterns observed during voluntary MU activity. Previously, the decomposition of slow MU contractions was performed in experiments on the rat medial gastrocnemius (Celichowski et al, 2014). Therefore, some of the results obtained in this study could be expected, and indeed several similarities were observed, namely, considerable diversity of either force and time parameters was common for the decomposed twitch-shape responses in each MU, and the first decomposed twitch was the weakest and the shortest.…”

“…For the medial gastrocnemius, the ratio of the force of the strongest decomposed twitch to the first twitch force was up to 6.9, the ratio for the contraction time was up to 3.5, and the ratio for the force-time area was up to 14.2 (Celichowski et al, 2014). This indicates that slow-type MUs in different muscles are not uniform with respect to mechanisms of summation into tetanic contractions, though they evidently have different properties to fast-type MUs in which this diversity of twitch force and time parameters is much smaller (Raikova et al, 2010;Celichowski et al, 2014).…”

“…The decomposed twitches of fast MUs have displayed up to 2.2 times higher forces, and up to 2.5 times longer contraction times than the single twitch. For slow MUs, the forces of decomposed twitches have reached even 7 times higher values, while the twitch contraction and relaxation parameters have been up to 3.5 times longer than those of the single twitch (Celichowski et al, 2014).…”

How many nerve fibres can be separated as donor from an integral nerve trunk when reconstructing a peripheral nerve trauma with amplification method by artificial biochitin conduit?

“…Overall, the complex movements of limbs, the maintenance of postures and the preservation of body temperature mainly depend on the tetanus contraction. The maximal tension produced by complete tetanic contraction is much larger than that of single contraction tension, and it reflects the number of functional motor endplates as well [13].…”

Spike-triggered averaging provides inaccurate estimates of motor unit twitch properties under optimal conditions

“…Under the assumption that the forces produced by different motor units are uncorrelated, a sufficient number of triggers should largely reduce this error. Moreover, a certain number of triggers is necessary to account for the natural short-term variability in the twitch shape (Celichowski et al, 2014). Consequently, most studies have imposed a lower limit for the number of triggers (trigger-threshold), but this limit varies largely across different study, e.g.…”