The tetanic depression in fast motor units of mammalian skeletal muscle can be evoked by lengthening of one initial interpulse interval

A lower than expected tetanic force (the tetanic depression) is regularly observed in fast motor units (MUs) when a higher stimulation frequency immediately follows a lower one. The aim of the present study was to determine whether prolongation of only the first interpulse interval (IPI) resulted in tetanic depression. The experiments were carried out on fast MUs of the medial gastrocnemius muscle in cats and rats. The tetanic depression was measured in each case as the force decrease of a tetanus with one IPI prolonged in relation to the tetanic force at the respective constant stimulation frequency. Force depression was observed in all cases studied and was considerably greater in cats. For cats, the mean values of force depression amounted to 28.64% for FR and 10.86% for FF MUs whereas for rats 9.30 and 7.21% for FR and FF motor units, respectively. Since the phenomenon of tetanic depression in mammalian muscle is commonly observed even after a change in only the initial interpulse interval within a stimulation pattern, it can effectively influence processes of force regulation during voluntary activity of a muscle, when motoneurones progressively increase the firing rate

Low-volume whole-body vibration lasting 3 or 6 months does not affect biomarkers in blood serum of rats

Whole-body vibration training (WBV) has been reported to improve both, bone strength and neuromuscular performance. Although changes in hormonal and immunological parameters following vibration exercises were reported, there are still few studies concerning the immune response with respect to different duration of WBV. In our study, we aimed to establish whether three and six months of specific, short-lasting WBV (four bouts lasting 30 s, 1 min rest intervals) influences blood cell counts as well as some immunological parameters in rats. Adult male Wistar rats were assigned randomly to two groups trained for three (WBV3mo) or six (WBV6mo) months and results were compared to the age matched control group (C). After the training period, red and white blood cells, lymphocytes, monocytes, granulocytes, hemoglobin, and hematocrit, as well as interleukin-1b, interleukin-10, interleukin-6, and vascular endothelial growth factor levels were determined. No significant differences between WBV3mo, WBV6mo, and C groups in complete blood counts or in immunological parameters were found, indicating that the whole-body vibration training used in this study did not disturb the balance of examined indices, directly or indirectly involved in inflammatory processes

Membrane receptors involved in modulation of responses of spinal dorsal horn interneurons evoked by feline group II muscle afferents

Modulatory actions of a metabotropic 5-HT(1A&7) membrane receptor agonist and antagonist [(+/−)-8-hydroxy-2-(di-n-propylamino)-tetralin; N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane-carboxamide] and an ionotropic 5-HT(3) membrane receptor agonist and antagonist [2-methyl-serotonin (2-Me 5-HT); N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-8-carboxamide hydrochloride] were investigated on dorsal horn interneurons mediating reflex actions of group II muscle afferents. All drugs were applied ionophoretically in deeply anesthetized cats. Effects of agonists were tested on extracellularly recorded responses of individual interneurons evoked by electrical stimulation of group II afferents in a muscle nerve. Effects of antagonists were tested against the depression of these responses after stimulation of raphe nuclei. The results show that both 5-HT(1A&7) and 5-HT(3) membrane receptors are involved in counteracting the activation of dorsal horn interneurons by group II afferents. Because only quantitative differences were found within the sample of the tested neurons, these results suggest that modulatory actions of 5-HT on excitatory and inhibitory interneurons might be similar. The relationship between 5-HT axons and axons immunoreactive for the 5-HT(3A) receptor subunit, which contact dorsal horn interneurons, was analyzed using immunofluorescence and confocal microscopy. Contacts from both types of axons were found on all interneurons, but their distribution and density varied, and there was no obvious relationship between them. In two of six interneurons, 5-HT(3A)-immunoreactive axons formed ring-like arrangements around the cell bodies. In previous studies, axons possessing 5-HT(3) receptors were found to be excitatory, and as 2-Me 5-HT depressed transmission to dorsal horn interneurons, the results indicate that 5-HT operates at 5-HT(3) receptors presynaptic to these neurons to depress excitatory transmission

A novel method to attain sinusoidal mechanical responses from single motor units.

INTRODUCTION: The relationship between output force and motor command depends on the intrinsic dynamic responses of motor units (MUs), which can be characterized by evoking accurate sinusoidal force responses at different frequencies. In this study we sought to determine whether sinusoidal modulation of the stimulation rate of single MUs results in reliable sinusoidal force changes. METHODS: Single axons of rat ventral roots were stimulated electrically by changing the pulse rate sinusoidally at different frequency modulation (0.4-1.0-2.0-4.0 Hz for slow, 1.0-2.0-4.0-7.0 Hz for fast MUs). The twitching sinusoidal force signal was interpolated. We calculated harmonic distortion (HD) and the correlation coefficient (r) between theoretical sines and interpolated signals. RESULTS: HD was always 0.97. CONCLUSIONS: The HD and r-values obtained indicate highly reliable sinusoidal responses, which supports the potential use of this method to further characterize the dynamic behavior of single MUs. Muscle Nerve 51: 134-136, 2015