Fatigue-related changes in discharge patterns of motor units in the inferior head of the lateral pterygoid muscle in humans

Influence of proprioceptive feedback on the firing rate and recruitment of motoneurons

We investigated the relationships of the firing rate and maximal recruitment threshold of motoneurons recorded during isometric contraction with the number of spindles in individual muscles. At force levels above 10% of maximal voluntary contraction, the firing rate was inversely related to the number of spindles in a muscle, with the slope of the relationship increasing with force. The maximal recruitment threshold of motor units increased linearly with the number of spindles in the muscle. Thus, muscles with a greater number of spindles had lower firing rates and a greater maximal recruitment threshold. These findings may be explained by a mechanical interaction between muscle fibres and adjacent spindles. During low-level (0% to 10%) voluntary contractions, muscle fibres of recruited motor units produce force twitches that activate nearby spindles to respond with an immediate excitatory feedback that reaches maximal level. As the force increases further, the twitches overlap and tend towards tetanization, the muscle fibres shorten, the spindles slacken, their excitatory firings decrease, and the net excitation to the homonymous motoneurons decreases. Motoneurons of muscles with greater number of spindles receive a greater decrease in excitation which reduces their firing rates, increases their maximal recruitment threshold, and changes the motoneuron recruitment distribution.

Does motoneuron adaptation contribute to muscle fatigue?

To help reduce the gap between the cellular physiology of motoneurons (MNs) as studied "bottom-up" in animal preparations and the "top-down" study of the firing patterns of human motor units (MUs), this article addresses the question of whether motoneuron adaptation contributes to muscle fatigue. Findings are reviewed on the intracellularly recorded electrophysiology of spinal MNs as studied in vivo and in vitro using animal preparations, and the extracellularly recorded discharge of MUs as studied in conscious humans. The latter "top-down" approach, combined with kinetic measurements, has provided most of what is currently known about the neurobiology of muscle fatigue, including its task and context dependencies. It is argued that although the question addressed is still open, it should now be possible to design new "bottom-up" research paradigms using animal preparations that take advantage of what has been learned with the use of relatively noninvasive quantitative procedures in conscious humans.

Is the human masticatory system devoid of recurrent inhibition?

The aim of the present study was to investigate the existence or otherwise of a functional recurrent inhibitory system (Renshaw cell system) in the motoneurons that innervate human masticatory muscles. In a previous study, L: -acetylcarnitine (L: -Ac), a substance known to potentiate recurrent inhibition in humans was found to alter, in a specific way, the discharge variability, and the synchronous activity of motor units depending on the presence or absence of recurrent inhibition in the corresponding motoneuron pool. Using a similar paradigm, we have recorded the tonic discharge activity of motor unit pairs from the masseter muscle during voluntary isometric contraction while subjects were undergoing continuous intravenous saline (SAL, NaCl 0.9%) perfusion. Following a brief baseline-recording period, the subjects were given a test injection of either L: -Ac or isotonic saline (SAL) in a double blind manner. The variability, synchronization, and coherence between the motor unit discharges were analysed during three successive periods: pre-injection, during injection, and post-injection, each lasting 2-3 min. Neither L: -Ac nor SAL injection induced a significant change in the inter-spike interval (ISI) or the coefficient of variation of the ISIs in the motor units tested. There were also no significant changes in the pattern of synchronous activity or in the coherence, which reflects the common frequency content of the unit discharges. Reminiscent of what had been observed previously with motoneurons without recurrent inhibition in the Abductor Digitorum Minimi muscle, the lack of effects of L: -Ac injection on the firing behaviour of masseter motoneurons may suggest that classical Renshaw cell inhibition is lacking in this motoneuron pool.

Physiological characteristics of motor units in the brachioradialis muscle across fatiguing low-level isometric contractions

The purpose of this study was to determine (i) if decomposition-based quantitative electromyography (DQEMG) could detect changes in motor unit potential (MUP) morphology and motor unit (MU) firing pattern statistics associated with muscle fatigue, (ii) if any detected changes are correlated with surface electromyographic (SEMG) signs of fatigue, and (iii) if significant fatigue-dependent changes are repeatable within individuals. Mean MU firing rates and the morphology of MUPs detected using needle and surface electrodes during constant-torque isometric contractions held until exhaustion were investigated in the brachioradialis (BR) muscle in 10 healthy volunteers (mean age=28.6 yr, SD+/-3.9). Time dependant changes were investigated using an analysis of variance with normalized time as a main effect. Partial correlation coefficients were computed using a repeated measures analysis of covariance to determine if changes in MU firing rates, needle-detected MUPs and surface-detected MUPs (SMUPs) were related to changes in SEMG signal amplitude and frequency parameters. Intraclass correlation coefficients (ICCs) were used to determine the within-subject repeatability of changes in MU firing rates, and MUP and SMUP parameters. Significant decreases in mean MU firing rates were found along with significant increases in various duration and area related parameters in both MUPs and SMUPs across the fatiguing contraction. The SEMG signal demonstrated the expected changes with fatigue: an increase in amplitude and a decrease in frequency content. SEMG amplitude was significantly positively correlated with SMUP peak-to-peak voltage (r=0.85, p<0.05), and SMUP area (r=0.86, p<0.05). Mean power frequency was significantly negatively correlated with SMUP negative peak duration (r=-0.74, p<0.05). The significant time-dependent changes were reliably observed (ICCs were 0.94 for MUP peak to peak amplitude, 0.97 for MUP area and 0.95 for MUP area to amplitude ratio, 0.95 for SMUP peak-to-peak voltage, 0.83 for SMUP area, 0.99 for SMUP negative peak amplitude and 0.88 for SMUP negative peak area). The decreases in mean MU firing rates measured along with the increases in amplitude, duration and area parameters of MUPs and SMUPs and their partial correlation with SEMG amplitude during submaximal fatiguing contractions of the BR, suggest that recruitment is a main cause of increased SEMG amplitude parameters with fatigue. We conclude that DQEMG can be effectively and reliably used to detect changes in physiological characteristics of MUs that accompany fatigue.