The Cellular Basis for the Generation of Firing Patterns in Human Motor Units

Acute intermittent hypoxia increases maximal motor unit discharge rates in people with chronic incomplete spinal cord injury

Acute intermittent hypoxia (AIH) is an emerging technique for enhancing neuroplasticity and motor function in respiratory and limb musculature. Thus far, AIH-induced improvements in strength have been reported for upper and lower limb muscles after chronic incomplete cervical spinal cord injury (iSCI), but the underlying mechanisms have been elusive. We used high-density surface EMG (HDsEMG) to determine if motor unit discharge behaviour is altered after 15 × 60 s exposures to 9% inspired oxygen, interspersed with 21% inspired oxygen (AIH), compared to breathing only 21% air (SHAM). We recorded HDsEMG from the biceps and triceps brachii of seven individuals with iSCI during maximal elbow flexion and extension contractions, and motor unit spike trains were identified using convolutive blind source separation. After AIH, elbow flexion and extension torque increased by 54% and 59% from baseline (P = 0.003), respectively, whereas there was no change after SHAM. Across muscles, motor unit discharge rates increased by ∼4 pulses per second (P = 0.002) during maximal efforts, from before to after AIH. These results suggest that excitability and/or activation of spinal motoneurons is augmented after AIH, providing a mechanism to explain AIH-induced increases in voluntary strength. Pending validation, AIH may be helpful in conjunction with other therapies to enhance rehabilitation outcomes after incomplete spinal cord injury, due to these enhancements in motor unit function and strength. KEY POINTS: Acute intermittent hypoxia (AIH) causes increases in muscular strength and neuroplasticity in people living with chronic incomplete spinal cord injury (SCI), but how it affects motor unit discharge rates is unknown. Motor unit spike times were identified from high-density surface electromyograms during maximal voluntary contractions and tracked from before to after AIH. Motor unit discharge rates were increased following AIH. These findings suggest that AIH can facilitate motoneuron function in people with incomplete SCI.

Developmental changes in motor unit activity patterns: child-adult comparison using discrete motor unit analysis

Using global surface electromyography (sEMG) and the sEMG threshold it has been suggested that children activate their type-II motor unit (MU) to a lesser extent compared with adults. However, when age-related differences in discrete MU activation are examined using sEMG decomposition this phenomenon is not observed. Furthermore, findings from these studies are inconsistent and conflicting. Therefore, the purpose of this study was to examine differences in discrete MU activation of the vastus lateralis (VL) between boys and men during moderate-intensity knee extensions. Seventeen boys and 20 men completed two laboratory sessions. Following a habituation session, maximal voluntary isometric knee extension (MVIC) torque was determined before completing trapezoidal contractions at 70% MVIC. sEMG of the VL was captured and mathematically decomposed into individual MU action potential trains. Motor unit action potential amplitude (MUAPamp), recruitment threshold (RT), and MU firing rates (MUFR) were calculated. We observed that MUAPamp-RT slope was steeper in men compared with boys (p < 0.05) even after accounting for fat thickness and quadriceps muscle depth. The mean MUFR and y-intercept of the MUFR-RT relationship were significantly (p < 0.001) lower in boys than in men. The slope of the MUFR-RT relationship tended to be steeper in men, but the differences did not reach statistical significance (p = 0.056). Overall, our results suggest that neural strategies used to produce torque are different among boys and men. Such differences may be related, in part, to boys' lower MUFR and lesser ability to activate their higher-threshold MUs. Although, other factors (e.g., muscle composition) likely also play a role.

How Can Proteomics Help to Elucidate the Pathophysiological Crosstalk in Muscular Dystrophy and Associated Multi-System Dysfunction?

This perspective article is concerned with the question of how proteomics, which is a core technique of systems biology that is deeply embedded in the multi-omics field of modern bioresearch, can help us better understand the molecular pathogenesis of complex diseases. As an illustrative example of a monogenetic disorder that primarily affects the neuromuscular system but is characterized by a plethora of multi-system pathophysiological alterations, the muscle-wasting disease Duchenne muscular dystrophy was examined. Recent achievements in the field of dystrophinopathy research are described with special reference to the proteome-wide complexity of neuromuscular changes and body-wide alterations/adaptations. Based on a description of the current applications of top-down versus bottom-up proteomic approaches and their technical challenges, future systems biological approaches are outlined. The envisaged holistic and integromic bioanalysis would encompass the integration of diverse omics-type studies including inter- and intra-proteomics as the core disciplines for systematic protein evaluations, with sophisticated biomolecular analyses, including physiology, molecular biology, biochemistry and histochemistry. Integrated proteomic findings promise to be instrumental in improving our detailed knowledge of pathogenic mechanisms and multi-system dysfunction, widening the available biomarker signature of dystrophinopathy for improved diagnostic/prognostic procedures, and advancing the identification of novel therapeutic targets to treat Duchenne muscular dystrophy.

Neuromotor control of spontaneous quiet breathing in awake rats evaluated by assessments of diaphragm EMG stationarity

The neuromotor control of the diaphragm muscle (DIAm) is dynamic. The activity of the DIAm can be recorded via electromyography (EMG), which represents the temporal summation of motor unit action potentials. Our goal in the present study was to investigate DIAm neuromotor control during quiet spontaneous breathing (eupnea) in awake rats by evaluating DIAm EMG at specific temporal locations defined by motor unit recruitment and derecruitment. We evaluated the nonstationarity of DIAm EMG activity to identify DIAm motor unit recruitment and derecruitment durations. Combined with assessments of root mean square (RMS) and sum of squares (SS) EMG, the durations of these phases provide physiological information about the temporal aspects of motor control. During eupnea in awake rats (n = 10), the duration of motor unit recruitment comprised 61 ± 19 ms of the onset-to-peak duration (214 ± 62 ms) of the DIAm RMS EMG. The peak-to-offset duration of DIAm EMG activity was 453 ± 96 ms, with a terminating period of derecruitment of 161 ± 44 ms. The burst duration was 673 ± 128 ms. Both the RMS EMG amplitude and the SS EMG were higher at the completion of motor unit recruitment than at the start of motor unit derecruitment, suggesting that offset discharge rates were lower than onset discharge rates. Our analyses provide novel insights into the time domain aspects of DIAm neuromotor control and allow indirect estimates of the contribution of recruitment and frequency to RMS EMG amplitude during eupnea in awake rats.NEW & NOTEWORTHY We characterized three phases of neuromotor control-motor unit recruitment, sustained activity, and derecruitment-based on statistical assessments of stationarity of the diaphragm muscle (DIAm) EMG activity in awake rats. Our findings may allow indirect estimates of the contribution of motor unit recruitment and frequency coding toward generating force and provide novel insights about the temporal aspects of DIAm neuromotor control and descending respiratory drive in unanesthetized animals.

Ageing reduces persistent inward current contribution to motor neurone firing: Potential mechanisms and the role of exercise

Nervous system deterioration is a primary driver of age-related motor impairment. The motor neurones, which act as the interface between the central nervous system and the muscles, play a crucial role in amplifying excitatory synaptic input to produce the desired motor neuronal firing output. For this, they utilise their ability to generate persistent (long-lasting) depolarising currents that increase cell excitability, and both amplify and prolong the output activity of motor neurones for a given synaptic input. Modulation of these persistent inward currents (PICs) contributes to the motor neurones' capacities to attain the required firing frequencies and rapidly modulate them to competently complete most tasks. Thus, PICs are crucial for adequate movement generation. Impairments in intrinsic motor neurone properties can impact motor unit firing capacity, with convincing evidence indicating that the PIC contribution to motor neurone firing is reduced in older adults. Indeed, this could be an important mechanism underpinning the age-related reductions in strength and physical function. Furthermore, resistance training has emerged as a promising intervention to counteract age-associated PIC impairments, with changes in PICs being correlated with improvements in muscular strength and physical function after training. In this review, we present the current knowledge of the PIC magnitude decline during ageing and discuss whether reduced serotonergic and noradrenergic input onto the motor neurones, voltage-gated calcium channel dysfunction or inhibitory input impairments are candidates that: (i) explain age-related reductions in the PIC contribution to motor neurone firing and (ii) underpin the enhanced PIC contribution to motor neurone firing following resistance training in older adults.

Estimates of persistent inward currents in lower limb motoneurons are larger in females than in males

Noninvasive recordings of motor unit (MU) spike trains help us understand how the nervous system controls movement and how it adapts to various physiological conditions. The majority of participants in human and nonhuman animal physiology studies are male, and it is assumed that mechanisms uncovered in these studies are shared between males and females. However, sex differences in neurological impairment and physical performance warrant the study of sex as a biological variable in human physiology and performance. To begin addressing this gap in the study of biophysical properties of human motoneurons, we quantified MU discharge rates and estimates of persistent inward current (PIC) magnitude in both sexes. We decomposed MU spike trains from the tibialis anterior (TA), medial gastrocnemius (MG), and soleus (SOL) using high-density surface electromyography and blind source separation algorithms. Ten participants of each sex performed slow triangular (10 s up and down) isometric contractions to a peak of 30% of their maximum voluntary contraction. We then used linear mixed-effects models to determine if peak discharge rate and estimates of PICs were predicted by the fixed effects of sex, muscle, and their interaction. Despite a lack of sex-differences in peak discharge rates across all muscles, estimates of PICs were larger [χ2(1) = 6.26, P = 0.012] in females [4.73 ± 0.242 pulses per second (pps)] than in males (3.81 ± 0.240 pps). These findings suggest that neuromodulatory drive, inhibitory input, and/or biophysical properties of motoneurons differ between the sexes and may contribute to differences in MU discharge patterns.NEW & NOTEWORTHY Sex-related differences in motoneuron analyses have emerged with greater inclusion of female participants, however, mechanisms for these differences remain unclear. Estimates of persistent inward currents (i.e., ΔF) in motoneurons of the lower limb muscles were larger in females than in males. This suggests neuromodulatory drive, monoaminergic signaling, intrinsic motoneuron properties, and/or descending motor commands may differ between the sexes, which provides a potential mechanism underlying previously reported sex-related differences in motoneuron discharge patterns.

A geometric approach to quantifying the neuromodulatory effects of persistent inward currents on individual motor unit discharge patterns

Objective.All motor commands flow through motoneurons, which entrain control of their innervated muscle fibers, forming a motor unit (MU). Owing to the high fidelity of action potentials within MUs, their discharge profiles detail the organization of ionotropic excitatory/inhibitory as well as metabotropic neuromodulatory commands to motoneurons. Neuromodulatory inputs (e.g. norepinephrine, serotonin) enhance motoneuron excitability and facilitate persistent inward currents (PICs). PICs introduce quantifiable properties in MU discharge profiles by augmenting depolarizing currents upon activation (i.e. PIC amplification) and facilitating discharge at lower levels of excitatory input than required for recruitment (i.e. PIC prolongation).Approach. Here, we introduce a novel geometric approach to estimate neuromodulatory and inhibitory contributions to MU discharge by exploiting discharge non-linearities introduced by PIC amplification during time-varying linear tasks. In specific, we quantify the deviation from linear discharge ('brace height') and the rate of change in discharge (i.e. acceleration slope, attenuation slope, angle). We further characterize these metrics on a simulated motoneuron pool with known excitatory, inhibitory, and neuromodulatory inputs and on human MUs (number of MUs; Tibialis Anterior: 1448, Medial Gastrocnemius: 2100, Soleus: 1062, First Dorsal Interosseus: 2296).Main results. In the simulated motor pool, we found brace height and attenuation slope to consistently indicate changes in neuromodulation and the pattern of inhibition (excitation-inhibition coupling), respectively, whereas the paired MU analysis (ΔF) was dependent on both neuromodulation and inhibition pattern. Furthermore, we provide estimates of these metrics in human MUs and show comparable variability in ΔFand brace height measures for MUs matched across multiple trials.Significance. Spanning both datasets, we found brace height quantification to provide an intuitive method for achieving graded estimates of neuromodulatory and inhibitory drive to individual MUs. This complements common techniques and provides an avenue for decoupling changes in the level of neuromodulatory and pattern of inhibitory motor commands.