Biology subjects

Trajano, G. S.

Publications and source records attributed to Trajano, G. S..

4 recordsLinked to original sources

bioRxiv2021

INTRINSIC MOTOR NEURONE EXCITABILITY IS REDUCED IN SOLEUS AND TIBIALIS ANTERIOR OF OLDER ADULTS

Age-related deterioration within both motor neurones and monoaminergic systems should theoretically reduce neuromodulation by weakening motor neuronal persistent inward current (PIC) amplitude. However, this assumption remains untested. Surface electromyographic signals were collected using two 32-channel electrode matrices placed on soleus and tibialis anterior of 25 older adults (70{+/-}4years) and 17 young adults (29{+/-}5 years) to investigate motor unit discharge behaviours. Participants performed triangular-shaped plantar and dorsiflexion contractions to 20% of maximum torque at a rise-decline rate of 2%/s of each participants maximal torque. Pairwise and composite paired-motor unit analyses were adopted to calculate delta frequency ({Delta}F), which has been used to differentiate between the effects of synaptic excitation and intrinsic motor neuronal properties and is assumed to be proportional to PIC amplitude. Soleus and tibialis anterior motor units in older adults had lower {Delta}Fs calculated with either the pairwise [-0.99 and -1.46 pps; -35.4 and - 33.5%, respectively] or composite (-1.18 and -2.28 pps; -32.1 and -45.2%, respectively) methods. Their motor units also had lower peak discharge rates (-2.14 and -2.03 pps; -19.7 and -13.9%, respectively) and recruitment thresholds (-1.50 and -2.06% of maximum, respectively) than young adults. These results demonstrate reduced intrinsic motor neurone excitability during low-force contractions in older adults, likely mediated by decreases in the amplitude of persistent inward currents. Our findings might be explained by deterioration in the motor neurones or monoaminergic systems and could contribute to the decline in motor function during ageing; these assumptions should be explicitly tested in future investigations.

physiology↗

bioRxiv2020

Do Motoneurons Slow With Aging? A Systematic Review And Meta-Analysis With Meta-Regression

Nervous system maladaptation is linked to the loss of maximal strength production and motor control with aging. Motor unit discharge rates are a critical determinant of force production; thus, lower discharge rates could be a mechanism underpinning maximal strength and motor control losses during aging. This meta-analysis summarized the findings of studies comparing motor unit discharge rates between young and older adults, and examined the effect of distinct muscles and contraction intensities on the magnitude of discharge rates difference between these two groups. Eligible studies were combined in a meta-analysis, including tested contraction intensities and muscles in different levels, to investigate whether there were differences in discharge rates between younger and older adults. Motor unit discharge rates were higher in younger adults compared to older adults, with a pooled standardized mean difference (SMD) for all studies of 0.63 (95%CI= 0.27 to 0.99). Contraction intensity had a significant effect on the pooled SMD, with a 1% increase in intensity associated with a 0.009 (95%CI= 0.003 to 0.015) change in the pooled SMD. These findings suggest that the reductions in motor unit discharge rates, especially at higher contraction intensities, may be an important mechanism underpinning age-related losses in maximal strength production.

physiology↗

bioRxiv2020

Persistent Inward Currents Increase With The Level Of Voluntary Drive In Plantar Flexor Low-Threshold Motor Units

This study tested the hypothesis that estimates of persistent inward currents (PICs) in the human plantar flexors would increase with the level of voluntary drive. Twenty-one participants volunteered for this study (29.2{+/-}2.6 years). High-density surface electromyograms were collected from soleus and gastrocnemius medialis during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise of 2%/s and 30-s duration) of each participants maximal torque. Motor units identified in all the contraction intensities were included in the paired-motor unit analysis to calculate delta frequency ({Delta}F) and estimate the PICs. Increases in PICs were observed from 10% to 20% ({Delta}=0.6 pps; p<0.001) and 20% to 30% ({Delta}=0.5 pps; p<0.001) in soleus, and from 10% to 20% ({Delta}=1.2 pps; p<0.001) but not 20% to 30% ({Delta}=0.09 pps; p=0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% (respectively, {Delta}=1.75 pps, p<0.001; and {Delta}=2.43 pps, p<0.001) and 20% to 30% (respectively, {Delta}=0.80 pps, p<0.017; and {Delta}=0.92 pps, p=002). The repeated-measures correlation identified associations between {Delta}F and increases in maximal discharge rate for both soleus (r=0.64; p<0.001) and gastrocnemius medialis (r=0.77; p<0.001). An increase in voluntary drive tends to increase PIC strength, which has key implications for the control of force but also for comparisons between muscles or between studies when relative force levels might be different. These data indicate that increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit firing, and thus force output modulation.

neuroscience↗

bioRxiv2020

Passive muscle stretching reduces estimates of persistent inward current strength in soleus motor units

Prolonged ([≥]60 s) passive muscle stretching acutely reduces maximal force production at least partly through a suppression of efferent neural drive. The origin of this neural suppression has not been determined, however some evidence suggests that reductions in the amplitude of persistent inward currents (PICs) in the motoneurons may be important. The aim of the present study was to determine whether acute passive (static) muscle stretching affects PIC strength in gastrocnemius medialis (GM) and soleus (SOL) motor units. We calculated the difference in instantaneous discharge rates at recruitment and derecruitment ({Delta}F) for pairs of motor units in GM and SOL during triangular isometric plantar flexor contractions (20% maximum) both before and immediately after a 5-min control period and immediately after five 1-min passive plantar flexor stretches. After stretching there was a significant reduction in SOL {Delta}F (-25.6%; 95%CI = -45.1 to -9.1 %, p=0.002) but not GM {Delta}F. These data suggest passive muscle stretching can reduce the intrinsic excitability, via PICs, of SOL motor units. These findings (1) suggest that PIC strength might be reduced after passive stretching, (2) are consistent with previously-established post-stretch decreases in SOL but not GM EMG amplitudes during contraction, and (3) indicate that reductions in PIC strength could underpin the stretch-induced force loss. SUMMARY STATEMENTMotoneurons require an amplification mechanism to operate within the firing frequencies observed during normal motor behaviour. Here we present evidence that this amplification mechanism is reduced after passive muscle stretching.

physiology↗