Facilitation-inhibition control of motor neuronal persistent inward currents in young and older adults

ABSTRACT A well-coordinated control of motor neuronal persistent inward currents (PICs) via diffuse neuromodulation and local inhibition is essential to ensure motor units discharge at required times and frequencies. Current best estimates indicate that PICs are reduced in older adults; however, it is not yet known whether PIC facilitation-inhibition control is also altered with ageing. We investigated the responses of PICs to i) a remote handgrip contraction, which is believed to diffusely increase serotonergic input onto motor neurones, and ii) tendon vibration of the antagonist muscle, which elicits reciprocal inhibition, in both young and older adults. High-density surface electromyograms were collected from soleus and tibialis anterior of 18 young and 26 older adults during triangular-shaped plantar and dorsiflexion contractions to 20% (handgrip experiments) and 30% (vibration experiments) of maximum torque (rise-decline rate of 2%/s). The paired-motor-unit analysis was used to calculate ΔF, which is assumed proportional to PIC strength. ΔF increased in both soleus (0.55pps, 16.0%) and tibialis anterior (0.42pps, 11.4%) during the handgrip contraction independent of age. However, although antagonist tendon vibration reduced ΔF in soleus (0.28pps, 12.6%) independent of age, less reduction was observed in older (0.42pps, 10.7%) than young adults (0.72pps, 17.8%) in tibialis anterior . Our data indicate a preserved ability of older adults to amplify PICs following a remote handgrip contraction, during which increased serotonergic input onto the motor neurones is expected, in both lower leg muscles. However, PIC deactivation in response to reciprocal inhibition was impaired with ageing in tibialis anterior despite being preserved in soleus . KEYPOINTS Motor neuronal persistent inward currents (PICs) are amplified via diffuse neuromodulation and deactivated by local inhibition to ensure motor units discharge at required times and frequencies, allowing a normal motor behaviour. PIC amplitudes appear to be reduced with ageing, however it is not known whether PIC facilitation-inhibition control is also altered. Remote handgrip contraction, which should diffusely increase serotonergic input onto motor neurones, amplified PICs similarly in both soleus and tibialis anterior of young and older adults. Antagonist tendon vibration, which induces reciprocal inhibition, reduced PICs in soleus in both young and older adults but had less effect in tibialis anterior in older adults. Our data suggest that older adults have preserved soleus PIC facilitation during lowintensity contractions, equivalent to activities such as standing and walking. However, a reduced reciprocal inhibition of PICs in tibialis anterior may contribute to locomotion impairments, such as increases in soleus-tibialis anterior co-activation during propulsion.