Neurophysiology; gain control; neuromodulation; serotonin; persistent inward currents.
Serotonin and norepinephrine amplify and prolong synaptic inputs in the spinal cord though the development of persistent inward currents (PICs). Previous studies showed that the increase of serotonin induced by upper limb task also increased the excitability of lower limb motor units since the projections of the brainstem are diffuse. This study aims to investigate if greater intensities and volume of handgrip generates greater excitability in tibialis anterior’s motor neurones. Will be recruited thirty apparently healthy young women and men (18–40 years), with no injuries in the tested limbs, not taking medications, with body mass index <30kg/m2. Participants will visit the laboratory once and perform handgrip to 40% of the maximal isometric voluntary contraction (MIVC) for 15s and 30s, and to 80% of MIVC for 15s. Motor neurone excitability will be tested by ipsilateral dorsiflexion ramp-shaped contractions to 20% of MIVC. High-density surface electromyography will be used to estimate the contribution of PICs to self-sustained firings in motor neurones through the difference (ΔF) of the firing frequency. Linear mixed-effect models will be used to compare ΔF, peak discharge rates and recruitment thresholds over time. Omega-square will be used to investigate effect size (0−0.01, very small;0.01−0.06, small; 0.06−0.14, moderate; and>0.14, large).