LAUSR

Persistent inward calcium and sodium currents (IP) activated during motoneuron recruitment help synaptic inputs maintain self-sustained firing until de-recruitment. Here, we estimate the contribution of the IP to self-sustained firing in human motoneurons of varying recruitment threshold by measuring the difference in synaptic input needed to maintain minimal firing once the IP is fully activated compared with the larger synaptic input required to initiate firing prior to full IP activation. Synaptic input to ≈20 dorsiflexor motoneurons simultaneously recorded during ramp contractions was estimated from firing profiles of motor units decomposed from high-density surface-EMG. To avoid errors introduced when using high-threshold units firing in their nonlinear range, we developed methods where the lowest-threshold units firing linearly with force were used to construct a composite (control) firing rate profile to estimate synaptic input to the higher-threshold (test) units. The difference in the composite firing rate (synaptic input) at the time of test unit recruitment and de-recruitment (ΔF=Frecruit-Fde-recruit) was used to measure IP amplitude that sustained firing. Test units with recruitment thresholds 1-30% of maximum had similar ΔFs, which likely included both slow and fast motor units activated by small and large motoneurons, respectively. This suggests that the portion of the IP that sustains firing is similar across a wide range of motoneuron sizes. Higher-threshold units had more prolonged accelerations in firing rate at the onset of recruitment compared to lower-threshold units, likely reflecting IP activation closer to firing onset in the higher-threshold units, but well before firing onset in the lower-threshold units.