Differences in Human Motoneuron Excitability Between Functionally Diverse Muscles

Abstract

Introduction: Spinal motoneurons (MN) transmit neural commands from the brain to the muscles they innervate and, as a result, produce functional movement. However, MNs are not simply passive conduits of these command and, instead, actively shape motor output through alterations in intrinsic excitability. We hypothesize that the excitability of MNs is not fixed across the body; instead, MNs are functionally tuned to the tasks they control. Here, we investigate this mapping of MN excitability across motor pools.
 Methods: High-density surface electromyography of the tibialis anterior (TA) and first dorsal interosseous (FDI) was recorded from four neurologically intact participants while they performed low-level, isometric contractions. The data were decomposed into underlying motor unit action potentials and paired motor unit analyses were subsequently performed on these spike trains to quantify MN excitability (ΔF).
 Results: 1,638 motor unit spike trains were extracted across all contractions. Mann-Whitney U test revealed that all subjects (4/4) had significantly higher maximal discharge rates in FDI, 19.13 [17.62 – 20.59] pps, when compared to the TA, 13.08 [11.51 – 15.46] pps. All subjects (4/4) had a higher ΔF in the TA (4.22 [2.89 – 5.61] pps) than the FDI (3.62 [1.23 – 5.94] pps), with 3/4 reaching statistical significance.
 Conclusions: Our findings suggest that the discharge rate and intrinsic excitability of human MNs differs across TA and FDI motor pools during similar isometric tasks. These results support the notion that motor pools are functionally tuned to their environmental demands.