Intrinsic excitability of human motoneurons in biceps brachii versus triceps brachii

Abstract

The intrinsic excitability of spinal motoneurons is mediated in part by the presence of persistent inward currents (PICs), which amplify synaptic input and promote self-sustained firing. Studies using animal models have shown that PICs are greater in extensor motoneurons over flexor motoneurons, but this difference has not yet been demonstrated in humans. The primary objective of this study was to determine whether a similar difference exists in humans by recording from motor units in biceps and triceps brachii during isometric contractions. We compared firing rate profiles of pairs of motor units, in which the firing rate of the lower-threshold "control" unit was used as an indicator of common drive to the higher-threshold "test" unit. The estimated contribution of the PIC was calculated as the difference in firing rate of the control unit at recruitment versus derecruitment of the test unit, a value known as the delta-F (ΔF). We found that ΔF values were significantly higher in triceps brachii (5.4 ± 0.9 imp/s) compared with biceps brachii (3.0 ± 1.4 imp/s; P < 0.001). This difference was still present even after controlling for saturation in firing rate of the control unit, rate modulation of the control unit, and differences in recruitment time between test and control units, which are known to contribute to ΔF variability. We conclude that human elbow flexor and extensor motor units exhibit differences in intrinsic excitability, contributing to different neural motor control strategies between muscle groups.

Direction-dependent arm kinematics reveal optimal integration of gravity cues

“…Although undeniably supportive, interpretation of these results is complicated by direction-dependent properties of the peripheral neuromuscular system (Enoka, 1996; Cotel et al, 2009; Wilson et al, 2015). As a further test, exploitation of microgravity environments (e.g., during the zero-G phase of parabolic flight) offers a powerful tool to interrogate neural vs. peripheral origins of the directional asymmetries.…”

“…In contrast, a peripheral origin of kinematic asymmetries leads to different predictions. Specifically, if directional asymmetries originate from flexor/extensor motoneuron properties, SR should not change during the zero-G phase of parabolic flight (Ishihara et al, 1996, 2002; Cotel et al, 2009; for a review see Nagatomo et al, 2014; Wilson et al, 2015). Else, if directional asymmetries originate from force production properties, SR asymmetries should be eliminated instantly, not gradually, in 0g (Enoka, 1996).…”

“…Results of this second experiment are of major importance to disentangle peripheral and neural mechanisms for direction-dependent kinematics. This is because, if the observed direction-dependent kinematics were due to properties of the neuromuscular system, either an abrupt change or no change at all would be expected in the new gravity environment (Enoka, 1996; Ishihara et al, 1996, 2002; Cotel et al, 2009; Nagatomo et al, 2014; Wilson et al, 2015). The fact that neither happens reveals a central mechanism.…”

“…Furthermore, upward/downward direction-dependent kinematics could also arise from the complex dynamics of the peripheral neuromuscular system. For example, the firing properties of extensor motoneurons (pulling the arm downwards when upright) are known to obey different rules from those of flexor motoneurons (pulling upwards when uptight; Cotel et al, 2009; Wilson et al, 2015). In addition, muscle force production generated by eccentric contraction (elongation, e.g.…”

Phase- and Workload-Dependent Changes in Corticospinal Excitability to the Biceps and Triceps Brachii during Arm Cycling

“…Increased input from Ia afferents is known to exert a strong excitatory influence on motoneurone excitability, which may lead to increased recruitment and/or firing rate by activating persistent inward currents (PICs), for example, which amplify synaptic inputs [34,35]. Wilson and colleagues (2015) [36] recently demonstrated, via indirect measures, that the contribution of PICs to motoneurone excitability was higher in the lateral head of the triceps brachii than the biceps brachii during isometric contractions. It is also noted that: (1) motoneurones with lower recruitment thresholds, such as those in the lateral head of the triceps brachii, also have a higher incidence of PICs; and (2) there is a higher incidence of PICs in extensor compared to flexor motoneurones [37,38].…”