TY - JOUR
T1 - Intrinsic excitability of human motoneurons in biceps brachii versus triceps brachii
AU - Wilson, Jessica M.
AU - Thompson, Christopher K.
AU - Miller, Laura C.
AU - Heckman, Charles J.
N1 - Publisher Copyright:
© 2015 the American Physiological Society.
PY - 2015/6/1
Y1 - 2015/6/1
N2 - 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 (∆F). We found that ∆F 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 ∆F 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.
AB - 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 (∆F). We found that ∆F 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 ∆F 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.
KW - Elbow extensors
KW - Elbow flexors
KW - Intrinsic excitability
KW - Motoneurons
KW - Neuromodulation
UR - http://www.scopus.com/inward/record.url?scp=84930838368&partnerID=8YFLogxK
U2 - 10.1152/jn.00960.2014
DO - 10.1152/jn.00960.2014
M3 - Article
C2 - 25787957
AN - SCOPUS:84930838368
SN - 0022-3077
VL - 113
SP - 3692
EP - 3699
JO - Journal of neurophysiology
JF - Journal of neurophysiology
IS - 10
ER -