Force Steadiness: From Motor Units to Voluntary Actions

Physiology ◽  
2021 ◽  
Vol 36 (2) ◽  
pp. 114-130 ◽  
Author(s):  
Roger M. Enoka ◽  
Dario Farina
Keyword(s):  
Motor Function ◽  
Motor Neurons ◽  
Motor Units ◽  
Isometric Contractions ◽  
Strongly Correlated ◽  
Synaptic Inputs ◽  
Force Steadiness ◽  
Spinal Motor Neurons ◽  
Spinal Motor

Voluntary actions are controlled by the synaptic inputs that are shared by pools of spinal motor neurons. The slow common oscillations in the discharge times of motor units due to these synaptic inputs are strongly correlated with the fluctuations in force during submaximal isometric contractions (force steadiness) and moderately associated with performance scores on some tests of motor function. However, there are key gaps in knowledge that limit the interpretation of differences in force steadiness.

scholarly journals Central and peripheral innervation patterns of defined axial motor units in larval zebrafish

2019 ◽  
Author(s):  
Saul Bello-Rojas ◽  
Ana E. Istrate ◽  
Sandeep Kishore ◽  
David L. McLean
Keyword(s):  
Motor Neurons ◽  
Muscle Fibers ◽  
Motor Units ◽  
Spinal Motor Neurons ◽  
Larval Zebrafish ◽  
Spinal Motor ◽  
Axon Collaterals ◽  
Peripheral Innervation ◽  
Innervation Patterns ◽  
Fast Twitch

AbstractSpinal motor neurons and the peripheral muscle fibers they innervate form discrete motor units that execute movements of varying force and speed. Subsets of spinal motor neurons also exhibit axon collaterals that influence motor output centrally. Here, we have used in vivo imaging to anatomically characterize the central and peripheral innervation patterns of axial motor units in larval zebrafish. Using early born ‘primary’ motor neurons and their division of epaxial and hypaxial muscle into four distinct quadrants as a reference, we define three distinct types of later born ‘secondary’ motor units. The largest are ‘m-type’ units, which innervate deeper fast-twitch muscle fibers via medial nerves. Next in size are ‘ms-type’ secondaries, which innervate superficial fast-twitch and slow fibers via medial and septal nerves, followed by ‘s-type’ units, which exclusively innervate superficial slow muscle fibers via septal nerves. All types of secondaries innervate up to four axial quadrants. Central axon collaterals are found in subsets of primaries based on soma position and predominantly in secondary fast-twitch units (m, ms) with increasing likelihood based on number of quadrants innervated. Collaterals are labeled by synaptophysin-tagged fluorescent proteins, but not PSD95, consistent with their output function. Also, PSD95 dendrite labeling reveals that larger motor units receive more excitatory synaptic input. Collaterals are largely restricted to the neuropil, however perisomatic connections are observed between motor units. These observations suggest that recurrent interactions are dominated by motor neurons recruited during stronger movements and set the stage for functional investigations of recurrent motor circuitry in larval zebrafish.

scholarly journals Spinal motor neurons and motor function in older adults

2018 ◽  
Vol 266 (1) ◽  
pp. 174-182 ◽  
Author(s):  
Aron S. Buchman ◽  
Sue E. Leurgans ◽  
Veronique G. J. M. VanderHorst ◽  
Sukriti Nag ◽  
Julie A. Schneider ◽  
...  
Keyword(s):  
Older Adults ◽  
Motor Function ◽  
Motor Neurons ◽  
Spinal Motor Neurons ◽  
Spinal Motor

scholarly journals Decrease in force steadiness with aging is associated with increased power of the common but not independent input to motor neurons

2018 ◽  
Vol 120 (4) ◽  
pp. 1616-1624 ◽  
Author(s):  
Anna Margherita Castronovo ◽  
Natalie Mrachacz-Kersting ◽  
Andrew James Thomas Stevenson ◽  
Ales Holobar ◽  
Roger Maro Enoka ◽  
...  
Keyword(s):  
Motor Unit ◽  
Motor Neurons ◽  
Synaptic Input ◽  
Force Control ◽  
Common Input ◽  
Low Frequencies ◽  
Force Steadiness ◽  
Spinal Motor Neurons ◽  
Spinal Motor ◽  
The Common

Declines in motor function with advancing age have been attributed to changes occurring at all levels of the neuromuscular system. However, the impact of aging on the control of muscle force by spinal motor neurons is not yet understood. In this study on 20 individuals aged between 24 and 75 yr (13 men, 7 women), we investigated the common synaptic input to motor neurons of the tibialis anterior muscle and its impact on force control. Motor unit discharge times were identified from high-density surface EMG recordings during isometric contractions at forces of 20% of maximal voluntary effort. Coherence analysis between motor unit spike trains was used to characterize the input to motor neurons. The decrease in force steadiness with age ( R2 = 0.6, P < 0.01) was associated with an increase in the amplitude of low-frequency oscillations of functional common synaptic input to motor neurons ( R2 = 0.59; P < 0.01). The relative proportion of common input to independent noise at low frequencies increased with variability (power) in common synaptic input. Moreover, variability in interspike interval did not change and strength of the common input in the gamma band decreased with age ( R2 = 0.22; P < 0.01). The findings indicate that age-related reduction in the accuracy of force control is associated with increased common fluctuations to motor neurons at low frequencies and not with an increase in independent synaptic input. NEW & NOTEWORTHY The influence of aging on the role of spinal motor neurons in accurate force control is not yet understood. We demonstrate that aging is associated with increased oscillations in common input to motor neurons at low frequencies and with a decrease in the relative strength of gamma oscillations. These results demonstrate that the synaptic inputs to motor neurons change across the life span and contribute to a decline in force control.

scholarly journals Translatomic analysis of regenerating and degenerating spinal motor neurons in injury and ALS

iScience ◽  
2021 ◽  
pp. 102700
Author(s):  
Jennifer L. Shadrach ◽  
Wesley M. Stansberry ◽  
Allison M. Milen ◽  
Rachel E. Ives ◽  
Elizabeth A. Fogarty ◽  
...  
Keyword(s):  
Motor Neurons ◽  
Spinal Motor Neurons ◽  
Spinal Motor

A new method of isolating spinal motor neurons from fetal mouse

2017 ◽  
Vol 288 ◽  
pp. 57-61 ◽  
Author(s):  
Weifang Wang ◽  
Bao Qi ◽  
Hui Lv ◽  
Fei Wu ◽  
Lulu Liu ◽  
...  
Keyword(s):  
Motor Neurons ◽  
New Method ◽  
Fetal Mouse ◽  
Spinal Motor Neurons ◽  
Spinal Motor
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