Theoretical Model and Experimental Validation of the estimated proportions of common and independent input to motor neurons

Abstract

Abstract-Motor neurons in the spinal cord receive synaptic input that comprises common and independent components. The part of synaptic input that is common to all motor neurons is the one regulating the production of force. Therefore, its quantification is important to assess the strategy used by Central Nervous System (CNS) to control and regulate movements, especially in physiological conditions such as fatigue. In this study we present and validate a method to estimate the ratio between strengths of common and independent inputs to motor neurons and we apply this method to investigate its changes during fatigue. By means of coherence analysis we estimated the level of correlation between motor unit spike trains at the beginning and at the end of fatiguing contractions of the Tibialis Anterior muscle at three different force targets. Combining theoretical modeling and experimental data we estimated the strength of the common synaptic input with respect to the independent one. We observed a consistent increase in the proportion of the shared input to motor neurons during fatigue. This may be interpreted as a strategy used by the CNS to counteract the occurrence of fatigue and the concurrent decrease of generated force.

Exogenous neuromodulation of spinal neurons induces beta-band coherence during self-sustained discharge of hind limb motor unit populations

“…The effect of methoxamine administration on each dependent variable was determined using separate linear mixed models. Independent variables in the model included drug (administered/not administered) and animal (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Drug was included in the model as a fixed, repeated factor, and animal was included as a random factor with a random intercept and random slope over the effect of drug.…”

“…Within each trial, iterative coherence analysis was performed between the cumulative spike trains (CSTs) derived from two equally sized groups of up to 10 unique motor unit spike trains using Welch's averaged periodogram method using 1-s nonoverlapping windows (46). Within each trail, the level of significance was defined as the peak of the coherence function for frequencies Ͼ1,000 Hz (2,8,36). The coherence functions were z-transformed to account for differing number of windows across trails (52) and averaged within each animal to provide a single coherence function for the control and/or postmethoxamine conditions for each animal.…”

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

“…This was accomplished by computing coherence functions between the CSTs for groups of motor units. Coherence estimates increase monotonically with the number of motor units used to derive each CST (Castronovo et al 2015b;Farina et al 2012;Negro et al 2016b), with a rate of change that depends on the ratio between the common and independent components of the synaptic input. Therefore, we analyzed the rate of increase in coherence peaks for CSTs based on one pair of motor units or two groups of five motor units (Negro et al 2016b).…”

Control of Spinal Motoneurons by Feedback From a Non-Invasive Real-Time Interface

“…The strength of common input received by the motoneurons in the Ctrl-Set and A-Set, as well as in the Ctrl-Set and B-Set was estimated by using the maximum-squared coherence analysis on the de-meaned unfiltered CSTs of the corresponding motor units [28]- [31]. Therefore, the power spectral densities of both CSTs ( ) and / ( ), as well as the cross power spectral density / ( ) (Hanning window of 1 s, 50 % overlap) determined the estimated coherence value / ( ), as follows:…”