Functional compartmentalisation of motor units during the stretch reflex
Year: 2022
Type: dissertation
Abstract: Within the biceps brachii muscle, the activation of anatomically localized subsets of motor units depends on the function the muscle is performing, either flexion, supination, or exorotation. However, the relationship between this functional compartmentalisation and the stretch reflex has not been characterised. This thesis aims to determine if the stretch reflex elicits a functionally-relevant subset of motor units. High density surface electromyography (HDsEMG) data were recorded from the biceps brachii muscle during voluntary isometric flexion and supination and evoked stretch reflex responses while we varied experimental task. Convolutive blind source separation software was used to extract individual motor unit action potential spike trains from the EMG and 2-D cross-correlation was used to track these motor units both within task and between tasks. Using group size comparison statistics as seen in previous related studies, medians of voluntary task-specific motor units (flexion-only or supination-only) were different from non-task-specific motor units (tracked across flexion and supination) (p = 0.039) and reflex-only motor units were different from five out of six groups of voluntary motor units (p = 0.013) across a range of contraction levels (10 – 70% MVC). We then used a probabilistic detection model to compare with our experimental data and found no degree of similarity between the model and our experimental data, providing evidence for the first time of functional compartmentalisation of stretch reflex motor units, as well as of voluntary motor units using HDsEMG. Histograms of cross-correlation coefficients as a measure of matching confirmed these findings, as we found a bimodal distribution among voluntary motor units that was not seen when comparing these motor units with reflex motor units. The stretch reflex is integral to motor control, from response to external perturbation to moment-to-moment limb correction in space. Characterisation of the stretch reflex at the level of the motor unit provides insight into how muscles physically enact the reflex spinal loop signals and contribute to our understanding of how these pathways and motor units are affected in disorders involving the stretch reflex, including spasticity after stroke and hyperreflexia in cerebral palsy. We provide evidence for the first time of functional compartmentalisation of motor units involved in the stretch reflex, and that this subpopulation of motor units differs from that active during voluntary contraction.
Author: Culberson, Danielle Lee
Language: English
Subfield: Biomedical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7066120604