Changes in Motor Unit Firing Patterns as a Function of Age, Muscle, and Following a Unilateral Brain Injury: Ionotropic and Metabotropic EffectsPublic
Coordinated movement relies on the precise and controlled activation of populations of motor units, which convert the commands of the nervous system into muscle forces. Motor unit firing patterns are often nonlinear and generated through the response to a combination of ionotropic excitatory and inhibitory commands, as well as metabotropic neuromodulatory inputs. Analysis of these motor unit firing patterns provides insight into the motor commands utilized for movement in both healthy and pathological states. Through a series of experiments, we sought to analyze motor unit firing patterns to understand the changes in both ionotropic and metabotropic motor commands that occur in various cohorts. This was accomplished through a comprehensive characterization of modern motor unit analyses methods in young, healthy adults, followed by an investigation into the changes in motor unit patterns associated with healthy aging, and finally the comparison of the motor unit firing patterns of neurologically-intact individuals and those who had suffered a unilateral brain injury, within the same age group. Novel techniques allow for the recording of populations of motor units concurrently. We investigated the distribution of motor units decomposed using these new methods and population recordings to conduct a sensitivity analysis of a common motor unit measure of persistent inward currents, which are modulated by metabotropic inputs. Together these studies enable us to take full advantage of the novel motor unit recording method and facilitate the comparison of the results presented in this dissertation with previous work using different computational or recording methods. In our third study we investigated the changes in motor unit firing patterns that occur with aging in the absence of disease. We found significant reductions in motor unit firing rates and estimates of persistent inward current amplitude in older adults. These findings were seen at both elbow flexor and extensor muscles. Further, this study also provided greater context to the changes in motor unit firing patterns observed following hemiparetic stroke, as most stroke survivors are aged individuals. To understand the effects of a unilateral brain injury on motor unit firing patterns, we quantified the firing patterns of elbow flexor and extensor motor units in both upper extremities of individuals with chronic hemiparetic stroke. Estimates of persistent inward currents were increased in both paretic and non-paretic limbs of stroke participants, while motor unit firing rates and rate modulation were primarily reduced in the paretic limb. Together these results suggest that changes in metabotropic drive to motoneurons following a stroke is systemic, while alterations in ionotropic neural drive are more selective. Finally, we further probed the changes in neural drive following unilateral brain injury through the comparison of motor unit firing patterns during voluntary and synergy-driven contractions of the biceps brachii. Rate modulation impairments were more pronounced during synergy-driven contractions of the biceps, than during voluntary activation. We postulate that the rate modulation impairments seen following stroke are due to increased use of the corticobulbar pathways, which play a larger role during the synergy-driven contractions.
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