Determining the contribution of the corticospinal and reticulospinal tract responses to resistive exercise? An exploratory study in non-trained and strength-trained participants.
It is well established that the human neuromuscular system can modify its function in response to physical activity or experience. This response has been termed ‘plasticity’ and involves reorganisation of neural circuits in the primary motor cortex (M1) that control movement. In several different ways, resistive exercise has been shown to influence plastic changes in the neuromuscular system.
However, little is known regarding the modulation and the plasticity of the neural pathways interconnecting elements of the neuromuscular system and skeletal muscle in resistant-trained individuals. Further, additional important descending motor pathways, such as the cortico-reticulospinal or cortico-propriospinal pathway, could also serve as important neural pathways that contribute to neural drive and thus force production. There is little information regarding these potentially important pathways in humans, and no information regarding the efficacy of strength training to affect these pathways. These pathways can be stimulated via transcranial magnetic stimulation (TMS), which produce muscle twitches referred to as a motor-evoked potential (MEP). The amplitude of a MEP is a crude measure of the excitability of the neurons that contribute to these descending motor pathways.
Typically, activation of the cortico-reticulospinal pathway produces MEPs ipsilateral to the motor cortex that is stimulated, suggesting that uncrossed descending motor pathways could be involved in the cortical control of ipsilateral muscle activity. Importantly, the cortical control of ipsilateral proximal upper limb muscles is rarely studied in healthy people, which is surprising, given that ipsilateral control of the upper limb is evident in healthy people and the recovery of motor function after adult stroke and cerebral palsy. Because strength training results in an increase in force output from the trained muscle(s), examining the ipsilateral MEPs between trained and non-strength trained individuals may provide important new knowledge on the neuromuscular mechanisms of strength development. Therefore, this project will examine the effects of resistive exercise on modulating the corticospinal and reticulospinal tract, which will provide new insights to the contribution of different descending motor pathways and their training-induced effects in increasing force output.
Keywords: Corticospinal; Exercise; Reticulospinal; Transcranial Magnetic Stimulation, Neuroplasticity
Does motor training in a mirror box attenuate the loss of motor function following short-term limb immobilization.
Short-term limb immobilization that reduces muscle use for 8–10 hours is known to reduce muscle strength. However, the mechanisms through which this is achieved, and whether these changes can be used to modify motor skill learning, are not known. We have recently shown that unilateral strength training of one limb maintains strength and motor cortical plasticity following short-term immobilization. Interestingly, observation of a motor act performed by oneself, observation of a motor act performed by someone else, and viewing a motor act in a mirror (which is often the case in sport practice) all activate the same neural structures as the actual movement execution, producing subliminal facilitation of neurons forming the motor neural network.
The subliminal engagement of neurons might have an adaptive role in motor learning, and therefore action observation seems to be a potential tool to facilitate motor learning during periods of musculoskeletal rehabilitation. A specific form of motor practice that makes use of action observation is mirror training. In mirror training, the practicing limb’s image is superimposed over the resting limb, creating the illusion in the mirror that the resting limb is moving. Mirror training is known to reduce phantom limb pain and enhance recovery of motor function of the paretic lower and upper extremity after a stroke and can also facilitate skill acquisition of the non-trained hand in healthy participants.
However, it remains unclear as to whether mirror-training differentially modulates the cross-transfer of strength and motor cortex plasticity following short-term unilateral limb immobilization. Therefore, the purpose of this project is to use a model that combines unilateral limb immobilization and contralateral strength training to determine if strength training of the free limb (with or without a mirror) can attenuate the strength loss acquired during short-term unilateral limb immobilization. These findings have important clinical implications in the management of musculoskeletal or neurological injury that results in limb immobilization.
Keywords: Immobilization, plasticity, strength training, cross-education
Does premotor transcranial direct current stimulation increase motor cortex excitability and improve motor function?
Transcranial direct current stimulation (tDCS) is a non-invasive technique that modulates the excitability of neurons within the primary motor cortex (M1), but might also induce effects in distant brain areas caused by activity of interconnected brain zones (known as functional connectivity). We have previously established protocols for delivery of tDCS, efficacy of tDCS on brain excitability and motor skill performance in healthy individuals. In these experiments, M1 excitability was tested using single-pulse transcranial magnetic stimulation (TMS) before and after 20 minutes of anodal-tDCS application over the left M1. Interestingly, brain excitability increased for both the stimulated and non-stimulated M1 suggesting that the two brain regions are interconnected, denoting the phenomena of functional connectivity.
Therefore, the aim of this research is to examine whether premotor (PM) tDCS can modify the excitability of the M1 via cortico-cortical connectivity and its effect on motor function. This study will provide a unique insight into the underlying neural mechanisms contributing to any changes in functional connectivity following tDCS.
Keywords: Neuroplasticity; motor control, motor control, transcranial stimulation
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