Date of Award

1-1-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Interdisciplinary Neuroscience

Department

Interdisciplinary Neuroscience

First Advisor

Susan E D'Andrea

Abstract

Transcranial direct current stimulation (tDCS) is a safe, tolerable method to modify corticospinal excitability in people. Sensorimotor skill acquisition can be enhanced with adjuvant tDCS in healthy people and those with CNS injury. The mechanisms that produce skill improvements with adjuvant tDCS are not completely understood. Spinal reflexes participate in motor behaviors such as standing and walking in a task-dependent manner. Loss of this reflex modulation can occur after CNS injuries such as spinal cord injury or stroke and contribute to abnormal movements. Therefore, understanding how tDCS affects spinal cord excitability is important for its efficacious use as an adjuvant to sensorimotor therapy. Currently, only a handful of studies have tested the effect of tDCS on spinal cord excitability in neurotypical people, primarily in resting muscle.To better understand how tDCS effects spinal cord excitability during sensorimotor skill acquisition, this project investigates its impact on Hoffmann-reflex excitability (H-reflex, the electrical analog of the stretch reflex) in the soleus muscle of healthy controls during standing and while learning the simple motor skill of reducing soleus H-reflex size through operant conditioning. Over thirty years of research has shown learning the skill of reducing reflex size induces spinal cord plasticity and requires supraspinal structures to acquire. Fourteen young adults with no known neurological injuries participated in the study. The Medical University of South Carolina was the IRB of record. Chapter 1, Introduction, provides background information about the plasticity induced during motor learning, spinal reflexes, reflex operant conditioning, tDCS and the specific aims of the project. Chapter 2, Effects of Active and Sham tDCS on soleus H-reflex size in standing, presents the first manuscript from this project. In this study, the effects of Active (anode placed over leg M1) and Sham tDCS on soleus H-reflex size was evaluated while young, healthy controls simply stood comfortably. The results showed both Active and Sham participants’ submaximal H-reflexes briefly increased by a small amount (~6%) immediately after tDCS dosage onset (1 minute) and returned to baseline before the end of the dosage. The Active stimulation group’s reflex size returned to the pre-stimulus reflex size before the Sham group. This phenomenon’s probable mechanisms and indications for future studies using tDCS as an adjuvant are discussed. The findings are in preparation for Experimental Brain Research. Chapter 3, Effects of tDCS on the plasticity induced by soleus H-reflex operant conditioning, presents the second publication from this project. The effect of Active and Sham tDCS on reflex operant conditioning soleus H-reflex decrease in healthy people over twelve sessions was evaluated. Linear mixed models revealed there was a significant cumulative effect of the Active tDCS condition on the across-session H-reflex size that was collected before practice began each session. Here, the total decease in Control H-reflex size was gained in the fist 4-5 sessions whereas in prior healthy control studies using the same method, the across-session H-reflex size decrease (i.e., Control reflex size) did not begin to manifest itself until after approximately 12 sessions. The ability to learn the reflex skill (within-session change in H-reflex size) was similar to the prior study without tDCS (Thompson et al., 2009) and not significantly different between Active or Sham. These results suggest Active tDCS combined with the ongoing skill acquisition (reduce H-reflex size through operant conditioning) enhanced the Control reflex plasticity over practice alone across multiple sessions. These results can be explained by the ability of tDCS to enhance cortical descending influence over the excitability of spinal cord inhibitory circuits. The results of the second study are in preparation for The Journal of Neuroscience. The findings of this project suggest Active tDCS enhances the cortical descending influence over presynaptic inhibition of soleus H-reflexes. In addition, the Sham protocol used in this study had effects on spinal cord excitability. In Chapter 4, Summary, the project results are summarized, limitations are noted, and the impact of the results on the potential therapeutic use of the tDCS dosage (both Active and Sham) are discussed. In addition, Chapter 4 suggests future directions of research based on the results here. The Appendices provide additional details about Methods (A), dissertation Abbreviations (B) and supplemental figures and tables (C).

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