Does Synaptic Depression Of The Cerebellar Vermis Modulate The Reflexive Muscular Or Full-Body Postural Responses To An Electrical Vestibular Perturbation In Healthy Young Adults?




Lam, Christopher

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University of Guelph


This thesis is an investigation of the relationship between the cerebellum and the vestibular system in postural control. When an electrical current is passed behind the ears, it results in a biphasic reflexive response in the muscles used for balance as well as a postural response, which occurs in the direction perpendicular to head orientation. Although our understanding of the postural and EMG responses to a vestibular perturbation is already quite detailed and continues to expand, there is still much to learn. One area of particular interest that remains irresolute is the origin of the vestibular reflexes and the involvement of other supraspinal structures in the modulation of these responses. The aim of this thesis is to further investigate the role of the cerebellar vermis in the modulation of the reflexive muscle responses and the direction of the postural responses to a vestibular perturbation. The current thesis is comprised of three studies, all of which had a similar experimental design. Each study consisted of two conditions: a TEST group, who received continuous theta burst stimulation (cTBS) to cause temporary synaptic depression of the cerebellar vermis; and a SHAM group, who received paired pulse transcranial magnetic stimulation (ppTMS), which had no effect on cerebellar function. Baseline vestibular responses were evaluated with an unaltered cerebellum before the intervention of either cTBS or ppTMS, then vestibular responses were reevaluated immediately after. The first experiment utilized a square wave, electrical vestibular stimulus to evaluate the electromyographic (EMG) reflex responses in the soleus muscles with a depressed cerebellum. It was found that the medium latency (ML) response of the EMG reflex significantly increased with cerebellar depression (cTBS), whereas there was no significant change with ppTMS. This demonstrated that the cerebellar vermis is involved in the modulation of the EMG reflexive response to a vestibular perturbation. In the second experiment, a stochastic vestibular stimulus was used to evaluate the effects of extra-vestibular sensory input (vision and light tactile input from the finger) on the ML vestibular reflex, while the cerebellar vermis was temporarily depressed. It was found that the dampening of vestibular response amplitudes, caused by the availability of vision and touch, was significantly lessened with cerebellar depression (cTBS), where there were no changes with ppTMS. These results demonstrate the involvement of cerebellar vermis and its role in sensory integration, an essential part of the response to a vestibular perturbation. In the final experiment, the objective was to determine the role of the cerebellum in developing appropriately directed postural responses to a vestibular perturbation under different orientations of head turn. We found that with the head facing over the shoulder, the postural response was no longer directed perpendicular to head orientation when the vermis was depressed with cTBS. The sway direction became more mediolateral, as the cerebellum was unable to integrate neck proprioceptive input with the vestibular signal to generate a postural response directed orthogonal to head orientation. Collectively, the current work contributes novel findings that the cerebellum is significantly involved in the modulation of both the muscle reflex and postural response to a vestibular perturbation.



Vestibular, Balance, Cerebellum, Posture, Reflexes