UBC Theses and Dissertations
Using galvanic stimulation to explore the role of vestibulospinal inputs on lower limb motoneurons Kennedy, Paul Michael
Galvanic vestibular stimulation (GVS) is a technique used to activate the human vestibular system. The stimulus acts by altering the firing rate of the vestibular afferents (Goldberg et al. 1984), which leads to a change in the descending spinal inputs in the lateral vestibulospinal and reticulospinal tracts. These pathways provide a source of input onto spinal neurons that can affect the excitability of lower limb motoneurons. Galvanic stimulation, therefore, may be used to study the role of vestibulospinal input on lower limb motoneurons. The excitability of the motoneuron pool is affected by the length of the muscle fibers, so GVS may have differing effects at different muscle lengths. The strategy the nervous system adopts to activate muscles at different force producing lengths is still largely unknown. The effect of muscle length on motor unit recruitment was examined in the gastrocnemius muscle in Experiment 1. At a shortened muscle length, the onset of gastrocnemius motor unit activity occurred at significantly higher levels of plantar flexor torque. This may reflect inhibition of gastrocnemius motors units at shortened, non-optimal lengths. There are several pathways that might be involved in regulating the inhibitory inputs that act on the lower limb motoneuron pool, including the vestibulospinal tract. Experiments 2 and 3 provided evidence that a change in the vestibulospinal activity modified the amplitude of the passive ipsilateral soleus H-reflex. It was unclear, however, if this inhibition reflected a change in the activity of the la afferent pathway or motoneuron excitability. Consequently, Experiment 4 examined the effect of vestibulospinal influences on the discharge properties of single motor units in the gastrocnemius muscle. Activation of the vestibular system, using GVS, modified the onset and initial firing frequency of individual motor units at the shortened length but not at the long muscle length. This finding may reflect a change in the presynaptic inhibitory mechanisms that act on the motoneurons innervating muscle fibers that reach non-optimal force producing lengths. The results clearly demonstrate that GVS can alter the excitability of the triceps surae motoneuron pool. The descending vestibulospinal inputs operate to regulate inhibitory influences that act on the motoneuron pool.
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