| Summary: | Galvanic vestibular stimulation (GVS) applied to humans engaged in a postural
task evokes a distinct biphasic response in the soleus muscle. The purpose of this study
was to investigate vestibular connectivity to individual soleus motor units in quiet
standing humans. Subjects were instructed to stand quietly with their heads facing
forward, eyes open, and feet together. GVS perturbations (4 mA, 30 msec pulses) were
delivered bilaterally to the mastoid processes in a bipolar, binaural configuration. Surface
and intramuscular wire electromyography (EMG) were recorded from the right soleus
muscle. Surface EMG responses were trigger-averaged to the onset of the GVS pulse and
quantified by determining onset latencies, peak-to-peak amplitudes, and peak latencies.
Single motor units were identified using a template-matching algorithm. Post-stimulus
time histograms (PSTHs) were created for each motor unit using GVS as the trigger (time
= 0). PSTHs were analyzed using 2 msec bins to determine the number of occurrences the
motor unit fired at specific post-trigger latencies. Individual motor unit responses to GVS
differed between various units recorded in soleus. Certain motor units were influenced by
GVS and exhibited a characteristic biphasic response in their PSTH at latencies of -80
and -120 msec. In contrast, other motor units were not influenced by GVS and therefore
exhibited a constant probability of firing. Motor unit triggered-averages (MUTAs) were
created from trigger-averaging the surface EMGs to the onset of the individual motor unit
firing. Motor units that were influenced by the descending vestibular volley produced
MUTAs that were 1.6x greater in peak-to-peak amplitude than the non-influenced motor
units. The observations suggest that in humans, vestibular input projects non-unifonnly
on the soleus motomeuron pool with a bias towards higher threshold motor units. === Education, Faculty of === Kinesiology, School of === Graduate
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