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Vestibular connectivity to soleus motor units during quiet stance Lee Son, Gregory Martin

Abstract

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.

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