Vestibular-dependent spinal reflexes evoked by brief lateral accelerations of the heads of standing subjects

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An impulsive acceleration stimulus, previously shown to activate vestibular afferents, was applied to the mastoid. Evoked EMG responses from the soleus muscles in healthy subjects (n = 10) and patients with bilateral vestibular dysfunction (n = 3) were recorded and compared with the effects of galvanic stimulation (GVS). Subjects were stimulated while having their eyes closed, head rotated and while tonically activating their soleus muscles. Rectified EMG responses were recorded from the leg contralateral to the direction of head rotation. Responses were characterised by triphasic potentials that consisted of short latency (SL), medium latency (ML) and long latency (LL) components beginning at (mean ± SD) 54.2 ± 4.8 ms, 88.4 ± 4.7 ms and 121 ± 7.1 ms respectively. Mean amplitudes across all conditions were 6.9 ± 3.2 % for the SL interval, 12.5 ± 6.8 % for ML interval, and 7.5 ± 4.1 % for the LL interval, compared to prestimulus values. Stimulus rise times of 14 and 20 ms evoked the largest ML amplitudes. GVS evoked biphasic responses (SL and ML) with similar latencies. Like GVS, the polarity of the initial interval was determined by the direction of head acceleration and the evoked EMG response was attenuated when subjects were seated. There was no significant EMG response evoked when subjects were stimulated using 500 Hz vibration or in patients with bilateral vestibular dysfunction. Our study demonstrates that a brief lateral acceleration, likely to activate the utricle, can evoke spinal responses with properties similar to those previously shown for vestibular activation by GVS. The triphasic nature of the responses may allow the nervous system to respond differently to short compared to long duration linear accelerations, consistent with their differing significance.
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Laube, Robyn
Govender, Sendhil
Colebatch, James
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Journal Article
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UNSW Faculty