The effects of low-frequency sinusoidal linear acceleration on skin sympathetic nerve activity in humans

  • Monique Foster

Western Sydney University thesis: Master's thesis

Abstract

Motion sickness, caused by actual or apparent motion, is a common disorder that is yet to be fully understood. It is a complex disorder that transcends the most common symptoms of nausea and vomiting, with several of the symptoms or symptom-complexes being under-researched or not well understood. One of these symptom-complexes is centred around the drowsiness, lethargy and irritability associated with typical motion sickness, is known as sopite syndrome. It has previously been shown that sopite syndrome can be caused by exposure to low-frequency motion over a longer duration. Additionally, it is known that the vestibular apparatus, being activated by motion, plays an important role in the pathogenesis of motion sickness, as robust vestibular modulation of skin sympathetic nerve activity (SSNA) has been demonstrated. However, there is currently no research quantifying the physiological changes that occur during sopite syndrome. Therefore, this study aims to quantify the physiological effects of low-frequency motion on SSNA, skin blood flow and heart rate variability (HRV), while also measuring self-reported sleepiness in subjects. The techniques used to obtain these measurements were: microneurography to record SSNA, infrared plethysmography to record skin blood flow, ECG to analyse and determine HRV and the Karolinska Sleepiness Scale to compare sleepiness before and after motion. Subjects were seated upright and exposed to a randomised sequence of five slow, sinusoidal motions on a motorised platform: 0.03 Hz at 0.5 mg, 0.05 Hz at 0.5 mg, 0.1 Hz at 0.5 mg, 0.1 Hz at 5 mg and 0.2 Hz at 5 mg. Cross-correlation analysis was used to calculate the vestibular modulation of SSNA, which was present at all frequencies and ranged from 30.7 ± 3.8 (0.2 Hz) to 37.2 ± 4.4 (0.1 Hz at 5 mg). There was no significant difference in the modulation indices when the magnitude of vestibular modulation was compared across all frequencies. However, the vestibular modulation of SSNA was greater than cardiac modulation at all frequencies, and the cardiac modulation during each frequency was less than the baseline value. Additionally, the skin blood flow was lower than the baseline at all frequencies and there was no statistical change in any of the HRV indices. Thus, it has been shown that very low-frequency sinusoidal motion can be used to activate the vestibular system causing modulation of SSNA. However, even though subjects reported feeling relaxed or drowsy upon completion of the protocol, with some even falling asleep throughout, it didn't appear that the motion stimulus applied caused the onset of physiological relaxation.
Date of Award2017
Original languageEnglish

Keywords

  • motion sickness
  • vestibular apparatus
  • sympathetic nervous system

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