PDF《The Role of炉he燨toliths》

13 Chapter

2 The Role of?the?Otoliths 2.

1? Which Functions Do the?Otoliths Fulfil? It has gradually been recognised that the gravitational vector, primarily mediated by the otolith organs in the inner ear, is utilised as a reference by diverse systems. Besides its obvious role in the vestibulo-ocular and vestibule-spinal responses (e.g. Yates et?al. 2014), it also subserves a number of systems ranging from those cogni- tive processes involved in spatial orientation and navigation (Wraga et?al. 2000;

McIntyre et?al. 2001;

Indovina et?al. 2005;

Besnard et?al. 2015) to the regulation of autonomic mechanisms (e.g. Watenpaugh et?al. 2002;

Salanova et?al. 2016). The importance of maintaining correct spatial orientation becomes apparent in a variety of extreme sporting activities such as surfing, motorcycle racing or ice skat- ing where it is notable that well-trained participants maintain their head in an upright position while performing their various activities (see Fig. 1.1). With the head axis parallel to gravity, the visual surround is optimally aligned with the vertical and horizontal meridia of the retina and is thus beneficial to visual acuity and cognitive processing. This phenomenon is also reflected in the recent findings of Mast and Meissner (2004), who demonstrate that retinal images of the environment are processed optimally, i.e. with a minimum of processing time with the head in an upright position. In the absence of the otolith-mediated gravity refer- ence, the astronaut must rely largely on the visual field for orientation (cf. Fig.?2.1). Fig. 2.1? In the prolonged absence of the gravitational reference during spaceflight, it is necessary for the spaceflight traveller to take reference from visual (exotropic) cues or from some internal or idiotropic vector (Glasauer and Mittelstaedt 1998)

14 In the meantime, an increasing body evidence demonstrates the role of the otolith afferent information in the regulation of other body functions, e.g. blood pressure (Yates et?al. 1999). This effect was demonstrated by the work of Denise and co-? workers (Etard et?al. 2004) who demonstrated that arterial blood pressure is modu- lated by changes in the gravitoinertial force during parabolic flight. This work contributes to the increasing evidence of the role of otolith afferences in the regula- tory mechanisms of the sympathetic nervous system, contributing to the control of vascular resistance and blood pressure. In this sense, it has been demonstrated that the afferent information from the otolith system plays a role in the maintenance of bone-muscle synergy (Luxa et?al. 2013;

Salanova et?al. 2016;

Vignaux et?al. 2015). A number of experiments have been performed that demonstrate altered spatial perception during prolonged microgravity and the nature of adaptation to the altered conditions. In one example reported by Clément et?al. (2001), subjects were required to indicate their perception of tilt during centrifugation (Fig.?2.2). Under one-g conditions, the tilt of the gravitoinertial vector―resulting from the additional centripetal acceleration generated by the short-arm centrifuge―was by and large correctly perceived. When performed onboard the Space Shuttle, the sub- jects'

perception was initially very similar to that under one-g test conditions, despite the fact that the bias of the otolith-mediated gravity vector was absent. However, over the course of the 16-day mission, a gradual adaptation was observed, during which the subjective perception became aligned with the direction of the predominant centripetal acceleration vector. Together with the subjective reports from space travellers, this demonstrates that transitions to and from microgravity radically alter the demands on sensorimotor coordination. This is reflected in the interaction between the vestibular and oculo- motor systems as manifested during those head movements involving changes in orientation to the gravity vector. This is illustrated by measuring eye and head move- ment while tilting the head to the shoulder. Under Earthbound, one-g ? conditions, the otolith organs continuously signal head orientation relative to the gravity vector. When the head is tilted, e.g. to the left, this reorientation with respect to gravity is perceived by the otolith organs, and via reflex pathways to the extraocular muscles, a Fig. 2.2? The short-arm centrifuge employed for the tilt-translation experiment onboard the Shuttle seen here in the training mock-up (from Clément et?al. 2001)