Understanding vestibular-related physiological functions could provide clues on adapting to a new gravitational environment

Abstract

The peripheral vestibular organs are sensors for linear acceleration (gravity and head tilt) and rotation. Further, they regulate various body functions, including body stability, ocular movement, autonomic nerve activity, arterial pressure, body temperature, and muscle and bone metabolism. The gravitational environment influences these functions given the highly plastic responsiveness of the vestibular system. This review demonstrates that hypergravity or microgravity induces changes in vestibular-related physiological functions, including arterial pressure, muscle and bone metabolism, feeding behavior, and body temperature. Hopefully, this review contributes to understanding how human beings can adapt to a new gravitational environment, including the moon and Mars, in future.

Keywords: Gravity sickness; Hypergravity; Hypophagia; Hypothermia; Microgravity; Muscle atrophy; Osteopenia; Vestibular system; Vestibulo-cardiovascular reflex.

Fig. 1

a Arterial and venous pressure change after supine-to-standing posture change. At the standing posture, the hydrostatic pressure of the foot increases to 96 mmHg while the venous pressure increases to 106 mmHg. Since the vein is more compliant, it dilates and 500 mL of blood is shifted to the lower body. b Block diagram of arterial pressure control at the standing posture. Revised Fig. 5 in reference 5

Fig. 2

a Arterial pressure (AP) and mean AP (MAP) responses to 60º head-up tilt (HUT) with (lower panel) and without (upper panel) galvanic vestibular stimulation (GVS). The magnitude of vestibulo-cardiovascular reflex can be estimated by the AP response difference between that without and that with GVS. b Sum of differences in Δ AP between the initial response (within the first 20 s) to HUT without and with GVS [(without GVS) − (with GVS)], at Pre (2–4 months before launch), Post-1 (1–4 days after return), Post-2 (11–15 days after return), and Post-3 (2 months ± 12 days after return). Data are shown as mean ± standard error of the mean for six participants. *P < 0.05 vs. Pre. Revised Figs. 1 and 3 in reference 23.

Fig. 3

Role of follistatin in the effects of gravity change on muscle and bone. Follistatin suppresses the action of myostatin, an inhibitor of skeletal muscle mass and a stimulator of bone resorption. Hypergravity enhances follistatin expression in skeletal muscles through the vestibular system in mice. Circulating follistatin induced by gravity change might be involved in hypergravity-enhanced bone mass by inhibiting myostatin-induced bone resorption

Fig. 4

a Numbers of vomiting episodes during 10-min 2g exposure in a musk shrew with (sham) or without (VL) peripheral vestibular apparatus. b Representative images of fos-expressing cells in the nucleus of the solitary tract (NTS). c Summarized data for the number of fos-expressing cells in the NTS. Data are shown as mean ± standard error of the mean. *P < 0.05 vs. Sham. Revised Figs.1, 2, and 3 in reference 67.

Fig. 5

Summary of this review