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NO GRAVITY, NO BALANCE
R. J. Wubbels, H. A. A. de Jong, H. W. Kortschot and W. J. Oosterveld
Vestibular Department ENT, Academic Medical Center, University
of Amsterdam
PO Box 22660, 1100 DD Amsterdam, The Netherlands (e-mail: r.j.wubbels@amc.uva.nl)
During the hundreds of millions of years in which life evolved on our planet, the circumstances have often changed. Transformations happened either all of a sudden or very gradually. Whatever the time scale, some changes had very dramatic effects on the life forms on earth. Photosynthesis of green plants increased the oxygen concentration in the atmosphere. Erosion made the oceans more saline. Global climate changes and tectonic activity in the earth’s crust had an impact on temperature, humidity, seasonal variability and gulf streams. A collision with a meteoroid wiped out an unknown number of species. Etcetera. Because the local circumstances vary we see a diversity of anatomical and physiological adaptations. Among all the physical constraints with which animals (and plants) had to cope, however, one has remained constant all the time: the earth’s gravitational field.
When man decided to leave the earth, it turned out that weightlessness
can have serious physiological consequences for the crew and also for animals
on a space flight mission. Difficulties may occur with the cardiovascular, renal
or neurological system or with blood chemistry or the psychological condition.
Locomotion becomes unpredictable because motor control has not been adjusted
to this situation, and the vestibular system is deprived of its usual input
(the gravitational force). This is thought to be the cause of motion sickness
from which more than 30% of the crew of space flight missions suffer. However,
the exact relationship between the absence of gravity and motion sickness is
still not understood.
In our group, research on the effects of short periods
of weightlessness during parabolic flights started in 1967. Humans and a variety
of animals (goldfish, cave fish, pigeon, pig, rabbit, and rat) were subject of
zero-gravity experiments. The effect of unilateral or bilateral loss of labyrinths
on eye movements and behaviour was investigated in animal experiments. On humans,
eye movements were measured while stimulating the vestibular system either mechanically
(rotating chair), or by a bilateral temperature difference (caloric test). Figure
1 shows an example of a caloric test. The dashed curve shows the slow-phase velocity
(SPV) of the caloric nystagmus on the ground. The solid curve shows the SPV when
gravity changes on a parabolic flight (adapted
from: de Jong, Kortschot and Oosterveld; Acta Otolaryngol. 109: 1-7, 1990)*.
It has also been demonstrated that pupil size depends
on the magnitude of gravity (Kortschot, de Jong and Oosterveld; Neuro-ophthalmol.
10: 45-51, 1990)*. 
Pigeons and fish with normal vestibular function were allowed to move freely in
air and water respectively and their locomotion behaviour was recorded. Figure
2 shows an example of the spiraling trajectory of fish during weightlessnes on
a parabolic flight (adapted from: de Jong, Sondag, Kuipers and Oosterveld;
Aviat. Space Environ. Med. 67: 463-466, 1996)*. Apparently, the sense of what
is ‘up’ and ‘down’ is completely lost. On another campaign, pigeons were allowed
to fly during weightlessness. The result was a tumbling animal through the air
which, nevertheless, managed to avoid collisions with the walls (Oosterveld
and Greven; Acta Otolaryngol. 79: 233-242, 1975) All these examples show that
the absence of vestibular input during a period, lasting several seconds, of zero-gravity
causes serious physiological effects.
In our latest experiment (June 1999), on board of an Airbus A300 during the 26th parabolic flight campaign organized by ESA, the effect of weightlessness on the compensatory eye movements of two groups of rats was studied*. The function of this eye movement reflex is to fix the visual image on the retina and thereby avoid blurring of the image when the animal moves its head or body. The first group of rats was born and raised under normal gravity, but the second group was born and raised under 2.5 times normal gravity conditions in a centrifuge. With these experiments, we hope to improve our understanding of the effects that the magnitude of the permanently present gravity force has during the embryonic development of the vestibular system. For instance, the comparison of results of zero-gravity experiments with those performed under normal gravity conditions may learn us more about the relative contribution of the different components of the vestibular system (two linear acceleration detectors, i.e. the saccule and the utricle, and three angular acceleration detectors, i.e. the semicircular canals) to the compensatory eye movements.
The experience of crew members of space flight missions and the results of zero-gravity experiments have made it clear that the force of gravity forms the basis of our orientation in space (i.e. the earth-bound space in which we live and in which species have evolved!). In essence, body posture is a constant process of keeping one’s balance relative to the ever-present force of gravity. The vestibular system was deviced for the incessant detection of gravity. When, for whatever possible reason, this sensory system is deprived of its normal input (for instance during a fall either on a parabolic flight or from the roof of your house, or because of a disease) this may cause a strong and often uncomfortable sense of complete disorientation.
*These projects have been financially supported by the Space Research Organization of the Netherlands (SRON).