Gravity is critical to our existence.
As Earthlings, we have come to rely on
Earth’s gravity as a fundamental
reference that tells us which way is
down. Our very survival depends on
our ability to discern down so that we
can walk, run, jump, and otherwise
move about without falling. To
accomplish this, we evolved specialized
motion-sensing receptors in our inner
ears—receptors that act like biological
guidance systems. Among other things,
these receptors sense how well our
heads are aligned with gravity. Our
brains combine these data with visual
information from our eyes, pressure
information from the soles of our feet
(and the seats of our pants), and
position and loading information from
our joints and muscles to continuously
track the orientation of our bodies
relative to gravity. Knowing this, our
brains can work out the best strategies
for adjusting our muscles to move our
limbs and bodies about without losing
our balance. And, we don’t even have
to think about it.
At the end of launch phase, astronauts
find themselves suddenly thrust into
the microgravity environment. Gravity,
the fundamental up/down reference
these astronauts relied on throughout
their lives for orientation and
movement, suddenly disappears.
As you might expect, there are a
number of immediate consequences.
Disorientation, perceptual illusions,
motion sickness, poor eye-head/eye-
hand coordination, and whole-body
movements are issues each astronaut
has to deal with to some degree.
One thing we learned during the shuttle
era, though, is that astronauts’ nervous
systems adapt very quickly. By the
third day of flight, most crew members
overcame the loss of gravitational
stimulation. Beyond that, most
exhibited few functionally significant
side effects. The downside to this rapid
adaptation was that, by the time a
shuttle mission ended and the
astronauts returned to Earth, they had
forgotten how to use gravity for
orientation and movement. So, for the
first few days after return, they suffered
again from a multitude of side effects
similar to those experienced at the
beginning of spaceflight. During the
Earth-readaptation period, these
postflight affects limited some types of
physical activities, such as running,
jumping, climbing ladders, driving
automobiles, and flying planes.
The Space Shuttle––particularly
when carrying one of its Spacelab
or Spacehab modules and during
the human-health-focused,
extended-duration Orbiter medical
missions (1989 through 1995)––
provided unique capabilities to study
neurological adaptation to space.
By taking advantage of the shuttle’s
ability to remove and then reintroduce
the fundamental spatial orientation
reference provided by gravity, many
researchers sought to understand
the brain mechanisms responsible for
tracking and responding to this