SPACE PHYSIOLOGY

In space, the normal, ever-present effects of gravity on our bodies disappear, and this change profoundly affects our perception of body orientation and equilibrium. This change is the most dramatic of all of the physiological changes experienced during space flight. As a result, astronauts often disoriented and, in many cases, sick. The part of the nervous system that is primarily responsible for the balance mechanisms in the body is called the n eurovestibular system. We will focus on that system during the remainder of this section.

Figure 9. There is no "up" or "down" in space.
We've already learned that the awareness and perception of our body's orientation on Earth is attributed, in part, to the detection of gravity by the otolith organs (the utricle and the saccule), and to the detection of head rotational movements by the semicircular canals, both of which are in the inner ear. Gravity sensors in the joints and touch sensors in the skin are also involved, and the eyes contribute by sensing the body's relationship to other objects. In space, however, the weightless environment provides a different stimulus to the otolith organs, and the resulting signals no longer correspond with the visual and other sensory signals sent to the brain. This signal conflict causes dis orientation. That is, your brain has difficulty making sense of the fact that, although you see the floor and the ceiling, there is no other sense realism connected to the concept of "up" and "down" (Figure 9). The touch and balance mechanisms are completely confusing the brain because you don't realize how quickly you'll bounce back from something that you touch because there is nothing to anchor your position. immediately upon experiencing weightlessness, you feel as though you are simply a floating set of eyeballs, not even knowing where your limbs are because they have no weight associated with them to give your brain clues about their whereabouts! The complete collection of environmental and physical data th at your brain is trying to comprehend does not make sense.
Figure 10. Floating in microgravity.

When "free floating" in microgravity, the otolith organs no longer provide a vertical reference signal to the brain; rather, they respond only to linear acceleration, forward and backward, up and down, left and right. As an astronaut begins to adapt after a few days, he can begin to process this strange information in an appropriate way. He learns to propel himself around by pushing off walls, ceiling and floors and he can "fly" through the Spacelab cabin (Figure 10). In an effort to reinterpret the meaning of the otolith signals, and to provide some sort of a reference "down" axis, which seems to be required for comfort by some people, the nervous system seems to adapt and respon d more to nonvesitibular signals, particularly visual, proprioceptive, and tactile cues.

The practical aspects of neurovestibular research in space include the prediction, prevention, and treatment of space motion sickness; the reduction of risk in an emergency egress (escape) in the event of an accident; and the long-term is sues of human adaptation to very long duration flights en route to Mars. The underlying research questions deal with the fundamental role of gravity in the development of specialized sensory organs, the neural connections that are associa ted with the vestibular systems under altered environmental conditions, and the levels of the brain at which these reversible adaptive processes take place.

In order to explore the full range of neurovestibular adaptation to space flight, a series of human experiments was performed on a variety of missions under the direction of Dr. Laurence Young from the Massachusetts Institute of Technology (MIT) in Cambri dge, Massachusetts. Dr. Young brought together a large team of U.S. and Canadian researchers who developed a set of investigations aimed at documenting both physical vestibular changes and perceptual changes and to invest igate the mechanisms involved in these changes. Through the results of these experiments and others, investigators hope to also understand the nature of vestibular adaptation to weightlessness and the relationship of head movement and flu id shift to space motion sickness symptoms.

As in all of the other chapters, we will examine some of the actual results from Dr. Young's experiments that were obtained on several different space flight missions. His original hypotheses for the study were simple but direct statement s about his expectations and predictions related to the outcome of the experiment. Dr. Young's hypotheses were as follows:

Hypothesis 1

In the process of vestibular adaptation in weightlessness, the incoming otolith signals are inhibited and replaced by an increasing influence of visual and somatosensory orientation cues.

Hypothesis 2

Space motion sickness is caused by the sensory conflict between otolith signals, which are altered in space, and visual, somatosensory, and semicircular canal cues, which remain unaltered in space.

Before we begin our examination of Dr. Young's space flight results, let's participate in three "Student Investigations" designed to clarify certain important concepts from this chapter. These activities will prepare you to understand more about your sens ory and balance system so that you will have a better background for understanding Dr. Young's space flight experiment.

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