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and the effects of microgravity on the kinetics and efficacy of psychopharmacological medications should be interdisciplinary in nature.
In those instances where existing countermeasures are found to be ineffective, new countermeasures should be developed that effectively contribute to optimal levels of crew performance, individual well-being, and mission success.
Fundamental Gravitational Biology
Mechanisms of Graviperception and Gravitropism in Plants
Multicellular plants respond to changes in the direction of the gravitational vector by altering the direction of growth of roots and stems. The gravitropic response requires (1) perception of the gravitational vector by gravisensing cells, (2) intracellular transduction of this information, (3) translocation of the resulting signal to the sites of reaction (i.e., sites of differential growth), and (4) reaction to the signal by the responding cells (i.e., initiation of differential growth). In some systems, the gravity-perceiving cell is also the site of reaction (e.g., in the Chara rhizoid).
Studies of graviperception should concentrate on three problems:
The identity of the cells that perceive gravity in multicellular plants;
The intracellular mechanisms by which the direction of the gravity vector is perceived; and
The threshold value for graviperception—this will require a spaceflight experiment.
Studies of gravitropic transduction should focus on the nature of the cellular asymmetry that is set up in a cell that perceives the direction of the gravity vector.
Studies on the translocation step should concentrate on the nature and mechanism of the translocation of the signals that pass from the site of perception to the site of reaction.
Studies on the reaction step should focus on the mechanism(s) by which gravitropic signals cause unequal rates of cell elongation, and on the possible effects of gravity on the sensitivity of these cells to the signals.
Mechanisms of Graviperception in Animals
Work on space research is concerned with whether those parts of the vestibular system that are gravity sensitive (otolith organs) can develop and function adequately in microgravity. In addition, it is important to determine whether gravity influences the sensory systems that depend for their development and function on vestibular input. This includes the other sensory systems that interact directly with the vestibular system, the multiple brain regions containing neural space maps, and finally those areas in the brain capable of responding to alterations in their activity by neuroplastic changes.
Space-based experiments are needed to test the role of gravity on the embryonic development and maintenance of the vertebrate vestibular system. Prior to this, ground-based studies are needed to identify the critical periods in vestibular neuron develpment. In both Earth and space-based studies, it