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[The cytoskeleton of the Mauthner neurons in the guppy after returning from an orbital flight].

The ultrastructural investigations of the Mauthner neurons (M-cells) of P. reticulata were performed on the return of the fishes after a two weeks orbital flight, the M-cells of fishes from different control groups being simultaneously examined. The structure of M-cells from the experimental fishes was shown to be severely damaged. The changes involved the synaptic apparatus, postsynaptic areas, mitochondria and other organelles of the cytoplasm and the nucleus. Particular changes were seen in the neuronal cytoskeleton. While in the M-cells of intact fishes these were seen as evenly distributed microfilaments and neurofilaments, in the M-cells of experimental fishes the cytoskeleton looked as alternating bundles of condensed filaments. The first indications of cytoskeleton damage appeared in M-cells of the guppy from transport control group and strengthened in fishes from the group of synchronous control. Thus, the data obtained give us a possibility to suggest that the return on the Earth and following readaptation after a prolonged flight at conditions of microgravity and consequent disfunction of vestibular apparatus may result in the damage of central neurons receiving vestibular afferentation (such as M-cells). That, in turn, may be a cause of functional disorders of the central nervous system, including the nerve centres maintaining the organs of motion. These facts are necessary to take into consideration when more prolonged cosmic flights are planned.