Gravity as a Factor in Evolutionary Adaptation of Animals to Living on the Earth

The review considers the current ideas about the role of the gravitational factor in activities of the sensorimotor and cardiovascular systems, as well as new basic problems and questions of space medicine and physiology. The review presents data on animal embryonic development in weightlessness, evolution of the motor and cardiovascular systems, and characteristics of their functions in conditions of normal and altered gravity. Much attention is paid to the results of unique studies with ground-based gravitational unloading models: head-down bed rest, dry immersion and hindlimb suspension, in which the mechanisms that regulate various body systems under conditions of altered gravity were studied. Terrestrial organisms have learned to live in the gravitational field. Almost all of their body systems are gravity dependent. However, the extent and mechanisms of this dependence remained unclear for a long time. Space flights opened up the possibility of studying the activity of living systems in the absence of gravity. Changes in activity of sensory systems are among the main factors that mediate the effect of weightlessness on the motor system. Under the Earth’s conditions, the afferent support of motor control is polyreceptive and involves vision, the vestibular apparatus, and support and muscular proprioception. In microgravity, activity of some channels is completely eliminated (support afferentation), distorted (vestibular apparatus), or weakened (proprioception). Similar processes occur in the cardiovascular system: loss of the gravity-dependent pressure gradient causes profound changes in the structure and function of the heart and vessels, including both resistive and capacitive ones. It is still an open question as to how much the various changes occurring in the cardiovascular system are associated with the disappearance of the gravity-dependent pressure gradient. Some questions of gravitational physiology are impossible to answer in space flights experiments. Various methods were therefore developed to simulate the effects of gravitational unloading on the Earth. New knowledge on the mechanisms of changes occurring in the sensorimotor system was gotten by comparing space flight data and data obtained in model experiments. A basic problem of gravitational physiology of the cardiovascular system is the degree of correspondence between changes observed in laboratory animals or under model conditions (bed rest, immersion, and hindlimb suspension) and changes recorded in humans during real space flights. The problem is specially discussed in the review. Many issues remain unresolved in the light of future inter-planetary missions, including the problems of post-flight readaptation of the motor and cardiovascular systems to normal gravity conditions. Another problem is preventing losses of strength, endurance, and orthostatic stability. The development and improvement of countermeasures for preventing the negative effects of space flight is impossible without understanding the mechanisms that underlie the observed changes.