One of the outstanding achievements of Russian science is undoubtedly the physiological underpinning of the concept of the unity of the body and its surrounding milieu, as developed by I. M. Sechenov and I. P. Pavlov.Sechenov was the first to pose the question of the reflex as an elementary act of adaptation of the body to its environmental conditions, which laid the basis for studies of the mechanisms of adaptive behavior. The ideas of Sechenov were developed by Pavlov and his school. In the 1930s, Pavlov laid down the principles for the dynamic equilibrium of the body with its environment; these reflected the central role of the nervous system in changing the behavior of the organism on adaptation to changes in its environmental conditions. There is now no doubt that there is a triad of adaptive responses: the genetic determination of an individual's nervous system, unconditioned reflexes, and conditioned reflexes. The mechanisms of these responses, their relative involvements, and their interactions in any given adaptive act remain far from completely understood. There is, however, considerable optimism that these questions will be resolved, largely because of advances in the areas of molecular-cellular and neurogenetic studies of adaptive behavior.Of the great variety of types of adaptive responses, it is logical to select a number of basic categories for studies of their underlying mechanisms. Pavlov noted that the highest form of adaptive response consists of acquisition of new habits, or learning. Learning can be non-associative or associative. Examples of non-associative learning include sensitization, acclimatization, and post-tetanic potentiation, and examples of associative learning include classical Pavlovian conditioning.A special position in adaptive behavior is held by a form of conditioning of the body to unfavorable environmental factors, such as hypoxia, toxic substances, and ecological and social stresses. However, adaptation to unfavorable factors in these cases is based on the acquisition of new habits, which involves identification of these factors followed by launching of the adaptive responses. This concept suggests the existence of specific as well as general universal molecular-cellular mechanisms of learning and adaptation to unfavorable environmental factors, and this idea is supported by experimental data.In the complex analysis of adaptive brain responses, the central position is taken by studies of the involvement in these processes of signal transduction at different levels of organization (the interneuronal, the cellular, the molecular, and the genetic) which are studied both at the level of the whole in situ brain and in comparatively simple models of neuronal networks (invertebrate nervous systems, live in vitro mammalian brain slices). Important elements of interneuronal transduction include the perception of signals and their transmission by electrical and chemical pathways to neuronal networks. A variety of neuromediator systems is responsible for the synaptic transmission of...
