The "neurosciences" is now a common 'term to describe several branches of the biology related to studies of the nervous system. In fact, this term also character­ izes a multidisciplinary approach to studies of probably the most complicated system of the most complicated living organisms. Investigation of the central ner­ vous system (CNS) is the most quickly developing branch of the neurosciences. According to Konorski's definition, the CNS "is endowed with two principal properties which are referred to as reactivity and plasticity. Reactivity of the sys­ tem is its capacity to be activated by stimulation of receptive organs; plasticity is the capacity to change its reactive properties as the result of successive activa­ tions" ([520], p. 7). According to Kostyuk [535], plastic reorganizations include "any changes in the efficacy or direction of neuronal connectivity which exceed in their duration usual synaptic and spike events" ([535], p. 25). Neuronal plasticity underlies adaptive modifications of various complexities both in normal CNS, e. g. its development, formation of memory traces and condi­ tioned responses (CRs) , as well as in brain pathology, e. g. reorganization and compensation of damaged nervous functions. Clarification of plasticity mecha­ nisms is of great cognitive importance and might be of practical value in the treatment of mental and especially memory disorders.