The functional role of an increase in cell excitability and synaptic efficiency in the new cortex during learning

Abstract

Data were obtained in experiments on nonimmobilized and nonanesthetized rabbits, during the development of an analog of a CR, with recording of the response of the pyramidal tract, suggesting temporal specificity in the manifestations of membrane and synaptic plasticity, the participation of these mechanisms in both representations of the combined stimuli, and primarily unidirectional changes in the degree of their participation in these points of the cortex. It is concluded that temporary membrane plasticity creates conditions through the mechanisms of synchronization and summation for the passage of excitation from the sensory link to the motor output of the new connection. The gradual reorganization of excitatory and inhibitory connections to the output elements of the conditioned reflex act, determined by the mechanisms of synaptic plasticity, determine and strengthen the specialized character of the developed reaction.

Synchronization and cooperative interaction in brain activity

“…The maximal degree of manifestation of synaptic plasticity during the development of a cellular analog of the CR in response to combined stimulation of two points of the cortex [22] has been observed following the highest value of the coefficient of correlation which appears following the presentation of several tens of combinations; this suggests the synchronous unfolding of the processes of membrane and synaptic plasticity in the mechanism of the CR. Stimulation of the LH as a reinforcement leads not only to a significantly more substantial increase in cellular excitability, but also to a greater degree of manifestation of a correlational association.…”

“…A specific contribution to the phenomenon of synchronization is made by the mechanisms of heightening of cellular excitability and intensification of synaptic efficiency [20,22,23]. M. N. Livanov, speaking of the mechanism of synchronization, used the term "cellular excitability," basing himself on the data relating to the reorganizations of impulse activity which include changes both in cell excitability and in the weights of the synaptic inputs.…”

“…On the one hand, the former represent the engagement of synaptic inputs of different mediator and functional (transmission and modulatory) nature, with the predominance of their varying ratio (of the excitatory inputs at the early stage and the inhibitory inputs at the late stage) at particular specific moments of the organization of the CR [22], and on the other hand, the latter represent the triggering, through depolarizational processes [74,92,126] and through the activation of NMDA receptors, of the cascade of intracellular enzymatic processes [101,103,104,106,115] that alter the activity of neurons at a particular time [70,72] in a manner analogous to the shifts seen in the development of the CR [69, I 15]. The role of the NMDA receptors has been investigated in such structures as the hippocampus and visual cortex in short-term potentiation, at the early and late stages of long-term potentiation, and in the case of interassociated changes in synaptic plasticity and learning [62,64,67,79,81,86,89,91,100,111,112,117].…”

“…During the development of a CR, the number of unidirectionally reacting pairs of neurons in the zone of the cortical representation of the signal stimulus increases as compared with the initial state; this may be represented as the result of the multiplication of the functional groups of neurons with synergic reactions to the CS [58]. Such a regularity in the course of the formation of the CR consists in an initial heightening of overall excitability of the cortical neurons with subsequent local enhancement of synaptic efficiency [22]. Ontogenetic development also moves from the prevalence of membrane mechanisms in participation in cortical plasticity to an increase in the significance of synaptic plasticity [21].…”

“…The increase in cellular excitability in the cortex which appears at the early stage of the development of the CR determines the dominant properties of the stage of generalization of the CR, and is replaced by a long-sustained enhancement of the synaptic efficiency of excitatory and inhibitory connections underlying the stage of specialization of the CR [23]. The initial parallel change in cellular excitability in the cortical projections of combined stimuli is characterized by an increase in cellular excitability up to the dominant level, with the summation of lateral excitations [22,24].…”