Cellular mechanisms of behavioral plasticity in simple nervous systems

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Abstract

Review of our own experimental studies of the cellular mechanisms of learning in the nervous systems of gastropod mollusks, along with published results, allows identification of a number of the principles of operation of nervous systems, which are important for descriptions of learning and memory processes: 1) the main plastic changes on learning occur at the level of interneurons; 2) learning results in selective alteration of the efficiency of particular synaptic inputs of command neurons; 3) reinforcement is not linked with neuron activity in the receptor-sensory neuron-interneuron-motoneuron-effector reflex arc, but is mediated by neurons which modulate this circuit, this involving a single neuron in some simple cases; 4) modulator neuron activity is required for the acquisition of plastic modifications to defensive behavior (including associative modifications) but is not necessary for the reproduction of acquired responses to a conditioned stimulus. At the same time, modulator neurons (comprising the reinforcement neuron system) are required for reproduction of contextual associative responses; and 5) changes resulting from learning occur at at least two independent loci in the nervous system.

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