Cellular mechanisms of behavioral plasticity in simple nervous systems

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.