Dynamics of learning-induced cellular modifications in the cortex.

This aim of this review is to describe the dynamics of learning-induced cellular modifications in the rat piriform (olfactory) cortex after olfactory discrimination learning and to describe their functional significance to long-term memory consolidation. The first change to occur is in the intrinsic properties of the neurons. One day after learning, pyramidal neurons show enhanced neuronal excitability. This enhancement results from reduction in calcium-dependent conductance that mediates the post burst after-hyperpolarization. Such enhanced excitability lasts for 3 days and is followed by a series of synaptic modifications. Several forms of long-term enhancement in synaptic connections between layer II pyramidal neurons in the piriform cortex accompany olfactory learning. Enhanced synaptic release is indicated by reduced paired-pulse facilitation. Post-synaptic enhancement of synaptic transmission is indicated by reduced rise time of post-synaptic potentials and formation of new synaptic connections is indicated by increased spine density along dendrites of these neurons. Such modifications last for up to 5 days. Thus, olfactory discrimination rule learning is accompanied by a series of cellular modifications which occur and then disappear at different times. These modifications overlap partially, allowing the maintenance of the cortical system in a 'learning mode' in which memories for specific odors can be acquired rapidly and efficiently.