Final answer:
Research on Aplysia shows that learning involves the creation of more neurotransmitter release points and receptor sites at synapses during conditioning, which indicates synaptic plasticity as a key process.
Step-by-step explanation:
Studies of the snail species Aplysia have indicated that during conditioning trials, the axon terminals of sensory neurons acquire more neurotransmitter release points and postsynaptic neurons develop more receptor sites. This process is a form of synaptic plasticity, where the repeated application of a stimulus (such as serotonin in tissue culture) leads to the strengthening of synaptic connections. Such changes facilitate the retention of a trained response even after the receptors have been destroyed, suggesting a structural change at the synaptic level that supports learning and memory.