Final answer:
The Aplysia sea slug shows increased sensitivity in the sensory neuron and increased synaptic connections between sensory and motor neurons.
Step-by-step explanation:
The Aplysia sea slug can be trained to withdraw its siphon tube. When serotonin is added to the tissue culture, it increases the number of synaptic connections and induces training in vitro. Even after the synaptic receptors are destroyed, the trained response is retained, indicating that the sensory neuron becomes more sensitive and the synaptic connection between the sensory and motor neurons increase.