Final answer:
The modulatory interneuron releases neurotransmitters like serotonin, which enhance the connection between the interneuron and motor neuron, affecting the Aplysia's tail response.
Step-by-step explanation:
After shocking the Aplysia's tail, the modulatory interneuron releases neurotransmitters onto other neurons in response to stimuli. Specifically, in Aplysia, which is often used as a model organism for studying reflexes and neuroplasticity, the modulatory neuron can release serotonin or other neuromodulators that enhance the signal between the interneuron and motor neuron. This modulation can influence the strength and efficacy of synaptic connections, thereby affecting the motor output such as the contraction strength of the Aplysia's tail.