The blanks in the sentence can be filled as follows: The terminal buttons of a neuron contain synaptic vesicles that house neurotransmitters, the chemical messengers of the nervous system. Explanation:

• Terminal buttons are the small knobs at the end of the axon branches of a neuron.
• Synaptic vesicles are tiny sacs within the terminal buttons that store neurotransmitters.
• Neurotransmitters are released from these synaptic vesicles to transmit signals across the synapse to other neurons or target cells.