Habituation response is a stimulus that weakens with repeated exposure to the same stimulus. For example, adaptation to traffic sounds by the people living in crowded cities.
The synapse serves as a basic site of learning. The changes occurring in the synapse, which accompany habituation occurs within the presynaptic axon terminal of the sensory neurons. The influx of calcium ions is occurs in response to the action potential, which causes release of neurotransmitter. However, this response grow less sensitive with repeated use.
Thus, the blanks can be filled with presynaptic axon terminal; sensory; calcium; ions.