Answer:
Dendrites.
Step-by-step explanation:
Dendrites is the part of neuron which is affected by the substance Y which leads to the decrease of action potential because dendrites is the part that obtain information from other cells and carry that information to the cell body of neuron which transfer this information to the neighboring neurons. If the dendrites is damaged or adversely affected due to chemicals then it is unable to perform its function properly and its action potential will be greatly affected.