A, neurotransmitters will decompose after they play a role, and A is correct;

B, the transmission of excitement at the synapse is unidirectional, so when B is stimulated, no potential change can be detected at D, and B is wrong;

C and II are postsynaptic membranes, and the potential changes on postsynaptic membranes are chemical signals → electrical signals, and C is wrong;

D, stimulation D, the direction of excitation conduction is from the excited area to the non-excited area, and the direction of current in the membrane is from the excited area to the non-excited area, with the same direction, and D is wrong.

So choose: a.