A, the transmission of excitement between neurons is conducted through synapses, which is unidirectional; Because neurotransmitters exist in synaptic vesicles of presynaptic membrane, they can only be released by presynaptic membrane and then act on postsynaptic membrane, so excitement can only be transmitted from axon of one neuron to cell body or dendrite of another neuron, and A is correct;

B and II represent postsynaptic membrane, and the signal changes from chemical signal to electrical signal, and B is wrong;

C, reflex needs a complete reflex arc to directly stimulate the nerve center. The reflex arc in this process is incomplete, and C is correct.

D, nerve impulses on nerve cells are conducted in the form of local current (or electrical signal), and D is correct.

Therefore, choose: B.