The postsynaptic neuron's dendrite receives neurotransmitter, rather in the manner of sniffing perfume, and changes the permeability of its membrane to certain ions, usually Na+, K+, Cl-, or Ca++ in some combination. Ions flowing through the membrane in turn produce the voltage change known as the postsynaptic potential (PSP). If excitatory, it is called the EPSP; if inhibitory, the IPSP.