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Excitatory signaling generally increases sodium or calcium entry, depolarizing the membrane toward the threshold for an action potential and producing an excitatory postsynaptic potential (EPSP). Inhibitory signaling can increase chloride entry or potassium efflux, moving the membrane away from threshold and producing an inhibitory postsynaptic potential (IPSP). Glutamate and aspartate are major fast excitatory transmitters; GABA and glycine are major fast inhibitory transmitters. Other transmitters—including acetylcholine, catecholamines, serotonin, histamine, and purines—can have varying effects depending on receptor subtype. The lesson also introduces the roles of neuropeptides and nitric oxide, the importance of excitation-inhibition balance, and examples of how drugs can modulate these pathways.