Albert Hilscher · Public domain

Otto Loewi proved that nerve cells communicate chemically rather than only electrically, demonstrating in a now-famous 1921 experiment — reportedly conceived in a dream — that stimulating the vagus nerve of one frog heart released a substance that could slow a second, disconnected heart. That substance turned out to be acetylcholine, the first neurotransmitter ever identified. He shared the 1936 Nobel Prize in Physiology or Medicine with Henry Dale, a lifelong friend who had helped inspire the experiment decades earlier.