Cajal’s Neuron Doctrine: 1898–

“[Neurons are] mysterious butterflies of the soul, the beating of whose wings might one day, who knows, reveal the secrets of mental life,” said Ramón y Cajal, in a poetic turn of phrase that captures the following century of research. The 20th century started with the neuron doctrine and ended with neural nets. Neurons are so completely different in appearance from other cell types, that previously it was not even clear whether they were in fact cells. By proposing the neuron doctrine, that neurons are discrete, independent cells, Cajal ushered in a revolution. Over the course of the twentieth century, different types of neurons were distinguished, including sensory neurons (which communicate with one another) and motor neurons (which signal to muscle). Neuron signaling was shown to take place by chemical or electrical transmission. The concept developed that the brain functions by means of the network of connections between its neurons. The model of the neuron as an input–output device was the spark for the development of A.I. The biology of the brain as seen today is truly the result of Cajal’s revolution.