In Cajal's work on Regeneration and Degeneration of the nervous centers, he realized that when he injured an axon of a projection cell, for example, a Purkinje cell in the cerebellum or a pyramidal cell in the cerebral cortex, this structure degenerated until one of its first collaterals, becoming in the main axonic trunk and transforming this projection cell into an interneuron. In this way, it changed its function, but maintained its existence. 3D print by Jeremy Swan based on an original illustration by Santiago Ramon y Cajal. Courtesy of the Cajal Institute, Spanish National Research Council or CSIC©