dendritic spines were first described at the end of the 19th century by santiago ramón y cajal on cerebellar neurons. ramón y cajal then proposed that dendritic spines could serve as contacting sites between neurons. this was demonstrated more than 50 years later thanks to the emergence of electron microscopy. until the development of confocal microscopy on living tissues, it was commonly admitted that spines were formed during embryonic development and then would remain stable after birth. in this paradigm, variations of synaptic weight were considered as sufficient to explain memory processes at the cellular level. but since about a decade ago, new techniques of confocal microscopy demonstrated that dendritic spines are indeed motile and dynamic structures that undergo a constant turnover, even after birth.