Neuron theory, the cornerstone of neuroscience, on the centenary of the Nobel Prize award to Santiago Ramón y Cajal.

Exactly 100 years ago, the Nobel Prize for Physiology and Medicine was awarded to Santiago Ramón y Cajal, "in recognition of his meritorious work on the structure of the nervous system". Cajal's great contribution to the history of science is undoubtedly the postulate of neuron theory. The present work makes a historical analysis of the circumstances in which Cajal formulated his theory, considering the authors and works that influenced his postulate, the difficulties he encountered for its dissemination, and the way it finally became established. At the time when Cajal began his neurohistological studies, in 1887, Gerlach's reticular theory (a diffuse protoplasmic network of the grey matter of the nerve centres), also defended by Golgi, prevailed among the scientific community. In the first issue of the Revista Trimestral de Histología Normal y Patológica (May, 1888), Cajal presented the definitive evidence underpinning neuron theory, thanks to staining of the axon of the small, star-shaped cells of the molecular layer of the cerebellum of birds, whose collaterals end up surrounding the Purkinje cell bodies, in the form of baskets or nests. He thus demonstrated once and for all that the relationship between nerve cells was not one of continuity, but rather of contiguity. Neuron theory is one of the principal scientific conquests of the 20th century, and which has withstood, with scarcely any modifications, the passage of more than a 100 years, being reaffirmed by new technologies, as the electron microscopy. Today, no neuroscientific discipline could be understood without recourse to the concept of neuronal individuality and nervous transmission at a synaptic level, as basic units of the nervous system.