This drawing of the rodent early visual system illustrates one of Cajal’s most important contributions to science: the law of dynamic polarization, wherein information flows from the receptive dendrites of a neuron, through its cell body, and down through its axon, which then makes synaptic contacts onto dendrites of subsequent neurons in the pathway. Here he shows two routes by which visual information is used to control movements of the body. Signals from the retina (A) enter the brain by the optic nerve (B) and terminate in two different brain regions – the lateral geniculate nucleus of the thalamus in the forebrain (D) and the superior colliculus on the roof of the midbrain (E). The superior colliculus, evolutionarily conserved across mammal, integrates visual and other inputs to provide signals (M) instructing physical movement and orientientation. Neurons in the visual thalamus (D) relay signals to the visual cortex (G), which sends projections to higher-order cortical areas (N) and also provides another set of descending motor commands (L). This diagram was submitted by Cajal as part of a joint application for the 1902 Martinez y Molina Award titled “Sensory Centers in Man and Animals” with his brother Pedro, also a noted histologist who specialized in amphibian and reptilian brain structures.

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©