This is a pyramidal neuron, so named for the pyramid-shaped body at the center of this drawing, from the cerebral cortex of a human. This outermost layer of the brain integrates information from sensory organs, commands movements and is the hub for higher brain functions, such as consciousness. In his drawing, Cajal gives the branches or dendrites different weights to show how the neuron extends in three-dimensional space. It’s likely that this represents a sort of idealized portrait of a pyramidal neuron, a synthesis of many observations. Courtesy Instituto Cajal del Consjo Superior de Investigaciones Científicas, Madrid, © 2017 CSIC

The images are iconic to anyone who has dipped a toe in the vast and enigmatic pool of neuroscience: A delicate branching pattern emanates from and surrounds a small black splotch. Varied line weights hint at the three-dimensional structure the drawing describes, reminiscent of a bare-branched tree in winter. These are Spanish neuroanatomist Santiago Ramón y Cajal’s drawings of neurons.

Over five decades of work, Cajal (1852-1935) created more than 2,900 drawings detailing the nervous system’s architecture. Countless hours of close observation informed those drawings and helped him realize two fundamental truths of neuroanatomy. First, he deduced that the brain was made up of many individual cells, called neurons, rather than a spider web-like structure of fused cells (the view of many of his contemporaries). Second, the electrical signals those cells generate flow through neurons in one direction: branching dendrites receive the pulse, pass it to the main cell body and then along their axons—long projections that can connect distant parts of the brain.

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