Cajal’s law of dynamic polarization, wherein information flows from the receptive dendrites of a neuron through its cell body and down through its axon (and on through the dendrites of the cells with which it forms synapses), is well illustrated in this figure above comparing the visual and olfactory systems. The olfactory and visual systems were integral to Cajal’s formulation of the law of dynamic polarization, since it was clear where the external stimulus originated and in which direction information ought to flow. Cajal was thus able to hypothesize that the dendrites (or, in photoreceptors, the pigmented outer segments) of the receptor neurons, which were closest to the stimulus, serve a receptive function, and thus, that the axons must be responsible for passing the information onwards. The olfactory system is shown in Fig 1: information originates in the olfactory mucosa (B), where olfactory receptor neurons (a) sense environmental stimuli and send signals to the olfactory bulb (A), which sends it further on to the olfactory cortex (C). This information transfer is unidirectional – signals originating in the olfactory cortex and relayed back to the olfactory bulb travel along a separate set of neurons (e,f,g). Similarly, in the visual system (Fig 2), information travels in the retina from the photoreceptors (a,m), through the bipolar cells (b,n) and ganglion cells (c,o) through the optic nerve to higher visual centers (B). Cajal showed that, in birds, information originating higher in the visual system travels back to the retina along a separate set of neurons (p,q,r).

3D model created by Jeremy Swan based on an original illustration by Santiago Ramon y Cajal