Cajal often used the olfactory system as a subject, finding it to be both easily accessible and regularly structured. Information derived from environmental stimuli that interact directly with olfactory receptors is transmitted to the main olfactory bulb (MOB) or accessory olfactory bulb (AOB) for processing before being transmitted deeper into the olfactory cortex. The MOB (B) receives input from the olfactory sensory neurons in the main olfactory epithelium, while the AOB (A) receives input from the vomeronasal organ via the vomeronasal nerve bundle (D). The AOB is thought to process input mainly from pheromones. Cajal noted that the AOB was composed of 4 layers: the glomerular layer (a), the mitral/tufted layer (b), the lateral olfactory tract (c), and the granular layer (d). Although the MOB and AOB shown above appear to be converging onto a common point, this is not the case – the mitral/tufted cells of the MOB synapse project to the main olfactory cortex, while the AOB mitral/tufted cells bypass the main olfactory cortex and project directly to the medial amygdala and hypothalamic nuclei.

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