Olfactory Bulb (1890)
The Olfactory Bulb (1890) conveys scent information from the nose to the brain through six layers of neurons: the olfactory nerve layer (not shown), glomerular layer (A), external plexiform layer (B), mitral cell layer (C), internal plexiform layer (D), and the granule cell layer (E), the deepest layer. In 1890, Cajal expanded on Schwabe's work to define these layers and discovered that, unlike other sensory systems, olfactory signals are transmitted from neuron groups within an olfactory fiber to multiple cells.
Cajal also observed that scent signals do not always follow a strict sequence. Mitral cells interact with olfactory fibers in the glomerular layer and relay output to deeper structures in the olfactory cortex. Though Cajal documented the granule cell layer's morphology, its function—providing inhibitory feedback to mitral cells—was only uncovered more recently.
