The olfactory receptor cells are actually neurons, or nerve cells, outfitted at their knobby ends with six to twelve hair-like cilia. The cilia dangle into the thin layer of mucus that coats the membrane and snag passing particles for smell analysis. Each receptor cell is connected by a single primary olfactory neuron to one of the brain's two olfactory bulbs. The primary olfactory neurons pass through holes in the cribriform plate, a penny-thin bone at the front of the cranial cavity upon which the olfactory bulb rests. The primary neurons come together in structures known as glomeruli and there meet secondary neurons.
Messages shoot across the synapses, the gaps separating the primary and secondary neurons.
Smell nerves fibers then wend their way in complex paths throughout the rest of the brain, particularly into the most primitive portions, evolutionarily speaking. Some fibers reach the hypothalamus, the center controlling appetite, anger, fear and pleasure, while others continue into the hippocampus, which regulates memories, or descend into the depths of the brain stem, where such basic functions as remembering to breathe are regulated. That's why odors can generate extremely powerful emotional responses. In Proust's masterwork, it probably wasn't the sweet taste of that madelaine that unleashed the remembrance of things past. Rather, it was likely to have been the buttery odor that stimulated the narrator's hippocampus and hypothalamus.