Each olfactory neuron in the nose has a long fiber, or axon , that pokes through a tiny opening in the bone above it, the cribriform plate, to make a connection, or synapse , with other neurons . This synapse actually forms in the olfactory bulb , which is a part of the brain. A round, knob-like structure, the olfactory bulb is quite large in animals with an acute sense of smell but decreases in relative size as this ability wanes. Thus, bloodhounds, which can follow the scent of a person's tracks for long distances over varied terrain, have larger olfactory bulbs than humans do, even though humans are more than twice the total size of these dogs. The result is a highly organized spatial map of information derived from different receptors.
Most odors consist of mixtures of odorant molecules.
Therefore, other odors would be identified by different combinations. Surprisingly, the spatial map is identical in the olfactory bulbs of all the mice that have been tested, Buck says. As she points out, this information provided the key to an ancient riddle.