Olfaction

Olfaction, the chemical sense of smell, begins when airborne molecules enter the nasal cavities. At the top of the uppermost cavity sits the olfactory epithelium, a small patch of sensory receptor neurons.

When odorants reach this area, a molecule stimulates one of thousands of receptors on a neuron. It doesn't matter which, because each neuron only has one type. The same type of molecules, though, can stimulate several different neurons.

When enough receptors are stimulated, the neurons generate action potentials, passing on the smell information into the olfactory bulbs, which sit just above the nasal cavity below the brain. Here, neurites from similar olfactory neurons meet in a relay-like station called the glomeruli, where mitral cells collate the converging information.

Details about the types of receptors and the strength of stimulation are unique for each odor molecule. This combinatorial diversity allows us to distinguish between millions of different odors.

The mitral cells then pass the collated information on to the olfactory cortex. From there it splits into two destinations, the thalamus, where it integrates with other sensory information to create the perception of smell or flavor, and the hippocampus, where odor information is linked to memory formation.