(a) The Olfactory System Inhaled through the nose or the mouth, airborne molecules travel to the top of the nasal cavity and stimulate the olfactory receptors. When stimulated, these receptor cells communicate neural messages to the olfactory bulb, which is part of the olfactory cortex of the brain, where the sense of smell is registered.

(b) The Olfactory Receptor Cells More than 5 million olfactory neurons make up the moist, mucus-bathed tissue at the back of the nose. Projecting from the olfactory neurons are the fiberlike olfactory hairs, which can be clearly seen in this photograph. Olfactory neurons are replaced every month or two.

Smneedham/Getty Images
Heide Benser/zefa/Corbis
Lennart Nilsson/ Scanpix