Smell begins when airborne molecules stimulate olfactory receptor cells. If a substance is somewhat volatile (that is, if it easily turns into a gas), it will give off molecules, or odorants. Nonvolatile materials like steel do not have a smell.
Temperature and humidity affect odor because they increase molecular volatility. This is why trash smells stronger in the heat and cars smell musty after rain. A substance's solubility also affects its odor. Chemicals that dissolve in water or fat are usually intense odorants.
When an air current sweeps an odorant up through the nostrils, the molecules hit the olfactory epithelium -- the center of olfactory sensation. The epithelium occupies only about one square inch of the superior portion of the nasal cavity. Mucus secreted by the olfactory gland coats the epithelium's surface and helps dissolve odorants.
Olfactory receptor cells are neurons with knob-shaped tips called dendrites. Olfactory hairs that bind with odorants cover the dendrites. When an odorant stimulates a receptor cell, the cell sends an electrical impulse to the olfactory bulb through the axon at its base.
Supporting cells provide structure to the olfactory epithelium and help insulate receptor cells. They also nourish the receptors and detoxify chemicals on the epithelium's surface. Basal stem cells create new olfactory receptors through cell division. Receptors regenerate monthly -- which is surprising because mature neurons usually aren't replaced.
While receptor cells respond to olfactory stimuli and result in the perception of smell, trigeminal nerve fibers in the olfactory epithelium respond to pain. When you smell something caustic like ammonia, receptor cells pick up odorants while trigeminal nerve fibers account for the sharp sting that makes you immediately recoil.
But how does odor actually become smell? In the next section, we'll learn more about olfactory receptors and odorant patterns.