- There is a main olfactory system, which detects volatile, airborne substances.
- There is secondary, or accessory, system. The accessory olfactory system senses fluid-based stimuli. Behavioral evidence indicates that most often, the stimuli detected by the accessory olfactory system are pheromones.
The olfactory system is often spoken as being similar to the gustatory system (taste system). Both are chemosensory senses. They change chemical signals into nerve impulses, which are translated into perceptions in the brain.
The part of the brain which deals with olfaction is the rhinencephalon, a small part in the human brain, but much more important in other vertebrates. The piriform cortex is probably the area most closely associated with identifying the odor. The medial amygdala is involved in social functions such as mating and recognising animals of the same species. The entorhinal cortex pairs odors with memories. The exact functions of these higher areas is still being researched.
Linda B. Buck and Richard Axel won the 2004 Nobel Prize in Physiology or Medicine for their work on the olfactory system. By analyzing rat DNA, they estimated that there were about one thousand different genes for olfactory receptors in the mammalian genome.
Damage to the olfactory system can happen through brain injury, cancer or toxic gases. The damage is usually measured by doctors giving the patient something to smell and having the patient try to guess what it is.
- Scott, J.; McBride, R.; Schneider, Stephen (1980). "The organization of projections from the olfactory bulb to the piriform cortex and olfactory tubercle in the rat". Journal of Comparative Neurology. 194 (3). doi:10.1002/cne.901940304.
- Buck L. & Axel R. 1991. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65 (1): 175–187.