Although the sense of smell (olfaction) is perhaps the least developed sense in humans, people are able to detect odors surprisingly well. For example, the chemical that gives bell peppers their odor (2-isobutyl-3-methoxypyrazine) can be detected at an air concentration of about one molecule per billion.
Odorants, the stimuli for the olfactory system, fall into two classes: water-soluble and fat-soluble. Water-soluble odorants (e.g. ethanol) can only be detected in high concentrations, but fat-soluble odorants (e.g. violet) can be detected at much lower concentrations because fat-soluble molecules, in essence, stick to the nose.
Anatomy of the Olfactory System
The act of smelling relies on the interaction between several cells and structures. The olfactory epithelium lays spread across the top of the nasal cavity and holds the olfactory receptor neurons. These neurons project olfactory cilia, hair-like protrusions from a knob at the end of the cell, into a layer of mucus that lines the inside of the nasal cavity. This mucus protects the cells of the olfactory epithelium.
The olfactory receptor neurons themselves are bipolar neurons that reach up through the cribriform plate, the horizontal plate of the ethmoid bone, toward the olfactory bulb. The olfactory bulb is the structure that ultimately tells the brain about the detected odor.
Transduction of Odor Stimuli by Olfactory Receptors
The presence of an odorant in the nasal cavity causes the olfactory receptor neurons to generate an electrical signal that is passed to the olfactory bulb. The cilia of the olfactory receptor neurons contain odorant receptors. In fact, Richard Axel and Linda Buck, winners of the 2004 Nobel Prize in Physiology and Medicine, found that humans have roughly 1,000 genes (almost 3% of all genes) devoted to olfactory receptors. This means that there are roughly 1,000 different olfactory receptors, giving humans the ability to smell 1,000 different odors.
When an odorant molecule binds to a receptor protein on the olfactory receptor neuron’s cilia, a G-protein associated with the particular odor activates an enzyme (adenylate cyclase) in the cell that converts the nucleotide ATP into cAMP, a kind of chemical messenger. When this happens, the cAMP opens protein channels in the cell that let calcium and sodium ions inside. Once inside, the calcium ions open up channels that let chloride ions in the cell.
With all of these charged ions moving inside the cell, the neuron gets charged and relays this electrical signal to the olfactory bulb. The process of this electricity-generation is eventually stopped with the help of a protein that exchanges calcium ions inside the cell for sodium ions outside the cell. This exchange process, however, is slow to change the charge of the neuron, so smells will persist for a while.
Anatomy of the Olfactory Bulb
The olfactory bulb is a layered structure that sends olfactory information to the brain. At the base of the olfactory bulb are spherical pockets that join incoming and outgoing olfactory receptor neurons. These are called glomeruli, and each glomerulus is concerned with a particular smell. The neurons that enter the olfactory bulb go to different glomeruli, depending on which odor their olfactory receptor specializes in.
In the glomeruli, the incoming olfactory receptor neurons meet any of three types of cells: mitral cells, tufted cells, and periglomerular cells. The mitral cells are the principle projection neurons, sending the odor signal to the brain. Tufted cells also send odor signals to the brain, but there are fewer of them. The periglomerular cells connect glomeruli together, allowing for comparison of input.
Processing the Sense of Smell in the Brain
Functional magnetic resonance imaging (fMRI) studies have found the olfactory bulb to be highly connected to the rest of the brain. The mitral cells from the olfactory bulb gather into the lateral olfactory tract that carries the electrical signal of odor detection to brain structures like the entorhinal cortex (a learning center in the brain), and the amygdala (where fear is processed).
One of the more prominent brain areas involved in the processing of olfaction is the pyriform cortex. This region sits in the temporal lobe and keeps the odor-specific mitral cells separate.
Pheromones, Estrogen, and the Loss of the Sense of Smell
There is still a lot that neurobiologists do not know about the sense of smell. Three phenomena involved with the sense of smell are pheromones, estrogen detection, and the loss of sense of smell.
Pheromones are odorants that are secreted by animals that trigger behavioral changes in other animals in the same species. In many animals, the secretion of pheromones influences social, reproductive, and parenting behaviors. Rats have been found to detect pheromones using structures called vomeronasal organs. These organs are present in large numbers in rodents, but only eight percent of humans have them, and it is not clear whether they have any function in humans. The evidence is inconclusive, however, that humans (via vomeronasal organs or not) can detect pheromones.
Many people will have heard the assertion — or will have experienced the phenomenon itself — that women who live together begin to synchronize their menstrual cycles. This may be due to smelling estrogen. Although people do not consciously smell anything, brain scans have shown specific activity in response to smelling estrogen. Specifically, females show increased activity in the anterior hypothalamus, and males demonstrate more activity in the posterior hypothalamus.
Finally, the term anosmia refers to the loss of the sense of smell. Some anosmias are specific to just a few odors, but others are characterized by severe loss of the sense. If severe enough, a person who has lost their sense of smell could suffer from a decreased appetite and could have difficulty identifying hazardous foods and substances that should be avoided. See “Causes of Loss of Sense of Smell and Taste” for more.
References
Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W., LaMantia, A., McNamara, J. O., & White, L. E. (Eds.). (2008).Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.
Herz, R. (2007). The scent of desire: Discovering our enigmatic sense of smell. New York, NY: HarperCollins.
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