March 31, 2012 § Leave a comment
Similar to the way road atlases map out different real-world locations, the brain configures itself a map of our physical world. One such map is the olfactory system, which represents one of the oldest sense models in the history of mammals. This system has been long believed to map in such a way that groups of chemically related odorants index with clusters of cells that are situated next to one another.
Researchers at the Stowers Institute for Medical Research found that this “chemotopic” theory of olfaction was deficient, and sought to develop a new model of how the sense of smell works. Their findings were published in the online March edition of the Proceedings of the National Academy of Sciences.
Associate investigator C. Ron Yu, Ph.D., who led the study, said, “When we mapped the individual chemical features of different odorants, they mapped all over the olfactory bulb, which processes incoming olfactory information. From the animal’s perspective that makes perfect sense. The chemical structure of an odor molecule is not what’s important to them. They really just want to learn about their environment and associate olfactory information with food or other relevant information.”
Embedded in the membrane of nasal cavity sensory neurons are olfactory receptors. The brain receives odor information from these receptors. When odor molecules communicate with receptors, electric signals travel to the glomeruli in the olfactory bulb. Each glomerulus picks up input from the receptor neurons, and expresses only one type of olfactory receptor. The activation patterns in the bulb are believed to signify the specific odors.
Yu explains that while, “chemotopy is a very attractive model,” it has not yet been accurately mapped. This is due to limitations with earlier technologies. Recent experiments suggest that the chemotopic model breaks down at the fine level. Yu and his team endeavored to increase the resolution of this olfactory map. They generated a new line of transgenic mice with outstanding sensitivity and then created equipment that would allow them to present a single mouse with hundreds of odor stimuli.
The team examined the activation pattern at the level of a single glomerulus. The researchers found that certain odors activated glomeruli in the definite area of the olfactory bulb, while others signaled to glomeruli all over the olfactory map. This means odors from different classes interacted too, implying that the glomeruli have not yet evolved to sense the chemical shapes of only specific odorants.
Limei Ma, Ph.D., research specialist at Stowers and first author of the study says that this makes sense because there are hundreds of thousands of different odors. She explains, “Many of them could be really novel to the organism, something they never encountered before. The system must have the capability to recognize and encode anything.”
If the chemotopic hypothesis is wrong, and glomeruli do not correspond steadfastly with certain specific molecular shapes, what unites them? The team generated a “tunotopic” olfactory system hypothesis, in which individual olfactory receptors are “tuned” during evolution to a variety of molecular odorants. They respond to the millions of smells in combination, where similar tuned glomeruli lay near each other. Ma explains that this arrangement enhances contrast among similar odors.
Yu says, “The evolution of these receptors is not dictated by the chemical structures that they recognize. Most of our receptors have descended from a few common ancestral genes. Initially, they are more likely tuned to similar odors. When receptors accumulate mutations, it adds to their repertoire of natural odors they recognize.”
This olfactory system operates similar to a musical symphony. If we think of chemotopy in terms of a symphony, the musicians are clustered according to their instruments, never playing with other instruments. Their sounds are limited. The tunotopic hypothesis has different instruments overlapping to create more and differing sounds. Yu’s team believes the tunotopic hypothesis better describes the visual, auditory, and somatosensory process. Tunotopy allows an animal to better distinguish between the variation and degrees of odor. From an evolutionary standpoint, this precision is useful for the animal to sort through its environment. It helps us adapt to the changing world.
“When you have a new chemical synthesized, like new perfumes and food flavors, you don’t have to create new brain regions to react to it,” says Ma. “What you do is use the existing receptors to sense all these chemicals and then tell your brain whether this is novel, whether it’s similar, or whether it’s something really strange.”
“Smell is a Symphony”. (March 19, 2012). Neuroscience News. March 24, 2012. http://neurosciencenews.com/olfactory-bulb-brain-model-odor-tunotopic-hypothesis/.
Vokshoor, Amir. Ed. Arlen D. Meyers. “Olfactory System Anatomy”. (June 27, 2011). Medscape. March 24, 2012. http://emedicine.medscape.com/article/835585-overview.