Posts tagged olfactory system
Articles and news from the latest research reports.
Swarming insect provides clues to how the brain processes smells
Our sense of smell is often the first response to environmental stimuli. Odors trigger neurons in the brain that alert us to take action. However, there is often more than one odor in the environment, such as in coffee shops or grocery stores. How does our brain process multiple odors received simultaneously?
Barani Raman, PhD, of the School of Engineering & Applied Science at Washington University in St. Louis, set out to find an answer. Using locusts, which have a relatively simple sensory system ideal for studying brain activity, he found the odors prompted neural activity in the brain that allowed the locust to correctly identify the stimulus, even with other odors present.
The results were published in Nature Neuroscience as the cover story of the December 2013 print issue.
The team uses a computer-controlled pneumatic pump to administer an odor puff to the locust, which has olfactory receptor neurons in its antennae, similar to sensory neurons in our nose. A few seconds after the odor puff is given, the locust gets a piece of grass as a reward, as a form of Pavlovian conditioning. As with Pavlov’s dog, which salivated when it heard a bell ring, trained locusts anticipate the reward when the odor used for training is delivered. Instead of salivating, they open their palps, or finger-like projections close to the mouthparts, when they predict the reward. Their response was less than half of a second. The locusts could recognize the trained odors even when another odor meant to distract them was introduced prior to the target cue.
“We were expecting this result, but the speed with which it was done was surprising,” says Raman, assistant professor of biomedical engineering. “It took only a few hundred milliseconds for the locust’s brain to begin tracking a novel odor introduced in its surrounding. The locusts are processing chemical cues in an extremely rapid fashion.”
“There were some interesting cues in the odors we chose,” Raman says. “Geraniol, which smells like rose to us, was an attractant to the locusts, but citral, which smells like lemon to us, is a repellant to them. This helped us identify principles that are common to the odor processing.
Raman has spent a decade learning how the human brain and olfactory system operate to process scent and odor signals. His research seeks to take inspiration from the biological olfactory system to develop a device for noninvasive chemical sensing. Such a device could be used in homeland security applications to detect volatile chemicals and in medical diagnostics, such as a device to test blood-alcohol level.
This study is the first in a series seeking to understand the principles of olfactory computation, Raman says.
“There is a precursory cue that could tell the brain there is a predator in the environment, and it has to predict what will happen next,” Raman says. “We want to determine what kinds of computations have to be done to make those predictions.”
In addition, the team is looking to answer other questions.
“Neural activity in the early processing centers does not terminate until you stop the odor pulse,” he says. “If you have a lengthy pulse – 5 or 10 seconds long – what is the role of neural activity that persists throughout the stimulus duration and often even after you terminate the stimulus? What are the roles of the neural activity generated at different points in time, and how do they help the system adapt to the environment? Those questions are still not clear.”
(Source: news.wustl.edu)
Neuron ‘claws’ in the brain enable flies to distinguish one scent from another
Think of the smell of an orange, a lemon, and a grapefruit. Each has strong acidic notes mixed with sweetness. And yet each fresh, bright scent is distinguishable from its relatives. These fruits smell similar because they share many chemical compounds. How, then does the brain tell them apart? How does the brain remember a complex and often overlapping chemical signature as a particular scent?
Researchers at Cold Spring Harbor Laboratory (CSHL) are using the fruit fly to discover how the brain integrates multiple signals to identify one unique smell. It’s work that has a broader implication for how flies – and ultimately, people – learn. In work published today in Nature Neuroscience, a team led by Associate Professor Glenn Turner describes how a group of neurons in the fruit fly brain recognize multiple individual chemicals in combination in order to define, or remember, a single scent.
The olfactory system of a fruit fly begins at the equivalent of our nose, where a series of neurons sense and respond to very specific chemicals. These neurons pass their signal on to a group of cells called projection neurons. Then the signal undergoes a transformation as it is passed to a body of neurons in the fly brain called Kenyon cells.
Kenyon cells have multiple, extremely large protrusions that grasp the projection neurons with a claw-like structure. Each Kenyon cell claw is wrapped tightly around only one projection neuron, meaning that it receives a signal from just one type of input. In addition to their unique structure, Kenyon cells are also remarkable for their selectivity. Because they’re selective, they aren’t often activated. Yet little is known about what in fact makes them decide to fire a signal.
Turner and colleague Eyal Gruntman, who is lead author on their new paper, used cutting-edge microscopy to explore the chemical response profile for multiple claws on one Kenyon cell. They found that each claw, even on a single Kenyon cell, responded to different odor molecules. Additional experiments using light to stimulate individual neurons (a technique called optogenetics) revealed that single Kenyon cells were only activated when several of their claws were simultaneously stimulated, explaining why they so rarely fire. Taken together, this work explains how individual Kenyon cells can integrate multiple signals in the brain to “remember” the particular chemical mixture as a single, distinct odor.
Turner will next try to determine “what controls which claws are connected, and how strong those connections are.” This will provide insight into how the brain learns to assign a specific mix of chemicals as defining a particular scent. But beyond simple odor detection, the research has more general implications for learning. For Turner, the question driving his work forward is: what in the brain changes when you learn something?
A shot of anxiety and the world stinks
In evolutionary terms, smell is among the oldest of the senses. In animals ranging from invertebrates to humans, olfaction exerts a primal influence as the brain continuously and subconsciously processes the steady stream of scent molecules that waft under our noses.
And while odors — whether the aroma of stinky socks or the sweet smell of baking bread — are known to stir the emotions, how they exert their influence biologically on the emotional centers of the human brain, evoking passion or disgust, has been a black box.
Now, however, researchers using powerful new brain imaging technologies are peeling back some of the mystery, revealing how anxiety or stress can rewire the brain, linking centers of emotion and olfactory processing, to make typically benign smells malodorous.
Writing today (Sept. 24, 2013) in the Journal of Neuroscience, a team led by Wen Li, a professor of psychology at the UW-Madison Waisman Center, reports that the brains of human subjects experience anxiety induced by disturbing pictures and text of things like car crashes and war transform neutral odors to distasteful ones, fueling a feedback loop that could heighten distress and lead to clinical issues like anxiety and depression.
The finding is important because it may help scientists understand the dynamic nature of smell perception and the biology of anxiety as the brain rewires itself under stressful circumstances and reinforces negative sensations and feelings.
"After anxiety induction, neutral smells become clearly negative," explains Li, who conducted the study with UW-Madison colleagues Elizabeth Krusemark and Lucas Novak, and Darren Gitelman of Northwestern University’s Feinberg School of Medicine. "People experiencing an increase in anxiety show a decrease in the perceived pleasantness of odors. It becomes more negative as anxiety increases."
Using behavioral techniques and functional magnetic resonance imaging (fMRI), Li’s group looked at the brains of a dozen human subjects with induced anxiety as they processed known neutral odors.
Functional MRI is a technology that enables clinicians and researchers to observe the working brain in action. Before entering the MRI where screens cycle through a series of disturbing pictures and text, subjects were exposed to and rated a panel of neutral smells.
In the course of the experiment, the Wisconsin team observed that two distinct and typically independent circuits of the brain — one dedicated to olfactory processing, the other to emotion — become intimately intertwined under conditions of anxiety. Subsequent to anxiety induction and the imaging process, subjects were asked again to rate the panel of neutral smells, most assigning negative responses to smells they previously rated as neutral.
"In typical odor processing, it is usually just the olfactory system that gets activated," says Li. "But when a person becomes anxious, the emotional system becomes part of the olfactory processing stream."
Although those two systems of the brain are right next to each other, under normal circumstances there is limited crosstalk between the two. However, under conditions of induced anxiety, the Wisconsin team observed the emergence of a unified network cutting across the two systems.
The results may have clinical implications in the sense that it begins to uncover the biological mechanisms at play during periods of anxiety. “We encounter anxiety and as a result we experience the world more negatively. The environment smells bad in the context of anxiety. It can become a vicious cycle, making one more susceptible to a clinical state of anxiety as the effects accumulate. It can potentially lead to a higher level of emotional disturbances with rising ambient sensory stress.”
Sex, Smell And Science – The Genetics Of Olfaction
No two people smell exactly alike. That is, noses sense odors in individual ways. What one nose finds offensive, another may find pleasant, while another might not smell anything at all. Scientists have long known the way things smell to us is determined by our genes.
Now, two studies appearing in the journal Current Biology (1, 2) have identified “the genetic differences that underpin the differences in smell sensitivity and perception in different individuals.” And while some of these differences merely help determine our culinary preferences, others appear to play a subconscious role in how we choose our sexual partners.
For the first study, 200 people were tested to determine their sensitivity to 10 different chemical compounds commonly found in foods. The researchers found four of the ten odors had a genetic association. These were malt, apple, blue cheese, and a floral scent associated with violets.
The research team, led by Sara Jaeger, Jeremy McRae, and Richard Newcomb of Plant and Food Research in New Zealand, used a genome-wide association study. Their first task was to identify which test subjects could smell each chemical compound and which could not. They then searched the subjects’ genomes for areas of DNA that differed between these people.
“We were surprised how many odors had genes associated with them. If this extends to other odors, then we might expect everyone to have their own unique set of smells that they are sensitive to,” explained McRae
“These smells are found in foods and drinks that people encounter every day, such as tomatoes and apples. This might mean that when people sit down to eat a meal, they each experience it in their own personalized way.”
They further found there is no regional differentiation. A person in one part of the world is just as likely to be able to smell a particular compound as a person in another part of the world. In addition, sensitivity to one compound does not predict the ability to smell another compound.
The genes that determine our ability to perceive certain odors all lie in or near the genes that encode olfactory receptors. These receptors occur on the surface of sensory nerve cells in the upper part of the nose. A particular smell is perceived when these receptor molecules bind with a chemical compound wafting through the nose, causing nerve cells to send an impulse to the brain and producing our sensation of smell.
For the violet smell, caused by a naturally occurring chemical compound known as β-ionone, the researchers were able to pinpoint the exact mutation in gene OR5A1 that determines whether the smell is perceived as floral, sour or pungent, and whether it is found to be pleasant.
These findings might have future marketing value. According to Richard Newcomb, “Knowing the compounds that people can sense in foods, as well as other products, will have an influence on the development of future products. Companies may wish to design foods that better target people based on their sensitivity, essentially developing foods and other products personalized for their taste and smell.”
SEXY OR STINKY?
A separate study was conducted by Leslie Vosshall of the Rockefeller University Hospital. Humans have about 1,000 genes that influence smell, and around 400 of these are responsible for sensing a particular odor molecule.
Testing 391 human subjects, Vosshall studied olfactory responses to two closely related steroids, androstenone and androstadienone, which are found in male sweat. People generally have strong reactions to these steroids, finding them either sweet and florally or rank and noxious. The gene 0R7D4 determines the intensity of these odors as well as the perception of them being either pleasant or repulsive.
According to Vosshall’s report: “People who found the smell repulsive were more likely to have two functional copies of OR7D4; those who perceived it as a more mild smell tended to have one or two impaired copies of the gene.”
This study is part of the larger goal of understanding how genetic and neuronal factors influence behaviors.
A 2002 study published in Nature Genetics provided more insight into the effect of male pheromones on women. This study looked at the link between women’s preferences for the odors given off by men and a group of genes called the Major Histocompatibily Complex (MHC) which contribute to a persons’ immune response.
In this experiment, a group of 49 women were asked to smell 10 boxes. Some of the boxes held t-shirts worn by men with different MHC genes, and others contained familiar household odors such as bleach or cloves.
The t-shirts were worn by men who slept in them for two nights and avoided contact with other scents during that time, even to the point of avoiding other people. According to the report, “the women were then asked to rate each scent based on their familiarity, intensity, pleasantness and spiciness, as well as choose the one odor which they would choose if they had to smell it all the time.”
What the researchers found was the women did not choose the scents of men whose genes were similar to their own, nor did they choose those whose genes were too dissimilar. The women showed no preference for odors from men who had the same genes as their mothers, but did show a preference for odors from men who shared genes they inherited from their fathers.
Scientists believe there are two reasons for preferring a mate whose MHC genes are different than one’s own. One is that it would tend to create offspring with more genetic diversity and thus more robust immune systems. The other is it helps to avoid inbreeding.
Of course, when people choose their mates, there are a number of social factors that come into play as well. However, studies have shown married people tend to have different types of genes than their spouses.
So, the next time you like the way a person smells, keep in mind it may mean you have complementary genes.
Temporal Processing in the Olfactory System: Can We See a Smell?
Sensory processing circuits in the visual and olfactory systems receive input from complex, rapidly changing environments. Although patterns of light and plumes of odor create different distributions of activity in the retina and olfactory bulb, both structures use what appears on the surface similar temporal coding strategies to convey information to higher areas in the brain. We compare temporal coding in the early stages of the olfactory and visual systems, highlighting recent progress in understanding the role of time in olfactory coding during active sensing by behaving animals. We also examine studies that address the divergent circuit mechanisms that generate temporal codes in the two systems, and find that they provide physiological information directly related to functional questions raised by neuroanatomical studies of Ramon y Cajal over a century ago. Consideration of differences in neural activity in sensory systems contributes to generating new approaches to understand signal processing.
Scientists Identify Critical Link In Mammalian Odor Detection
Researchers at the Monell Center and collaborators have identified a protein that is critical to the ability of mammals to smell. Mice engineered to be lacking the Ggamma13 protein in their olfactory receptors were functionally anosmic – unable to smell. The findings may lend insight into the underlying causes of certain smell disorders in humans.
“Without Ggamma13, the mice cannot smell,” said senior author Liquan Huang, PhD, a molecular biologist at Monell. “This raises the possibility that mutations in the Ggamma13 gene may contribute to certain forms of human anosmia and that gene sequencing may be able to predict some instances of smell loss.”
Odor molecules entering the nose are sensed by a family of olfactory receptors. Inside the receptor cells, a complex cascade of molecular interactions converts information to ultimately generate an electrical signal. This signal, called an action potential, is what tells the brain that an odor has been detected.
To date, the identities of some of the intracellular molecules that convert odor information into an action potential remain a mystery. Suspecting that a protein called Ggamma13 might be involved, the research team engineered mice to be lacking this protein and then tested how the ‘knockout’ mice responded to odors.
Importantly, because the Ggamma13 protein plays critical roles in other parts of the body, the Ggamma13 ‘knockout’ was confined exclusively to smell receptor cells. This specificity allowed the researchers to characterize the effect of Ggamma13 deletion on the olfactory system without interference from changes in other tissues.
Both behavioral and physiological experiments revealed that the Ggamma13 knockout mice did not respond to odors. The findings were published in The Journal of Neuroscience.
In behavioral tests, control mice with an intact sense of smell were able to detect and retrieve a piece of buried food in less than 30 seconds. However, mice lacking Ggamma13 in their olfactory cells required more than 8 minutes to perform the same task. Both sets of mice were able to quickly locate the food when it was placed in plain sight.
A second set of experiments measured olfactory function on a physiological level. Using olfactory tissue from knockout and control mice, the researchers recorded electrical responses to 15 different odors. Responses from the Ggamma13 knockout mice were greatly reduced, suggesting that the olfactory receptors of these mice were unable to translate odor signals into an electrical response.
Together, the findings demonstrate that Ggamma13 is essential for mammals to smell odors and extend the current understanding of how olfactory receptor cells communicate information about odors to the brain. Future studies will seek to identify how Ggamma13 interacts with other molecules within the olfactory receptor.
“Loss of olfactory function can greatly reduce quality of life,” said Huang. “Our findings demonstrate the significant consequences when just one molecular component of this complex system does not function properly.”
(Source: monell.org)
Sniffing Out Schizophrenia
Neurons in the nose could be the key to early, fast, and accurate diagnosis, says a TAU researcher
A debilitating mental illness, schizophrenia can be difficult to diagnose. Because physiological evidence confirming the disease can only be gathered from the brain during an autopsy, mental health professionals have had to rely on a battery of psychological evaluations to diagnose their patients.
Now, Dr. Noam Shomron and Prof. Ruth Navon of Tel Aviv University’s Sackler Faculty of Medicine, together with PhD student Eyal Mor from Dr. Shomron’s lab and Prof. Akira Sawa of Johns Hopkins Hospital in Baltimore, Maryland, have discovered a method for physical diagnosis — by collecting tissue from the nose through a simple biopsy. Surprisingly, collecting and sequencing neurons from the nose may lead to “more sure-fire” diagnostic capabilities than ever before, Dr. Shomron says.
This finding, which was reported in the journal Neurobiology of Disease, could not only lead to a more accurate diagnosis, it may also permit the crucial, early detection of the disease, giving rise to vastly improved treatment overall.
From the nose to diagnosis
Until now, biomarkers for schizophrenia had only been found in the neuron cells of the brain, which can’t be collected before death. By that point it’s obviously too late to do the patient any good, says Dr. Shomron. Instead, psychiatrists depend on psychological evaluations for diagnosis, including interviews with the patient and reports by family and friends.
For a solution to this diagnostic dilemma, the researchers turned to the olfactory system, which includes neurons located on the upper part of the inner nose. Researchers at Johns Hopkins University collected samples of olfactory neurons from patients diagnosed with schizophrenia and a control group of non-affected individuals, then sent them to Dr. Shomron’s TAU lab.
Dr. Shomron and his fellow researchers applied a high-throughput technology to these samples, studying the microRNA of the olfactory neurons. Within these molecules, which help to regulate our genetic code, they were able to identify a microRNA which is highly elevated in those with schizophrenia, compared to individuals who do not have the disease.
"We were able to narrow down the microRNA to a differentially expressed set, and from there down to a specific microRNA which is elevated in individuals with the disease compared to healthy individuals," explains Dr. Shomron. Further research revealed that this particular microRNA controls genes associated with the generation of neurons.
In practice, material for biopsy could be collected through a quick and easy outpatient procedure, using a local anesthetic, says Dr. Shomron. And with microRNA profiling results ready in a matter of hours, this method could evolve into a relatively simple and accurate test to diagnose a very complicated illness.
Early detection, early intervention
Though there is much more to investigate, Dr. Shomron has high hopes for this diagnostic method. It’s important to determine whether this alteration in microRNA expression begins before schizophrenic symptoms begin to exhibit themselves, or only after the disease fully develops, he says. If this change comes near the beginning of the timeline, it could be invaluable for early diagnostics. This would mean early intervention, better treatment, and possibly even the postponement of symptoms.
If, for example, a person has a family history of schizophrenia, this test could reveal whether they too suffer from the disease. And while such advanced warning doesn’t mean a cure is on the horizon, it will help both patient and doctor identify and prepare for the challenges ahead.
(Source: aftau.org)
When our noses pick up a scent, whether the aroma of a sweet rose or the sweat of a stranger at the gym, two types of sensory neurons are at work in sensing that odor or pheromone. These sensory neurons are particularly interesting because they are the only neurons in our bodies that regenerate throughout adult life—as some of our olfactory neurons die, they are soon replaced by newborns. Just where those neurons come from in the first place has long perplexed developmental biologists.
Previous hypotheses about the origin of these olfactory nerve cells have given credit to embryonic cells that develop into skin or the central nervous system, where ear and eye sensory neurons, respectively, are thought to originate. But biologists at the California Institute of Technology (Caltech) have now found that neural-crest stem cells—multipotent, migratory cells unique to vertebrates that give rise to many structures in the body such as facial bones and smooth muscle—also play a key role in building olfactory sensory neurons in the nose.
"Olfactory neurons have long been thought to be solely derived from a thickened portion of the ectoderm; our results directly refute that concept," says Marianne Bronner, the Albert Billings Ruddock Professor of Biology at Caltech and corresponding author of a paper published in the journal eLIFE on March 19 that outlines the findings.
The two main types of sensory neurons in the olfactory system are ciliated neurons, which detect volatile scents, and microvillous neurons, which usually sense pheromones. Both of these types are found in the tissue lining the inside of the nasal cavity and transmit sensory information to the central nervous system for processing.
In the new study, the researchers showed that during embryonic development, neural-crest stem cells differentiate into the microvillous neurons, which had long been assumed to arise from the same source as the odor-sensing ciliated neurons. Moreover, they demonstrated that different factors are necessary for the development of these two types of neurons. By eliminating a gene called Sox10, they were able to show that formation of microvillous neurons is blocked whereas ciliated neurons are unaffected.
They made this discovery by studying the development of the olfactory system in zebrafish—a useful model organism for developmental biology studies due to the optical clarity of the free-swimming embryo. Understanding the origins of olfactory neurons and the process of neuron formation is important for developing therapeutic applications for conditions like anosmia, or the inability to smell, says Bronner.
"A key question in developmental biology—the extent of neural-crest stem cell contribution to the olfactory system—has been addressed in our paper by multiple lines of experimentation," says Ankur Saxena, a postdoctoral scholar in Bronner’s laboratory and lead author of the study. "Olfactory neurons are unique in their renewal capacity across species, so by learning how they form, we may gain insights into how neurons in general can be induced to differentiate or regenerate. That knowledge, in turn, may provide new avenues for pursuing treatment of neurological disorders or injury in humans."
Next, the researchers will examine what other genes, in addition to Sox10, play a role in the process by which neural-crest stem cells differentiate into microvillous neurons. They also plan to look at whether or not neural-crest cells give rise to new microvillous neurons during olfactory regeneration that happens after the embryonic stage of development.
Highly developed antennae containing different types of olfactory receptors allow insects to use minute amounts of odors for orientation towards resources like food, oviposition sites or mates. Scientists at the Max Planck Institute for Chemical Ecology in Jena, Germany, have now used mutant flies and for the first time provided experimental proof that the extremely sensitive olfactory system of fruit flies − they are able to detect a few thousand odour molecules per millilitre of air, whereas humans need hundreds of millions − is based on self-regulation of odorant receptors. Even fewer molecules below the response threshold are sufficient to amplify the sensitivity of the receptors, and binding of molecules shortly afterwards triggers the opening of an ion channel that controls the fly’s reaction and flight behaviour. This means that a below threshold odor stimulation increases the sensitivity of the receptor, and if a second odour pulse arrives within a certain time span, a neural response will be elicited.
It is amazing how many fruit flies (Drosophila melanogaster) find their way to a rotting apple. It is known that insects are able to detect the slightest concentrations of odour molecules, especially pheromones, but also “food signals”.
Dieter Wicher, Shannon Olsson, Bill Hansson and their colleagues at the Max Planck Institute for Chemical Ecology were looking for answers to the question why insects can trace odour molecules so easily and at such low concentrations in comparison to other animals. They focused their attention on odorant receptor proteins in the antenna, the insects’ nose. These insect proteins are pretty young from an evolutionary perspective and their molecular constituents may be the basis for the insects’ highly sensitive sense of smell.
Insect odorant receptors form a receptor system that consists of the actual receptor protein and an ion channel. After binding of an odour molecule, receptor protein and ion channel trigger the neural electrical response. This mechanism was recently described in the receptor system Or22a-Orco. Apart from functioning as so-called ionotropic receptors, which enable ion flow through membranes after binding of odour molecules, odorant receptors also elicit intracellular signals. These stimulate the formation of cyclic adenosine monophosphate (cyclic AMP or cAMP), which activates an ion flow through the co-receptor Orco. The role and relevance of this weak and slow electrical current, however, was until now unclear.
Merid N. Getahun, a PhD student from Ethiopia, and his colleagues have conducted numerous experiments on Drosophila olfactory neurons. They injected tiny amounts of compounds that stimulate, inhibit or imitate cAMP formation directly into the sensory hairs housing olfactory sensory neurons on the fly antenna. The researchers tested the flies’ responses to ethyl butyrate, which has a fruity odour similar to pineapple, and measured activity in the sensory neurons by using glass microelectrodes. As a control, they used genetically modified fruit flies where the co-receptor Orco had been inactivated. “The fact that these mutants are no more able to respond to cAMP or the inhibition/activation of the involved key enzymes, such as protein kinase C and phospholipase C, shows that the highly sensitive olfactory system in insects is regulated intracellularly by their own odorant receptors,” says Dieter Wicher, the leader of the research group.
The combination of odorant receptor and co-receptor Orco can be compared to a transistor, Wicher continues: A weak basic current is sufficient to release the main electric current that activates the neuron. The process can also be seen as a short-term memory situated in the insect nose. A very weak stimulus does not elicit a response when it first occurs, but if it reoccurs within a certain time span it will release the electrical response according to the principle “one time is no time, but two is a bunch.”
More than a century after it was first identified, Harvard scientists are shedding new light on a little-understood neural feedback mechanism that may play a key role in how the olfactory system works in the brain.
As described in a December 19 paper in Neuron by Venkatesh Murthy, Professor of Molecular and Cellular Biology, researchers have, for the first time, described how that feedback mechanism works by identifying where the signals go, and which type of neurons receive them. Three scientists from the Murthy lab were involved in the work: Foivos Markopoulos, Dan Rokni and David Gire.
"The image of the brain as a linear processor is a convenient one, but almost all brains, and certainly mammalian brains, do not rely on that kind of pure feed-forward system," Murthy explained. "On the contrary, it now appears that the higher regions of the brain which are responsible for interpreting olfactory information are communicating with lower parts of the brain on a near-constant basis."
Though researchers have known about the feedback system for decades, key questions about its precise workings, such as which neurons in the olfactory bulb receive the feedback signals, remained a mystery, partly because scientists simply didn’t have the technological tools needed to activate individual neurons and individual pathways.
"One of the challenges with this type of research is that these feedback neurons are not the only neurons that come back to the olfactory bulb," Murthy explained. "The challenge has always been that there’s no easy way to pick out just one type of neuron to activate."
To do it, Murthy and his team turned to a technique called optogenetics.
Using a virus that has been genetically-modified to produce a light-sensitive protein, Murthy and his team marked specific neurons, which become active when hit with laser light. Researchers were then able to trace the feedback mechanism from the brain’s processing centers back to the olfactory bulb.
Reaching that level of precision was critical, Murthy explained, because while olfactory bulb contains many “principal” neurons which are responsible for sending signals on to other parts of the brain, it is also packed with interneurons, which appear to play a role in formatting olfactory information as it comes into the brain.
(Image: BigStock)