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Sensing the way through olfactory map
Chemicals, called odourants, are detected in the nose by roughly 1, 000 different odour receptors. .- Pic by M. Moorthy
EACH DAY, we use our noses to help make sense of our surroundings. We may not be as dependent on our olfactory capabilities as dogs or mice, but we are able to recognise and``assign an odour''to many thousands of chemicals in our enviironment.
Understanding how signals from those receptors are arranged in higher regions of the brain to yield diverse odour perceptions has been a longstanding goal for researchers.
Now, researchers have taken a step toward that goal with a series of experiments that shows how signals from different odour receptors are arranged in the brain's olfactory cortex.
The findings provide new insights into the processes that underlie odour perception.
In an article published in the journal Nature, researchers at Harvard Medical School reported genetic studies in mice that uncovered a precise sensory map in the olfactory cortex.
They also showed that this map is virtually identical in different individuals. Odour molecules that enter the nose are detected by odour receptors located on the surface of olfactory neurons.
There are about five million olfactory neurons, which are located in the olfactory epithelium on the wall of the nasal cavity.
Each of these neurons extends a long process, called an axon, to the olfactory bulb of the brain. Once in the olfactory bulb, the axon enters a spherical structure, called a glomerulus, where it makes contact with neurons in the bulb. The bulb neurons, in turn, extend axons to make contact with neurons located in the olfactory cortex. When odour receptors on an olfactory neuron detect an odourant, the neuron is activated.
This sets off a chain reaction whereby signals are transmitted from the neuron in the nose to connected neurons in the bulb and then to neurons in the olfactory cortex.
The team found that each olfactory neuron in the nose has only one of the 1,000 different types of odour receptor. They also found that one odourant can be detected by several different receptors. They then showed that different combinations of receptors detect different odourants.
In the nose, the 5,000 or so neurons with the same receptor are scattered in one of four spatial zones.
In the olfactory bulb, however, the axons of neurons with the same receptor converge in a few invariant glomeruli, creating a map of odour receptor inputs that is nearly identical in different individuals.
But what happens to odour signals at higher levels of the nervous system to generate different odour perceptions and emotional responses? It had been shown that if one places a tracer in one region of the olfactory cortex, it would back-label neurons in many parts of the bulb.
These findings made it clear that there was no point-to-point connection between the bulb and cortex. So, some suggested there may be little or no organization of inputs to the olfactory cortex. The most extreme view was that the inputs were random and the brain was able to take that random information and sort out different smells.
To find out how signals derived from different odour receptors are represented in the olfactory cortex, the research team engineered mice to express a tracer, called barley lectin (BL), along with just one type of odour receptor at a time.
The BL tracer crosses gaps between connected neurons, called synapses, and labels chains of connected neurons. The scientists genetically inserted the BL tracer into the odour receptor gene M5 or M50 - choosing those two receptor genes because they are expressed in different zones of the olfactory epithelium.
For the sake of comparison, the researchers also engineered mice to express the tracer gene in all olfactory sensory neurons.
When the researchers used the tracer to follow the wiring of the M5BL or the M50BL mice, they found that the neuronal connections extended through the olfactory bulb and showed up in the olfactory cortex as discrete clusters in different regions.
In contrast, the mice in which all olfactory neurons were engineered with BL showed labeling throughout the olfactory cortex. A particular surprise, however, came when the researchers compared different mice with the same odour receptors labelled.
The fact that this information is highly organized in the olfactory cortex, and is the same in different individuals, implies something about the perception of odours among different individuals. It provides a potential explanation as to why the odour of, say, a skunk smells bad to all people, and roses smell sweet.
The tracer studies also provided information about the neural processing of odours. It appears that information from different receptors is being combined in the cortex, while that's not true in the nose or the olfactory bulb.
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