Researchers found that approximately six types of mammals from mice to cats distinguish odours in almost the same way by using circuitry in the brain.
"The study yields insights into organisational principles underpinning brain circuitry for olfaction in mammals that may be applied to other parts of the brain and other species," said Charles Stevens, co-author of the research published in the journal of Current Biology.
In brief, the study revealed that the size of each of the three components of the neural network for olfaction scales about the same for each species, starting with receptors in the nose that transmit signals to a cluster of neurons in the front of the brain called the olfactory bulb which, in turn, relayed the signals to a "higher functioning" region for odor identification called the piriform cortex.
"These three stages scale with each other, with the relationship of the number of neurons in each stage the same across species. So, if you told me the number of neurons in the nose, I could predict the number in the piriform cortex or the bulb," said Shyam Srinivasan, the paper's co-author.
The new study revealed that the average number of synapses connecting each functional unit of the olfactory bulb (a glomerulus) to neurons in the piriform cortex is invariant across species.
"It was remarkable to see how these were conserved," said Stevens.
Specifically, identification of individual odors is linked to the strength and combination of firing neurons in the circuit that can be likened to music from a piano whose notes spring from the depression of multiple keys to create chords or the arrangement of letters that form the words on this page.
"The discrimination of odors is based on the firing rate, the electric pulse that travels down the neuron's axon. One odor, say for coffee, may elicit a slow response in a neuron while the same neuron may respond to chocolate at a faster rate," said Srinivasan.
This code used for olfaction is different than other parts of the brain.
"We showed that the connectivity parameters and the relationship between different stages of the olfactory circuit are conserved across mammals, suggesting that evolution has used the same design for the circuit across species, but just changed the size to fit the animals' environmental niche," said Stevens.
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