Know How Brains of Mammals Evolved To Distinguish Odors

The world is full of millions and millions of different smells, but how the brains of mammals evolved to distinguish them is a mystery.
Now, two neuroscientists from the Salk Institute and the University of California at San Diego have discovered that at least six types of mammals, from mice to cats, distinguish odors in approximately the same way, using circuits in the brain that are evolutionarily conserved in all the species.
"The study offers information on the organizational principles that underlie brain circuits for mammalian sniffing that can be applied to other parts of the brain and other species," says Charles Stevens, distinguished professor emeritus at Salk's Laboratory of Neurobiology and co-author of the research published in July 18, 2019 edition of Current Biology.
In summary, the study reveals that the size of each of the three components of the neuronal network for smell is approximately the same for each species, beginning with the receptors in the nose that transmit signals to a group of neurons in the front of the brain called the olfactory bulb which, in turn, transmits the signals to a region of "superior functioning" for the identification of the odor called pyriform cortex.
"These three stages are scaled together, and the relationship between the numbers of neurons in each stage is the same in all species," says Shyam Srinivasan, assistant project scientist with the Kavli Institute for Brain and Mind at UC San Diego, and co-author of the article. "Then, if you told me the number of neurons in the nose, I could predict the number in the piriformis cortex or in the bulb."
The current study is based on the same duo's research, published in 2018, which described how the brains of mice process and distinguish odors using what are known as "distributed circuits." Unlike the visual system, for example, where information is transmitted in an orderly fashion to specific parts of the visual cortex, the researchers discovered that the olfactory system in mice is based on a combination of connections distributed through the pyriform cortex.
After that article, Stevens and Srinivasan tried to determine if the distributed neural circuit revealed in mice is similar in other mammals. For the current work, the researchers analyzed the brains of mammals of different sizes and types. Their calculations, plus previous studies in recent years, were used to estimate brain volumes. Stevens and Srinivasan used a variety of microscopy techniques that allowed them to visualize different types of neurons that form synapses (connections) in the olfactory circuit.
"We couldn't count every neuron, so we did a survey," says Srinivasan. "The idea is to take samples from different areas represented, so irregularities are detected."
The new study revealed that the average number of synapses that connect each functional unit of the olfactory bulb (a glomerulus) to neurons in the piriformis cortex is invariant in all species.
"It was amazing to see how they were preserved," says Stevens.
Specifically, the identification of individual odors is linked to the strength and combination of the firing neurons in the circuit that can be compared to the music of a piano whose notes arise from the depression of several keys to create chords, or the arrangement of the letters that form the words in this page.
"Odor discrimination is based on the firing rate, the electrical pulse that travels through the axon of the neuron," says Srinivasan. "An odor, say for coffee, can cause a slow response in a neuron, while the same neuron can respond to chocolate at a faster rate."
This code used for olfaction is different from other parts of the brain.
"We demonstrate that the parameters of connectivity and the relationship between the different stages of the olfactory circuit are preserved among mammals, suggesting that evolution has used the same design for the circuit between species, but only changed the size to adapt to the niche environmental of animals” says Stevens.
In the future, Stevens plans to examine other regions of the brain for other distributed circuits whose function is based on the similar coding found in this study.
Srinivasan says he will focus on how noise or variability in odor coding determines the balance between discrimination and learning, explaining that the variability that the duo is finding in their work could be a mechanism to distinguish odors, which could be applied to improve machine learning or artificial intelligence systems.

By: Preeti Narula
Content: https://www.sciencedaily.com/releases/2019/07/190718112446.htm