Although we have long known that the sense of smell resides in an area of the brain called the piriformis cortex, its function was until recently an enigma. And it is that this region that we share mammals, amphibians and reptiles is, apparently, somewhat disorganized, since it appears as a tangle of neurons that form a jumbled mess.
Now, a team of scientists from the University of Salk (USA) has just shed light on the matter and reveals that this apparently random distribution of the piriform cortex is key for the brain to correctly distinguish odors and, what’s more, it succeeds in an extremely precise way.
“The current paradigm tells us that the information that reaches the brain is transmitted by neurons that are activated when receiving a stimulus,” explains Charles Stevens, one of the authors of this new work. For example, in the case of vision, a part of the eyes is activated and transmits the information to a specific part of the visual cortex which in turn carries the information to the brain. However, it seems that the olfactory system does not follow this sequence: “What matters here is not which cells are active, but how many cells are active and to what degree they are,” explains Stevens.
When aromatic molecules bind to receptors in the nose, the signal is transmitted to the olfactory bulb and from there to the piriform cortex. “Until now we had not been able to distinguish any order in the connections of the piriform cortex of any species studied,” explains Shyam Srinivasan, another of the co-authors.
To study how odor information is encoded and verify whether their connections are truly random, the team of scientists analyzed the piriformis cortex of several mice using various staining and microscopy techniques that allowed them to visualize different types of cells in this region of the brain. The first goal was to quantify the number and density of neurons , and the researchers concluded that, in mice, the pyramidal cortex contains approximately half a million cells that divide into two zones: the larger and less dense posterior cortex, and the anterior cortex, smaller and denser.
Using this information and the knowledge yielded by previous studies on the synapses – neuronal connections – of the olfactory bulb with the piriformis cortex, the researchers made a surprising finding: each of the neurons of the olfactory bulb is connected to almost all the neurons of the cortex. piriformis.
“Every cell gets information from virtually every odor receptor out there,” explains Stevens. “There is not a ‘coffee smell’ neuron and a ‘chocolate smell’ neuron, but a bunch of coffee and chocolate neurons.” That is to say: instead of having a single receptor that detects an odor and illuminates a group of neurons, each odor has a kind of fingerprint that is based more on the strength of the connections. While the smell of coffee can activate almost the same neurons in the piriform cortex as the smell of chocolate, each neuron will fire to a different degree.
” An advantage of this system is that it can encode very complex information ,” explains Srinivasan. “On the other hand, it also has a greater capacity to respond to possible interferences: if a neuron sends a signal that is louder than necessary, this noise is canceled thanks to the fact that many other neurons are sending more precise simultaneous signals.”
The results of this work have been published in the Journal of Neuroscience . Now, the scientists want to repeat the study in other animals to look for possible similarities or differences, and to investigate other areas of the brain governed by seemingly random connections to see if they are organized in the same way as the piriformis cortex.
Piriformis cortex and long-term memory
This is not the only surprising discovery related to the piriformis cortex: a few months ago, a team of German researchers revealed that this region of the brain is also involved in the storage of long-term memories through a mechanism mediated by the orbitofrontal cortex. The results of their work were published in the journal Cerebral Cortex .
References:
Srinivasan & Stevens. 2018. The distributed circuit within the piriform cortex makes odor discrimination robust. Journal of Neuroscience https://doi.org/10.1002/cne.24492
Strauch & Manahan-Vaughan. 2018. In the Piriform Cortex, the Primary Impetus for Information Encoding through Synaptic Plasticity Is Provided by Descending Rather than Ascending Olfactory Inputs. Cereb Cortex. 2018 Feb 1; 28 (2): 764-776. doi: 10.1093 / cercor / bhx315.
