Biological complexity: insect smell receptors (Introduction)

by David Turell , Monday, June 21, 2021, 20:33 (1041 days ago) @ David Turell

Very complex but partially described:

https://www.quantamagazine.org/secret-workings-of-smell-receptors-revealed-for-first-ti...

"Smell, rather than sight, reigns as the supreme sense for most animals. It allows them to find food, avoid danger and attract mates; it dominates their perceptions and guides their behavior; it dictates how they interpret and respond to the deluge of sensory information all around them.

***

"Yet olfaction might also be the least well understood of our senses, in part because of the complexity of the inputs it must reckon with. What we might label as a single odor — the smell of coffee in the morning, of wet grass after a summer storm, of shampoo or perfume — is often a mixture of hundreds of types of chemicals. For an animal to detect and discriminate between the many scents that are key to its survival, the limited repertoire of receptors on its olfactory sensory neurons must somehow recognize a vast number of compounds. So an individual receptor has to be able to respond to many diverse, seemingly unrelated odor molecules.

***

"In insects, olfactory receptors are ion channels that activate when an odor molecule binds to them. They may be the largest and most divergent family of ion channels in nature, with millions of variants across the world’s insect species. And so they must carefully balance generality against specificity, staying flexible enough to detect an enormous number of potential odors while being selective enough to reliably recognize the important ones, which could differ considerably from one species or environment to another.

***

"A receptor built around a single binding pocket, with a response profile that can be retuned by the smallest of tweaks, could speed up evolution by freeing it to explore a broad spectrum of chemical repertoires.

"The architecture of the receptor also supported this view. Ruta and her colleagues found that it consisted of four protein subunits loosely bound at the channel’s central pore, like the petals of a flower. Only the central region needed to be conserved as the receptor diversified and evolved; the genetic sequences governing the rest of the receptor units were less constrained. This structural organization meant the receptor could accommodate a wide degree of diversification.

"Such light evolutionary constraints at the receptor level probably impose substantial selective pressure downstream on the neural circuits for olfaction: Nervous systems need good mechanisms for decoding the messy patterns of receptor activity. “Effectively, olfactory systems have evolved to take arbitrary patterns of receptor activation and endow them with meaning through learning and experience,” Ruta said. (my bold)

"Intriguingly, though, nervous systems don’t seem to be making the problem easier for themselves. Scientists had widely supposed that all the receptors on an individual olfactory neuron were of the same class, and that neurons for different classes went to segregated processing regions of the brain. In a pair of preprints posted last November, however, researchers reported that in both flies and mosquitoes, individual olfactory neurons express multiple classes of receptors. “Which is really surprising, and would increase the diversity of sensory perception even more,” Barber said.

"The findings from Ruta’s team are far from the last word on how olfactory receptors work. Insects use many other classes of ion channel olfactory receptors, including ones that are much more complex and much more specific than those of the jumping bristletail. In mammals, the olfactory receptor is not even an ion channel; it belongs to an entirely different family of proteins."

Comment: Amazingly complex as suggested. Note my bold. Odors are learned over time. Literally these receptors fondle the shape of the protein molecules they receive and gradually learn to understand what teh odor means and signifies.