If you know even a little about the olfactory system, you get the general idea that at the very beginning of any perceptible smell are numerous events in which odorous molecules bind to receptor proteins. This ultimately results in a neural signal being transmitted into the brain via the olfactory bulb. This was my starting point, too, but I decided it would be interesting to know how all that actually happened.
After a deep dive through the science that took several weeks and required the creation of the diagram included here, I finally got a grip on the basics. And like almost everything about our senses, it’s incredibly complex. The G protein-coupled receptors presented here are the basis of most of our olfactory receptors, but they can’t act alone. They require the assistance of a set of chemical players called “second messengers,” plus five types of ion channels that act as gates that selectively admit positively charged cations: calcium, sodium, potassium, and uniquely in olfaction, expel negative anion, chloride. The entire sequence is called transduction. After it’s done, there’s another sequence of events that sets the receptors back to their resting state, ready for their next binding event.
The point of all this complexity is to amplify the tiniest little twitch by the underbelly of the receptor molecule into a whole-cell chemical spasm called depolarization, leading to neurotransmitters being released to nearby nerves. To do that takes a lot of amplification, as you can imagine that a single molecule nestling into a binding “pocket” for less than a millisecond is a vanishingly small event. But inside the cell, these second messengers act just like an electrical amplifier circuit: a small change in status of whatever is regulating the gating mechanism—as one leg of a transistor does—leads to a much bigger flow through the gate. Just like that amplifier, this occurs in several stages within our olfactory neurons, as you can see from the diagram.
To widen out a bit, olfactory receptors are located on tiny hairlike extensions of olfactory neurons called cilia, poking out of a membrane near the top of our nasal cavity, and bathed in mucus. Most of our olfactory receptors are GPCRs, or G protein-coupled receptors, an ancient and widespread family of receptors serving many functions in vertebrates. The G proteins themselves cluster in groups of three (“trimers”) at the roots of the receptor proteins. They are the mechanism that starts the signaling cascade inside the cell. One action leads to another, then another and so on until you have three stages of amplification.
Olfactory transduction’s use of chloride ions makes it different from most GPCRs. This keeps things running smoothly in the highly unstable environment of the threadlike cilia, greatly enhances sensitivity and helps prevent adaptation: reduction of signaling with continued stimulation that commonly occurs in other types of sensory receptors.
Because this is all occurring at the molecular scale, things happen blazingly fast and in massive quantities. As many as 35 identical receptor binding events for one odorant have to take place within 50 milliseconds to create any kind of response.
I included a very similar diagram of a bitter receptor cell in the book, but we decided we didn’t have room for this one, since many of the mechanics of this molecular pinball machine are quite similar.
So what are the takeaways here? First is the sheer wonder of this system that has served our vertebrate ancestors for hundreds of millions of years. With our human insecurities around the sense of smell, it’s reassuring to know we have such an exquisitely sensitive instrument working for us. As a practical matter, it’s informative that smell, unlike most senses, has no cellular mechanism for adaptation. Instead, it relies on habituation—a neural process that happens on many different timescales.
Key to the Illustration
1) We start at the left with the receptor protein in a resting state.
BINDING AND TRANSDUCTION:
2a) the odorant/ligand binds to the receptor, 2b) changing the receptors shape by spreading its helices, opening the protective cocoon around the molecule of guanosine diphosphate (GDP), which becomes unstable; 2c) Instantly, the GDP is swapped for guanosine triphosphate (GTP); 2d) allowing the G-protein trimer to split into two chunks.
3) One of those G proteins, G-alpha-olf, activates an enzyme called AC3, which by its production of cAMP, opens the CNG ion channel, allowing sodium and calcium ions to trickle in. The calcium influx opens a chloride channel, TMEM16B, allows chloride to flow out of the cell.
4) This loss of chloride initiates cell depolarization: a wave of electrical activity that propagates along the length of the cell. When this begins, sodium (Nav1.7) and calcium (Cav2.1 or Cav2.2) channels open, and these ions boost the wave of depolarization to a higher level. If you’re wondering where all the chloride comes from, its level is maintained by yet another ion channel (not shown).
5) The influx of calcium causes glutamate-containing vesicles to merge into the cell wall and open up to the outside, spilling their neurotransmitter contents and signaling to the next neurons in the chain, located in the olfactory bulb.
SHUTDOWN AND RESET:
6) The recent influx of sodium opens a potassium channel (KvA), admitting potassium that ends the activity by repolarizing the cellular membrane.
7) A protein called calmodulin blocks the CNG calcium-specific ion channel, shutting it down. The lack of calcium shuts off the TMEM16B chloride channel, ending the outward flow of CL– ions.
8) The odorant, no longer useful, departs from the binding pocket. Two types of receptor enzymes (kinases) turn GTP back to GDP, making the receptor attractive to a protein called arrestin.
9) The arrestin quickly fits into the slot once occupied by the G-protein trimer, inactivating the receptor. If the receptor is damaged or in poor condition, it enters the cell’s interior for recycling.
10) If deemed fit for continued duty, the G-protein trimer swaps places with the arrestin and the receptor is ready for the next odor.
References for text and visuals:
Colten K. Lankford, “A Comparison of the Primary Sensory Neurons Used in Olfaction and Vision,” Frontiers in Cellular Neuroscience 14 (2020), https://doi.org/10.3389/fncel.2020.595523.
Elizabeth A. Corey, “Inhibitory signaling in mammalian olfactory transduction potentially mediated by Gαo,” Molecular and Cellular Neuroscience 110, January (2021): 103585, https://doi.org/10.1016/j.mcn.2020.103585.
Rong-Chang Li, “Cyclic-nucleotide–gated cation current and Ca2+-activated Cl current elicited by odorant in vertebrate olfactory receptor neurons,” PNAS 113, No. 40 (2016): 11078-11087, https://doi.org/10.1073/pnas.1613891113.