THE SCIENCE OF TASTE AND SMELL
2.8 OLFACTION, THE OTHER WAY TO TASTE:
BASICS OF SIGNAL TRANSDUCTION
Most of us remember a time when we had a serious head cold or sinus infection. The familiar taste of everything from our coffee to our favorite comfort food was wrong or simply missing. This is not because colds and sinus infections alter our taste buds;
this is due to the fact that much of what we consider our sense of taste actually comes from olfaction.
A human’s ability to smell chemicals is better developed than our ability to taste chemicals. It is estimated that humans can smell more than 400,000 different sub-stances with the great majority being unpleasant smells (80%). We smell with receptor cells located in the olfactory epithelium located in the superior portion of the nasal cavity with projections through the cribriform plate to the olfactory bulb (Fig. 2.2). The olfactory receptor cells both structurally and functionally are different from taste receptor cells. Olfactory receptors are modified neurons, not modified epi-thelial cells. The key difference here is that olfactory receptors have their own axons that send electrical signals to the brain. They do not have to release neurotransmitter to stimulate the nerves to the brain.
The olfactory epithelium consists of olfactory receptor cells that have cilia, which is the location of the receptor proteins for smell. These cilia protrude into a mucus layer covering the epithelium. Support cells are also present that help maintain an appropriate environment for the receptor cells including the production of mucus.
Finally, there are basal cells that are responsible for the production of new receptor cells. Basal cells are needed because the olfactory epithelial cells have a life span of 4–8 weeks and thus need to be replaced regularly.
The surface area of the human olfactory epithelium is approximately 10 cm2 and, while not very large, it is sufficient to allow us to detect odorants at concentrations of only a few parts per trillion. This is equivalent of being able to find one specific drop of water in an Olympic size swimming pool. It is the size of the olfactory epi-thelium and the number of receptor cells that determine our olfactory acuity. For example, we all have heard of tracking dogs that can detect the scent of someone who had passed by a location hours earlier. The surface area of the olfactory epithelium of most dogs is greater than 170 cm2, more than 17 times larger than
that of a human. Additionally, dogs have approximately 100 times more receptor cells per cm2 than humans. For dogs with the highest sense of olfactory acuity like the bloodhound, this means a 10,000,000‐fold greater olfactory acuity than an average human.
With the human’s ability to smell over 400,000 chemical compounds, you might think that signal transduction in the olfactory system would be quite complicated.
It is not. The variation in olfactory sensory transduction comes in the number of odorant receptors and not the intracellular signaling process used. Humans have over 350 genes that code for odorant receptor proteins. The extracellular binding domains of these odorant receptor proteins have odorant binding sites with each receptor type being subtly different from the others. Each odorant receptor cell expresses a single type of odorant receptor protein, and each type of receptor protein binds to a specific type of odorant. This means that the activation of each type of receptor cell corresponds to a specific odor.
Odorant receptor proteins function as GPCRs similar to those involved with sweet, umami, and bitter tastes. When you inhale, odorant molecules dissolve in the mucus of the olfactory epithelium. The odorant diffuses through the mucus to the surface of the olfactory receptor cilia and find an odorant receptor. The sequence of events from that point on is the same for all odorants (Fig. 2.18).
The steps involved in olfactory sensory transduction are:
1. The odorant binds to a specific olfactory receptor protein in the plasma mem-brane of the cilia of an olfactory epithelial cell.
2. The receptor that is a GPCR stimulates the activation of GOlf, the heterotrimeric G protein that functions in the olfactory system.
3. The GOlfα subunit releases GDP and binds to GTP in the activation process.
4. α‐GTP dissociates from the βγ subunits.
5. α‐GTP binds to and activates the enzyme adenylate cyclase.
6. Adenylate cyclase converts ATP to the second messenger cAMP.
7. cAMP binds to the cAMP‐dependent cation channel causing the channel to open.
8. The cAMP‐dependent cation channel is a nonspecific channel that changes membrane permeability to Na+ and Ca2+.
9. The net current flow of Na+ and Ca2+ entering the cell leads to membrane depolarization and an increase in Ca2+ concentration.
10. The increase in Ca2+ opens a Ca2+‐dependent Cl− channel. This allows Cl− to leave the cell causing an even greater depolarization.
11. The olfactory receptor cells are modified neurons. Thus when the membrane depolarization reaches threshold, the receptor cells fire an action potential that runs into the olfactory bulb then to the brain.
12. Olfactory transduction ends when the odorant diffuses away from the protein receptor and is broken down by enzymes in the mucus. The cAMP production in the cell stops, and the existing cAMP is broken down.
Influx of sodium and efflux chloride ions results in a depolarization sending a signal through the cilia axon to the olfactory body Adenylate
cyclase GTP GTP
GPCR receptor Olfactory epithelium
Cilia
GDP GDP
Golf G protein - GDP bound “inactive”
ATP cAMPcAMP
+ +
channel channel -gated Cl–
Cl–
Cilia membrane
Na+/Ca2+
Na+
Ca2+
Ca2+
odorants
To olfactory bulb
Mucus layer
FIGURE 2.18 Sensory signaling in olfactory epithelium. (a) Odorant compounds bind to receptors in the nasal passage activating neural signals to the olfactory bulb. (b) Odorants bind to specific classes of GPCRs initiating a sig-naling cascade leading to a depolarization of membrane potential.
BOx 2.3 CAN YOU TASTE THAT SMELL?
As presented earlier, the entire experience most of us refer to as taste is better described as flavor. We also know that flavor preferences are very personal and diverse. I know several people to whom having chocolate or strawberry ice cream instead of vanilla is a walk on the wild side. I also know many people who “use both spices on their food, salt and pepper.” On the other end of the spectrum, I know many people who cannot eat chili without adding extra Tabasco sauce first;
and of course, there are those people who stand in line for the opportunity to sign a waiver to try the newest super atomic hot sauce as the local barbecue rib festival.
The flavors we like are very personalized.
To demonstrate the relationship between taste and smell, you can do a simple experiment using hard candies. Can you tell whether a candy is sweet or sour when your eyes are closed and nose plugged? Can you determine whether a candy is lemon, lime, or apple with your eyes closed and nose plugged? What happens if you hold your breath instead of plugging your nose?
Materials
• Five pieces of hard candy that have the same shape and size. Jolly Ranchers are one example.
• A partner.
Process
• The two partners will take turns being the test subject and the experimenter.
• Each partner will taste two candies.
Experiment 1
• The test subject will keep their eyes closed throughout the experimental process.
• The test subject will have to plug their nose for this first experiment.
• The experimenter will open a piece of candy, making sure that the test subject does not see the color of the candy or label of the wrapper.
• The experimenter will give the candy to the test subject.
• The test subject puts the candy in their mouth, closes their mouth, and tastes the candy with their nose plugged.
• The experimenter should ask whether the subject can tell: (yes this happens with your eyes closed and nose plugged).
• Is the candy sweet or sour?
• What is the candy flavor?
• Next the test subject should stop plugging their nose but keep their mouth closed.
• What is the flavor of the candy?
• How were you able to tell the flavor?
• What happens if you plug your nose again?