Smell is the oldest and most basic sense. Smell is the detection of external chemicals, which bacteria, without even having neurons (because they are one-celled), are able to do with ease. Taste is a mere spin-off of smell, as it is also about detection of chemicals, just those in a little higher concentrations a little closer to the body.
A new paper by Shah and colleagues blurs the already fuzzy line between small, taste, and even nociception (detection of tissue damaging stimuli). They examined skin cells in the interior of the throats and lungs of humans. These cells are not neurons. There are many examples of skin (epithelial) cells generating electrical activity (“skin pulses”) that resembles the action potentials of neurons, however.
The skin cells on inside of the airway have lots of little hairlike cilia (pictured), beating away to keep nasty bits out. Traditionally, it’s been thought these just beat, but some cilia have sensory jobs, and the authors decided to see if these might do both.
This project should the advantage of high-throughout molecular techniques. They were able to look for a whack of genes for bitter taste receptors in one go, using microarrays: little chips with dots that “light up” if the right molecule is present. They found four bitter receptors being expressed in these ciliated skin cells.
It’s not fair to the authors to say it was all downhill from there, as there was clearly a lot of hard work, but I think the authors had a pretty easy time conceptually from here on out.
You’ve got receptors in the cells: where are they, specifically? With antibodies, you can show they’re found in the cilia.
You got receptors: are they physiologically active? By put on a bitter chemical on these cells, you can see all kinds of calcium, which is important in cell signaling, running into these skin cells (using fluorescent molecules that detect influxes of calcium ions).
You’ve got a physiological change inside the cell: does that change the beating of the cilia? Yes, you can increase the beating about 25%. I’m guessing this was just detected with regular old optical microscopes.
The last logical question is the one that the paper doesn’t have data to answer directly. If you get one of those bitter chemicals in your lungs, how does the beating cilia get rid of it? This is one question that can’t be answered with the cultured cells that were used for all the experiments above.
Shah, A., Ben-Shahar, Y., Moninger, T., Kline, J., & Welsh, M. (2009). Motile Cilia of Human Airway Epithelia Are Chemosensory Science, 325 (5944), 1131-1134 DOI: 10.1126/science.1173869
Picture from here.