Originally published 5 November 1990
An old vaudeville joke goes like this:
“I’ve got a goat without a nose.”
“You’ve got a goat without a nose? How does it smell?”
“Awful.”
OK, go ahead and laugh, but I don’t think it’s funny. I’m the goat without a nose. Born without a sense of smell. Never smelled a rose. Never smelled a piney forest. Never smelled a goat.
A few years ago National Geographic Magazine conducted a smell survey among its readers. Six scratch-‘n‑sniff scents were provided in an issue of the magazine with an invitation for readers to report what their noses detected. About 1 out of 100 respondents reported no odors at all, an affliction called total anosmia. That’s me.
Maybe its because I can’t smell that I keep an eye out for olfactory research in the scientific literature. There’s not much. The aural and visual senses get lots more attention from the psychologists of perception, just as they get more attention from novelists and poets. Smell is apparently not terribly important to humans, maybe because our noses are so far from the ground. If hound dogs and foxes produced poets and psychologists, you can bet their literature would be full of olfaction.
A bump in the brain
Only neurologists and brain researchers are inclined to give smell its due. It turns out that the olfactory pathway is a bee-line to the brain. Between the nerve endings in the nose that detect scents and the core of the brain there is only a single synapse, or place where nerve cells join, in a bump at the front of the brain called the olfactory bulb.
In fact, the 10 million or so odor detectors in the nose are bare nerve endings, unlike the other senses where there is a kind of membrane or window pane between the transmitting nerves and the outside world. In a real sense, the inside of the nose is naked brain.
The other end of the olfactory pathway, deep in the brain, is in close contact with the limbic system, the part of the brain involved with emotions and memory, which in turn connects to the glands that control sex, appetite and so on. All of this elaborate sniffing circuitry is apparently very ancient from an evolutionary point of view, but more or less expendable for humans. A moth needs the sense of smell to find a mate; humans can manage without it.
Odors are just volatile molecules floating in the air, and — as the old vaudeville joke implies — we emit them as well as detect them. Several hundred kinds of molecules have been detected in the gases exuded by the human body. Most of this stuff, like nitrogen and carbon dioxide, is odorless. Some of it (ammonia) is pungent, and some of it (hydrogen sulfide) is very much less than pleasant.
No one is completely sure how odor molecules activate olfactory nerves, but it seems likely it’s mostly a matter a shape. If a molecule has the right shape to fit the shape of the detector, bingo, odor. A few dozen types of partial anosmia — specific odor blindness — have been detected in humans, so it seems we must have about that many different shaped keyholes in the nose just waiting for the right key. A zig-zag-shaped keyhole detects a fatty smell because that’s the shape of fat molecules. Camphor keyholes are star-shaped, jasmine keyholes are shaped like wriggly worms, and so on.
The chemical composition of odor molecules seems less important than their shape. A carvone molecule with a right-handed twist smells of spearmint. A carvone molecule with a left-handed twist smells of lemon. Same atoms, same molecules, different shaped key.
I sometimes think about all those tens of millions of molecular keys clicking away in my nose, but nothing opens. No feelings of lust, appetite, annoyance, nostalgia. Nothing. And the doctors can’t tell me why. I suspect the problem is in that relay station at the front of the brain, the olfactory bulb, probably damaged in the trauma of birth.
Salamander sensations
A few years ago John Kauer of the Tufts Medical School published an article in Nature that I read with keen interest. Kauer made videos of activity in the olfactory bulb. He took live salamanders and coated their surgically-exposed olfactory bulbs with a voltage-sensitive dye. Then he electrically stimulated the salamander’s olfactory nerve and filmed the olfactory bulb with a video camera as the dyes reacted to electrical activity.
In a sequence of color-coded frames we see the olfactory bulb in action. Following the stimulus, a little firestorm of activity spreads through the salamander’s olfactory bulb, igniting and subsiding. This flare of activity flashes on Kauer’s TV screen. In a thousandth of a second it’s all over, presumably having evoked in the salamander’s brain some peculiarly salamanderish appetite or sensation.
If my guess is right, that’s where my own sniffer goes awry. I would love to invite Kauer to apply his video technique to my olfactory bulb, so that I might watch as that incipient fire storm of signal processing is snuffed out by faulty circuits, circuits that were probably irreversibly damaged at the very moment I entered into the world of smells.
