Here is an ABC article of a few weeks ago, which reports on our underrated sense
of smell.
It seems that at its most basic molecular level, the sense of smell might be just as
irreducibly complex as the sense of sight:
"It used to be that the olfactory system was a second-rate place to do
neuroscience," says Chuck Stevens, a neuroscientist at the Salk Institute in
La Jolla, Calif. "But that's changed in the last five years because the
molecular biologists have figured out what the receptors are like."...
Receptors, proteins embedded in the nose's neurons, latch onto bits of the
object you're smelling, like a key fitting into a lock...Writing in the journal
Cell, the researchers report that each receptor is like a key on a typewriter,
and each molecule types on several letters to produce a word, the odor.
"Just like letters are used over and over to define different words, each
receptor is used over and over to define an odor," Buck said in a statement.
The "words" are then sent to the olfactory bulb, a pine nut-sized part of the
brain right above the nose, where the words are turned into olfactory
sentences."
Reading this I am reminded of something Denton wrote about the "complexity and
ingenuity... the sheer brilliance apparent in the design of so many physiological
adaptations...that they appear to represent the very epitome of perfection":
"It is easy to sympathize with Darwin. Such feelings have probably occurred to
most biologists at times, for to common sense it does indeed appear absurd to
propose that chance could have thrown together devices of such complexity and
ingenuity that they appear to represent the very epitome of perfection. There can
hardly be a student of human physiology who has not on occasion been struck by
the sheer brilliance apparent in the design of so many physiological adaptations.
Like, for example, in the elegance manifest in the design of the mammalian kidney
which combines so many wonderfully clever adaptations to achieve water and salt
homeostasis and the control of blood pressure while at the same time
concentrating and eliminating from the body urea, the main end product of
nitrogen metabolism. Or like the choice of the bicarbonate buffer system as the
body's main defence against the accumulation of metabolic acids." (Denton M.J.,
"Evolution: A Theory in Crisis", 1985, p326)
Steve
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http://abcnews.go.com/sections/science/DailyNews/smel1990823.html
ABCNEWS
Sniff, What's that Odor?
Researchers Watch the Brain Smell
Different concentrations of a chemical can trigger very different responses in
the brain. Click to see how that can happen.
(ABCNEWS.com)
By Susan Conova
ABCNEWS.com
Most creatures that wiggle, slither, swim, walk or just plain sit survive by their
noses. Mother rats can identify their babies by smell, oysters use smell to get
lucky, and even lowly bacteria track down food via scent.
Though smell is our oldest sense, it remains the most mysterious.
"It used to be that the olfactory system was a second-rate place to do
neuroscience," says Chuck Stevens, a neuroscientist at the Salk Institute in
La Jolla, Calif. "But that's changed in the last five years because the
molecular biologists have figured out what the receptors are like."
How You Smell
Receptors, proteins embedded in the nose's neurons, latch onto bits of the
object you're smelling, like a key fitting into a lock. When you smell the
fragrance of a flower, bits of the flower are going up your nose. When you
smell garbage, same thing.
But fundamental questions remain. Like, how come the smell of mothballs
conjures up memories of childhood visits to Grandma's house?
Scientists struggling to answer these questions start with smaller ones.
One such mystery: How can one molecule cause two smells?
Take camphor. Isolated from the bark of the camphor tree, camphor, an
ingredient in products including insect repellents and Vicks VapoRub, usually
smells woody. But a funny thing happens when the camphor molecules thin
out. The odor changes - to something more like urine.
Photos of a rat's olfactory bulb responding to amyl acetate, a chemical that
smells like bananas. At low concentrations (top), two parts of the bulb,
indicated by the dark regions, recognize the chemical. At higher
concentrations (bottom), a third region also detects the odor. That can
completely change the smell perceived by the brain. (Howard Hughes Medical
Institute)
Don't blame camphor; it hasn't changed at all. Rather, it is the nose's odor
receptors that cause the distress.
Linda Buck, a neuroscientist at the Howard Hughes Medical Institute at
Harvard Medical School, and colleagues recently found that a variety of odor
receptors can capture a single type of molecule.
If the concentration of camphor is low, only a few of these receptors catch
the molecules, and they spark a signal to the brain that says, "Eeew, what
peed on my sofa?" When camphor concentration increases, additional
receptors get into the act and change the signal to, "Aahh, smells like a
forest."
Nose Like a Typewriter
Writing in the journal Cell, the researchers report that each receptor is like a
key on a typewriter, and each molecule types on several letters to produce a
word, the odor.
"Just like letters are used over and over to define different words, each
receptor is used over and over to define an odor," Buck said in a statement.
The "words" are then sent to the olfactory bulb, a pine nut-sized part of the
brain right above the nose, where the words are turned into olfactory
sentences. Researchers can't directly intercept the neural messages being
sent from nose to brain, but they can take pictures of them.
Duke University researchers, reporting in the journal Neuron, removed the
bone over the olfactory bulb in a rat then pointed a high-resolution video
camera at the bulb.
When part of the bulb is actively analyzing an odor, it draws more oxygen
and blood flow, making it appear darker and redder than surrounding inactive
parts.
The images of the pine nut-sized organ are startling, even in black and
white. Three seconds after the rat smells the odor, a pattern of dark dots
appears on the video, indicating which parts of the olfactory bulb are
analyzing the odor.
Different concentrations of the same odor molecule, or different molecules
that vary in structure by as little as a single atom, produce unique pictures of
dots.
Patterns of Odors
Information about these dots is sent to the brain's cortex, where smells are
distinguished and identified based on the pattern of neuron firings in the
olfactory bulb.
"One of the big questions in neurobiology is how information is coded in
the brain," says Stevens. "We smell things, and those things are represented
in the brain with the firing of neurons. This paper doesn't answer the question
but it's a big step forward. It tells you what information is coming to the
olfactory cortex. The next step is to figure out how that information is sorted
out in the cortex."
"Part of why it's exciting to us is the doors it opens," adds lead author Ben
Rubin, a graduate student at Duke.
One of these doors may lead to understanding why the smell of tequila,
years after a bad hangover, still makes people sick. "We learn many
memories like that," says Larry Katz, a neuroscientist at the Duke University
Medical Center. "We're looking at how these associations change [by looking
at] how the [smell] pattern changes."
And then maybe scientists will answer the most perplexing question of all:
Why does Grandma use so many mothballs in the first place?
SEARCH ABCNEWS.com FOR MORE ON
SUMMARY
Scientists have taken pictures that show the smelling part of the brain in
action.
[...]
"We smell things, and those things are represented in the brain with the firing
of neurons."
Chuck Stevens, Salk Institute
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"It is true that both genuine homologous resemblance, that is, where
phenomenon has a clear genetic and embryological basis (which as we have
seen above is far less common than is often presumed), and the hierarchic
patterns of class relationships are suggestive of some kind of theory of
descent. But neither tell us anything about how the descent or evolution
might have occurred, as to whether the process was gradual or sudden, or
as to whether the causal mechanism was Darwinian, Lamarckian, vitalistic
or even creationist. Such a theory of descent is therefore devoid of any
significant meaning and equally compatible with almost any philosophy of
nature." (Denton M.J., "Evolution: A Theory in Crisis", Burnett Books:
London, 1985, pp154-155)
Stephen E. Jones | sejones@iinet.net.au | http://www.iinet.net.au/~sejones
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