HOW WE SCENT & SMELL OUR WAY
by Jeanne Rose
"Scents fit into
the scent receptors the same way that a key fits into a lock. And
the flavor of food is a combined response to two of the chemical
senses; taste and odor. The sense of odor is more sensitive even in
human beings that the sense of taste. The sense of odor is the major
contributor to the perception of flavor.
A molecule has an odor
is dependent on if it can excite and stimulate the olfactory nerve
endings inside the nose. In humans, these nerve endings occupy an
area of yellow-brown colored epithelium that is about 5 sq. cm.
(square centimeters or 1 inch X 1 inch). It is in wafting a scent
utilizing eddy currents that this area is able to perceive odor.
When one sniffs and
sniffs, it is the eddy currents, not direct blasts, that carry the
molecules to this area."
WAFT, DON'T DRAFT
is the rule of the aromatherapy enthusiast.
"The Sense of Smell is an interesting phenomenon. There are 50
million or so receptors that compose the olfactory epithelium and
these are all bare nerve endings. There is no buffer between the
outside world and these bare nerve receptor of the olfactory
epithelium. Thus the nervous system is in direct contact with the
outside world: the brain is exposed in the nose. This part of the
brain, the smell brain or reptilian brain with its exposed sense of
smell suggests that it is the oldest and most primitive of the
senses. We smell therefore we are. Smell is linked in terms of
physical closeness with one of the more primitive parts of the
brain, the limbic system which is the seat of memory and learning,
the place and home and seat of the control of emotions. We can
perceive odors therefore we are alive and in direct contact with
that which has made the human race human. This limbic system is the
seat of the control of emotions and this may be the reason that
scent can have such a powerful impact on our psyche.
Smelly molecules are
called odor vectors or osmophores (smell carrier). (Osmo = smell in
Greek and phore = to bear or to be borne). It is unknown what the
relationship is between the molecular structure of the molecules and
the sensation they create that we call 'its scent'.
An odor vector must be
volatile to reach the nose and secondly, it must be slightly soluble
in water in order to dissolve in the mucus. It is possible that
these molecules act as detergents in order to carry insoluble
molecules into the receptor sites. It must interact with a protein
molecule in the olfactory nerve endings, be able to modify its
shape, and thus stimulate the nerve cell to send a smell (or other)
message to the brain.
It is probable that the
same lock-and-key mechanism of taste operates the sense of smell. A
molecule of a particular shape can attach to a given protein
molecule so long as it matches its shape in some respect.
About 30 types of
anosmia exist, which suggests at least 30 different types of locks
that can be opened with the correct key (molecule). Only a part of a
molecule needs to fit snugly into a site to trigger a scent signal.
If it is flexible it can fit into more than one site and excite a
Lurking in the olfactory
epithelium, among the mucus-exuding cells, are cells that are part
of the system that innervates the face (trigeminal nerve). It
is suspected that pungent and putrid molecules penetrate them,
interact with their proteins, and stimulate them to fire. Thus,
there are two types of olfaction: first smell, the ordinary type for
specific odors, and second smell for nonspecific pungency and
The color of the smell area is important as well. "Found at the
upper end of each nostril, the olfactory regions are yellow, richly
moist, and full of fatty substances. We think of heredity as
ordaining (such physical characteristics as) how tall one will be,
the shape of the face, and the color of the hair. Heredity also
determines the shade of yellow of the olfactory area. The deeper the
shade, the keener and more acute the sense of smell. Albinos have a
poor sense of smell. Animals, which can smell the beatific grandeur,
have dark-yellow olfactory regions; ours (humans) are light yellow.
The fox is reddish brown, the cat's an intense mustard brown."
Thus these animals have a more pronounced ability to detect odors.
P.W. Atkins: MOLECULES; Freeman; 1987.
Ackerman, Diane, THE COLOUR OF SMELL from the Natural History of the
ROSE, Jeanne. Smell Notes from Blending Class 2000
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be used without prior permission from Jeanne Rose.
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