Smell Ability
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 mixed response.

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 putridity."

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.

Reference:
P.W. Atkins: MOLECULES; Freeman; 1987.
Ackerman, Diane, THE COLOUR OF SMELL from the Natural History of the Senses.
ROSE, Jeanne. Smell Notes from Blending Class 2000
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