The Smell of a Rose
Posted February 1, 1998
Humans can recognize about 10,000 scents, ranging from the pleasant odor of a rose to the repulsive smell of a skunk. The initial detection of these odors, which often
consist of more than one type of molecule, occurs when the molecules bind to a specialized region at the back of the nasal cavity. This region consists of specialized neurons that have
receptor proteins in their plasma membranes. The nose contains millions of these specialized neurons. Each neuron has only one kind of receptor. Each receptor binds only to a specific
part of an odor molecule. Once bound, the receptor initiates an electrical signal that travels to the cell body of the neuron and then down the axon fiber, which passes through a hole
in the skull into the olfactory bulbs of the brain. These blueberry-sized bulbs are located where the signal synapses with a second neuron that carries the signal to the olfactory cortex
in the brain.
How does the brain identify the odor? It depends on which molecules are in the odor and which receptor cells they stimulate. A rose odor may have molecules "a", "b",
and "c" which each bind to a separate neuron. Recall that each neuron contains only one type of receptor. Thus a neuron may contain numerous "A" receptors that will bind only to "a"
odor molecules from the rose scent. In the same way, "B" receptors will bind only to "b" odor molecules, and so on. Another scent such as mint may have molecules "b", "d", and "e". The
brain identifies the particular smell interpreted as "rose" or "mint" from the combination of receptors activated.
Using molecular probes that tag receptors, researchers have begun to untangle the wiring of the olfactory bulbs in rats. They have demonstrated that small areas in
the olfactory bulbs called glomeruli get signals from one kind of receptor which may be located in different areas of the nasal cavity. A sort of "filing system" occurs in the olfactory
bulbs. As the excited neuron signals enter the olfactory bulb, there is an orderly placing of signals in a particular glomerulus. Like signals are brought together. If an "a" molecule
stimulates an "A" receptor in one part of the nasal cavity and another "a" molecule stimulates an "A" receptor in a different part of the nasal cavity, both of these signals will end
up at the same glomerulus in the olfactory bulb. Thus, the jumble of neurons that leave the nose are sorted into glomeruli in the olfactory bulbs. Each receptor and each glomerulus responds
to just one type of smell molecule. The brain then receives a signal from the various activated glomeruli and interprets the smell. The next mystery to be unraveled is to map the pathway
between the glomeruli and the olfactory cortex where the odor is interpreted in the brain.
References
Richardson, S. "The Smell Files," Discover, August 1995, pp. 30-31.
Axel, R. "The Molecular Logic of Smell," Scientific American, Vol. 274:10, October 1995, pp. 154-159.
Brownlee, S. and R. Watson. "The Senses," U. S. News and World Report, January 31, 1997, pp. 51-59.
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