Biology 202
2004 Second Web Paper
On Serendip

Of the five senses, smell is perhaps the least understood both mechanistically and behaviorally. There are many questions as to why people react differently, if at all, to certain smells. This difference in behavior may be interpreted as being due to a physical characteristic of the human body. However, it remains to be seen what is responsible for this difference in behavior, the brain or an alternative organ?

Some elementary conclusions regarding olfaction can be made using general observations, however such conclusions give little insight into the actual mechanism of olfaction and behavioral responses to smell. Regardless, they are a good starting point in exploring these issues. First, we can conclude that odors and smells are perceived in humans through a common pathway. We know this because on some basic level, all humans can agree that certain things smell. For example, we can all agree for the most part that a rose smells-we may not all agree on what a rose smells like, but it does have a scent. Along these lines we can also conclude, generally, that there are distinct odors which differ somehow in their chemical components causing them to be received differently. For example, the smell perceived from an orange can easily be identified as different from that of gasoline.

In addition to expanding and understanding further the aforementioned conclusions, this paper seeks to understand how humans can receive the same odor and behave or respond differently to it. Gasoline is one example of an odor that elicits different behaviors in different people. Many people despise the smell of gasoline saying it causes feelings of nausea, and avoid smelling gasoline as much as possible. Yet there are others who find the smell somewhat pleasant, and go out of their way to smell more of it by taking longer to pump gas or by taking deeper breaths while doing so. This particular phenomenon intrigues me. More broadly speaking, what is responsible for the behavioral response to odor?

In order to fully explore this question, a better understanding of the mechanism of olfaction is needed. Odorants are collected in the sensory epithelia of humans, located in the upper regions of the nasal cavity ((1)). Odorant molecules are absorbed in the mucus layer of the sensory epithelia where they then travel to receptor cells, which are located on the cilia that line the nasal cavity. These cilia each comprise receptor proteins that are specific to certain odorant molecules ((1)). Binding of an odorant to a receptor protein results in an activation of a second messenger pathway. There are two known pathways to date. The most common, involves the activation of the enzyme adenyl cyclase upon the binding of an odorant molecule ((2)). This enzyme catalyzes the release of cyclic AMP (cAMP). The increase in cAMP levels causes ligand-gated sodium channels to open, causing a depolarization of the membrane. This depolarization results in an action potential ((2)). These electric signals are carried by olfactory receptor neurons through the olfactory bulb. The olfactory bulb then relays information to the cerebral cortex resulting in sensory perception of smell ((2)).

On average, humans can recognize up to 10,000 separate odors ((3)), yet only have about 1,000 different olfactory receptor proteins ((4)). Clearly, there is a step in the pathway of olfaction that allows for combinations of odorant molecules to be organized. This step was found to take place in the olfactory bulb. Within this organ, the activity of different olfactory receptors in combination is used to signal the brain for specific smells ((4)). Richard Axel M.D., an investigator at Columbia University College of Physicians and Surgeons and a pioneer in the field of olfactory research, explains this processing role of the olfactory bulb best:
"The brain is essentially saying... 'I'm seeing activity in positions 1, 15, and 54 of the olfactory bulb, which correspond to odorant receptors 1, 15 and 54, so that must be jasmine ((4))."

Knowledge about the mechanism of olfaction now allows us to explore what is responsible for behavioral responses to odor. My initial answer to this question was the brain. One thing I have learned in our class discussions was that for the most part, behavior is the result of inputs and outputs from the brain and how they are processed. Accordingly, the brain should be responsible for the different behaviors observed in response to smell. However, after exploring and learning about olfaction on a more detailed level, I now believe that the source of behavioral response to odor may lie within the olfactory bulb. One role of the olfactory bulb is to receive signals from odorant receptors and relay that information to the brain. In this way the olfactory bulb is functioning to process and interpret the input signals from odorant receptors and produce corresponding output signals for the brain to subsequently interpret. It seems logical that in processing the inputs from odorant receptors, the olfactory bulb is also producing some type of output that results in a behavioral response.

Further investigation revealed evidence that may support this hypothesis. Signals sent from the olfactory bulb are sent not only to the cerebral cortex, which is responsible for conscious thought processes; but also signals the limbic system, which generates emotional feelings ((5)). This leads me to question whether the signals sent to the cortex and limbic system are identical or similar in any way? Also, is there a difference in the number of signals sent between the two locations in response to odorant reception? Meaning, do more signals get sent to the cortex when a person smells oranges, compared to the limbic system? All of these questions are worth pursuing; perhaps it is information in the signals sent to the limbic system, which is responsible for the behavioral responses to odor.

There is much about olfaction that remains unclear, particularly about the relationship between behavior and olfaction. To date, there is little evidence that suggests what portion of the body is responsible for behavioral response to odors. Further investigations involving the olfactory bulb may prove worthwhile in determining what is responsible for the behavioral response to smell.

References

1)Monell Chemical Senses Center, an overview of olfaction

2) Lancet, Doron. "Vertebrate Olfactory Reception." Ann. Rev. Neurosci. 9 (1986): 329-355.

3)The Mystery of Smell: The Vivid World of Odors

4)The Mystery of Smell: How Rats and Mice-and Probably Humans-Recognize Odors

5)Sensing Smell


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