October 6, 2003



Do you know anyone who always smells like very strong perfume or cologne? Do you think they can smell how strong it is? Have you ever noticed that smells seem to disappear after you have been exposed to them for a while? For example, perfumes and colognes that you put on in the morning may smell quite strong to you at first, but:

These are some of the questions you will be exploring during this activity.

Background Information

Olfaction refers to the sense of smell, which has similarities in all terrestrial and many aquatic vertebrates. The mechanisms that control olfaction are divided into distinct regions. The olfactory epithelium, located in the roof of each nasal cavity in humans, is the organ of smell.

Special olfactory receptor cells, numbering about 25 million, make up the bulk of this epithelium. Smells, in the form of indi­vidual molecules, are bound to receptor molecules in the membranes of the cilia that extend into the fluid mucus layer that coats the epithelium. The membranes of the nose secrete these fluids to keep the molecules in solution to facilitate smell.

 Axons from the receptor cells lead to and synapse with the olfactory bulb that lies just under the frontal lobe of the brain. Special cells in the bulb are activated by smell stimuli and carry impulses toward the olfactory tubercle, where the impulse is sent to the limbic system, thalamus, and cortex. In evolutionary terms, the limbic system is one of the oldest parts of the brain. It contains such structures as the hippocampus, amygdala, fornix, and mammillary bodies, and is extremely important to the emotional aspects of our experiences. This fact explains why smells often evoke emotional memo­ries. Other fibers terminate in the olfactory cortex at the front of the cerebrum and are interpreted as the tens of thousands of chemical scents that humans are capable of identifying.

 Exactly how smells are perceived, stored in memory, and then recalled many years later is not yet fully understood. Much of what we know about olfac­tion has been learned from animal studies that may or may not be applicable to olfaction in humans. It is thought that the memory of odors is important for several survival functions such as avoiding danger, seeking food, fight­ing, and mating. Memory of odors is thought to be a function of both the limbic structures of the brain and the cerebral cortex.

Damage to the olfactory receptors or nerves, obstruction of the air passages, and either permanent or transient chemical interference with olfactory receptors may result in the permanent or temporary loss of olfaction. The general term used to describe the loss of all ability to smell is anosmia, while specific anosmia refers to the loss of olfaction for one or a few related com­pounds. Other common terms related to olfaction are hyposmia, a mild loss of olfactory sensitivity, such as when one experiences a head cold; and hyperosmia that occurs when one is overly sensitive to some or all smells. Among individuals with normal olfaction, sensitivity can vary a thousand­fold (Dodd & Castellucci, 1991).

Olfaction is reported to be linked to the endocrine system (Engen, 1982). A classic example that links the loss of olfaction to the endocrine system is Kallman's syndrome. The syndrome is inherited through an autosomal dominant gene with incomplete expressivity and affects mainly men. Kallman's appears to be related to problems in the hypothalamus, the brain center essential to normal production of the sex hormones and the reproduc­tive cycle. Individuals with this syndrome have poorly developed genitals (hypothalamic hypogonadism) and usually exhibit anosmia. Magnetic resonance imaging (MRI) studies indicate that the "interruption or total absence of the olfactory sulcus is a primary defining characteristic of the disorder" (Klingmuller et al., 1987).  The olfactory sulcus is necessary for the detection of odor. When missing or interrupted, anosmia occurs.

Other olfactory dysfunctions are reported to occur with the following dis­eases:

Parkinson's  disease ‑ Studies by Ansari and Johnson in 1975 showed deficits in the ability to smell in individuals with Parkinson's disease. This was reinforced by the study of Ward et al. in 1983 that showed decreased ability to smell in their patients. In one test, 36 out of 72 patients could not smell coffee. Doty et al. in 1988 confirmed deficits in odor detection by Parkinson's patients.

Addison's disease.‑ Henken and Bartter's study in 1966 showed that individuals with Addison's disease may be as much as 100,000 times more sensitive to odors than individuals without the disease due to their adrenal insufficiency. In addition to the detection of smell, the trigeminal (V cranial) nerve endings in the nasal cavities are stimulated by some irritating or painful chemical stimulants. Stimulation of the trigeminal receptors elicits some of the strongest physiological responses in the body. A question for students might be, "Can you think of a reason why trigeminal nasal sensations may be helpful for detecting some types of chemicals?" An answer to the student question would be that this response helps us avoid injury from noxious chemicals such as acids, and perhaps acrid smoke. In a study done by Cain and Murphy in 1980 on individuals with normal trigeminal nerves, "the degree of irritation was found to increase with repeated inhalation." This response differs from olfactory fatigue and may be explained by the fact that the trigeminal receptors are deeper in the skin than the olfactory receptors.

Olfactory fatigue can be explained partially by phenomena occurring within actively working nerves. When a series of stimuli of similar strength bom­bard nerve receptors, the nerves become accustomed to the stimulus. This phenomenon appears to happen because the rate of change within the nerve's membrane is inadequate to keep up with continuous stimulation. Examples of olfactory fatigue in everyday life are the smoking odors in a person's house or on his/her clothing that go unnoticed by the smoker, but are detected easily by the nonsmoker. Explanation of the exact biochemical basis for these changes is beyond the scope of this activity.

Fatigue of sensory receptors can be demonstrated easily with other senses, such as touch. Our clothing is in constant contact with the nerves of our skin that respond to the stimulus of touch, but people rarely think con­sciously about their clothing being in contact with their skin. When we put on different clothes, the level of stimulus changes and we are more aware of our clothing next to our bodies. The senses of taste, hearing, vision, and smell respond similarly. The first taste of a food or drink is the most acute and dulls within several bites. We are aware of music when we first turn on the radio, and then it tends to blend into the background until there is a news bulletin. A person becomes accustomed to the smell of a new car until someone reminds him/her of the smell, and the individual becomes aware of it once more.

Lab Protocol

This laboratory activity demonstrates the concept of olfactory fatigue and the relationship between smell and memories using aromatic oils.

1.       Work in pairs: One person will act as the subject while the other will keep track of time using a lab timer or stopwatch and record the data.

2.       On the lab bench you will bottles of two aromatic oils, vanilla and cloves. For each aromatic oil, there are several different concentrations.

3.       The subject should close the left nostril by pressing his/her left index finger against the outside of the nostril. The timekeeper should indicate to the subject to begin smelling the oil through the open nostril. The data recorder should record this time as the starting time in his/her data journal. The subject should keep the oil at a consistent distance (about 30 cm) from his/ her nose with his/her mouth closed.

4.       The subject should continue to smell the oil until the odor is no longer noticeable. At this time, he/she should indicate this to the timekeeper and the data recorder should record this as the ending time.

5.       Steps 3 and 4 should be repeated immediately with the same subject using different aromatic oil.

6.       After completing Steps 3 and 4 for the different aromatic oil, the subject should release the closed nostril and waft the scent of the second aro­matic oil under the newly opened nostril and indicate if it is difficult to detect the odor. These observations should be recorded.

7.       Repeat this process with each of the different concentrations.

In small groups, try to develop explanations for what occurred in the laboratory activity. Consider the following questions about olfactory fatigue:

          How long did it take before you could no longer smell the first aro­matic oil?

          Could you smell the second oil when it was wafted to your nose immediately after the first oil?

          How do the fatigue times of the two odors compare when you smell one immediately after the other?

          Did opening the second nostril allow you to smell the aromatic oil? What might account for your observations?

          Explain what happened to the smell over a period of time.

          Did the smell remind you of anything?

          Give examples of smells that go unnoticed by some individuals, but that are detected easily by others, such as the smell of cooking oil on a fast‑food restaurant worker's uniform.


Next, try to build on your data to extend the experi­ment and learn more about olfaction. Talk with other lab groups about their results. Are there any trends you see? Work with your lab partner to design and conduct an experiment of your own design and analyze then analyze your data. Afterwards, each group should share its results with other members of the class. Sug­gested experiments you may wish to investigate include the following:

          Does gender affect the ability to detect odor?

          At what level of concentration are certain common aromatic oils detectable?

          Does time of exposure until fatigue vary by gender?

          Does a certain odor cause the recall of the same memories in all individuals?