It's Right Under Our Noses: The Importance of Smell to Science and our Lives

Biology 103
2000 Third Web Report
On Serendip

It's Right Under Our Noses: The Importance of Smell to Science and our Lives

Robin Reineke

The olfactory system, which senses odors, is vital to our lives, and comprises one of the most primal parts of the brain. Just like our sense of hearing, sight, touch or taste, the sense of smell is a way for us to gather messages about the environment around us. The difference between the senses of taste and smell from our other senses is the fact that they are "chemical senses", in other words, the clues that they send to our brains come in the form of chemicals found in the surrounding environment. With every breath we take, we are sampling our environment for dangers, food, or other individuals. Because we have two nostrils, and thus two "smell systems" (they actually are not identical), we are able to tell from which direction a smell is coming from. Our noses lead us into environments, like little feelers. Smells, unlike tastes, can be detected from a distance, giving our brains an advanced warning about something before we actually put it into our bodies. Our sense of smell not only provides us with warnings about the environment, but also plays an important role in how we recognize each other, communicate with each other, and recall memory.

Often called "the forgotten sense," the way we smell is not well understood. Our "smell centers" are located just below our brains, and above our nasal passages, in a region called the nasal mucosa. Within this region is located our olfactory epithelium. The olfactory epithelium is covered with mucus that is secreted by nearby glands. This mucus contains mucopolysaccharides, immunoglobulins, proteins (e.g. lysozyme) and various enzymes (e.g. peptidases) (3). The nasal mucosa also contains a pigmented-type of epithelial cell. The color of the cell varies from light yellow (in humans) to dark brown (in dogs) and may have to do with olfactory sensitivity. Finally, the nasal mucosa contains receptor cells, or sensory nerve neurons-some 10 million in humans-that are raised above the surface tissue. Each neuron has 8 to 20 olfactory cilia (3). The cilia, hair-like receptors that extend from the receptor cells, are covered with thin, clear mucus. We breathe in airborne molecules that travel to and combine with the mucus covering the cilia on the receptor neurons. The mucus dissolves odor molecules not already in vapor form (3). When the mucus becomes too thick, it can no longer dissolve the molecules-this is why we cannot smell when under the affliction of a bad head cold. At the other end of the receptor cells are axons-nerve fibers-that project into the olfactory bulb. There is an olfactory bulb in each nasal cavity (i.e. one per nostril) (1).

Within the olfactory bulb there are mitral cells, which connect to these axons. The axons of the olfactory receptor cells and the mitral cells merge to form the lateral olfactory tract. The tract leads directly to the neocortex section of the brain (1). Primates have a pathway that runs from the limbic brain to the hypothalmus and is involved with mood and memory. The olfactory tract projects to the limbic system, an ancient region of the brain concerned with emotion, pleasure, memory (specific types), and motivation (3).

Though it is known that odors come in the form of airborne molecules that travel to the olfactory receptors, the issue of how smell is actually received by the olfactory system is still debated. "The average person can discriminate between 4,000 to 10,000 different odor molecules. Much is unknown about exactly how we detect and discriminate between various odors", (2). There are several theories that form the major arguments in this debate.

The "lock and key" theory claims that the molecular shape of a specific odor identifies it from other odors. The theory also states that there are 7 primary odors that have their own receptor sites. The primary 7 odors, according to the theory, are camphor, musk, floral, peppermint, ether, pungent and putrid. "These 7 primary odours were proposed to have different shaped receptors corresponding to the shape of the molecules," (1). Another theory suggests that the olfactory molecule diffuse across the surface of the receptor cell that creates an ion pore. The time it takes for diffusion determines how strong the smell is. But the theory does not address differences in smell. Yet another theory, which has been virtually discounted by modern scientists, held that the pigments in olfactory cells in combination with odors gave rise to a semiconductor current. One theory tied the reception of smell to molecular vibrations. It was suggested that because the frequency of many smells is in the infrared, differing vibrations of infrared distinguished different smells. Several problems with this theory are that the body has its own natural infrared heat, and also, many different smells have the same infrared vibrations, (1).

So what allows for such a wide range of odor perception? In studying vision, one can link all colors perceived by the eye back to three primary colors. The nose can detect thousands of distinct odors. Recently, researchers found a large family of genes that possibly codes for odor receptors. The gene family is one of the largest discovered-programming 500 to 1,000 different types of receptors. These receptors are randomly arranged throughout the olfactory bulb. Scientists have shown that every neuron in the olfactory bulbs participates simultaneously in the act of smelling. When information about the stimulus hits the olfactory bulb, the entire bulb is active, rather than just a few neurons. When the actual stimulus is received, however, the bulb takes on different patterns according to different smells. This forms a sort of map or code that the brain recognizes as a unique scent (3). The bulbular pattern may even be different each time an odor is received, depending on what odors were received before it. This is what allows for odor conditioning and also explains why we are ultra-sensitive to odors we have never previously experienced (1).

The specific processes of odor reception are interesting to scientists because of the amazing property that the receptor cells have-the ability to regenerate. The study of how the connections are made between olfactory neurons and other neurons provides information about how neural connections are made in other parts of the brain. Scientists hope to understand the process of regeneration among olfactory receptor cells in order to find ways for neurons to re-grow in other parts of the nervous system (3). They also hope this area of study will aid in finding treatments for those who have lost their sense of smell as a result of age, disease, or sensory damage.

Not much is known about chemical sensing in general. Genes very similar to those for odor receptors control chemical sensing such as taste, and the ability of sperm to locate an egg. There is a region of the nose called the vomeronasal organ, which is where the sensing of chemical signals called pheromones is thought to take place. Pheromones appear to have an effect on hormone release, mating and communication among animals, and probably also among humans (3). As I mentioned before, the olfactory system sends messages to the limbic system, a primitive brain structure that governs emotions, memory storage and behavior (3). It also sends messages to the cortex, where conscious thought occurs. Scientists have only begun to tap into the valuable information in their noses! Though still greatly a mystery, the olfactory system may help answer questions about our thoughts, emotions and behavior (3).

Chemical Communication

Especially in the animal world, smell plays an important role in identification and communication. Animals often recognize hierarchy and territory through odor. Each individual has a distinctive smell, or odortype (4). Babies, from the time of birth, can identify their mothers on the basis of smell alone. Conversely, after less than 24 hours, mothers can identify their infants by smelling them. Siblings can recognize each other by smell, and dogs can smell the difference between twins (however, not identical twins, which indicates that identical twins have identical smells) (1). Studies have even shown that rodents, and possibly even humans, prefer a mate with a different odortype-possibly a way of promoting genetic diversity within a population (4). Smell is the most honest way we have of communicating, though most humans are not "tuned in to it" or have lost the acute sense of smell that animals still possess.

Studies at the Monell Chemical Senses Center in Philadelphia have shown that odortypes, or individual odors are coded by the genes of the major hostcompatibility complex (8). These are the same genes responsible for accepting or rejecting skin grafts or other transplanted organs. Although it is not understood how these genes code for odor, researchers have discovered groups of volatile organic acids which, when mixed differently, apparently produce an individual's specific odor (8). These odortypes effect mate selection, sexual arousal, physiology, and mother-infant recognition. In less than 50 hours, an infant can discriminate between a gauze pad worn by his or her own breast-feeding mother, and than of another lactating mother. There is evidence from animal studies that the sense of smell is often the only identifying characteristic that an infant is able to recognize. When a mother rat's nipple is washed, the pup is no longer attached (1).

Recent studies on rodents have shown that they can detect fear through scent alone (1). When the odor of rats that had been mildly electrically shocked was presented to rats that had not been shocked, they behaved in a nervous manner. Interesting studies done on humans at Monell by Denise Chen, and at Rutgers University by Jeanette Haviland have shown that humans are able to recognize fear or happiness in fellow human beings.

In the study, an equal number of men and women wore absorbent underarm pads while watching clips of either scary or funny movies. They separated the "happy armpit pads" from the scared ones and added in some neutral pads before asking about 80 men and women to smell the samples and attempt to identify "happy smells" and "scared smells". Although most subjects said they didn't smell anything, their guesses were very accurate. Women were apparently especially good at picking out the scared male samples. Three quarters of the women, and half the men were able to identify the one "fearful male odor jar" out of six. "But men could not detect the happy smell of other males or fear from females", Chen said (5).

There was another study done by Chen and Haviland on how the scent of other individuals influenced a person's mood. Volunteers in three age groups, between 3 and 8, in their early 20's, and elderly individuals in their 70's agreed to use no deodorants, perfumes, or soaps and wear an armpit pad for 10 hours. About 300 university students were asked to report on their moods before and after smelling samples taken from the armpits of various people. Apparently, the smell from elderly women produced an uplifting effect, and the scent from young males produced a depressing effect. Haviland gave possible explanations of this phenomenon-that the scent of elderly females conveyed approachability and familiarity, whereas the scent from young males conveyed aggression (5).

One of the most prominent mysteries in the realm of pheromones is the phenomenon that when women live together, their menstrual cycles tend to regulate and become synchronized. A study done in 1998 by psychologists Martha McClintock and Kathleen Stern that discovered two human pheromones that had an effect on women's' menstrual cycles. Similar to the Chen/Haviland experiment discussed above, women were exposed to arm pads worn by other women. Women who were exposed to the pheromone pads of women ovulating had longer cycles and women who were exposed to pads of women who had not yet entered ovulation had shorter menstrual cycles (7).

Though these experiments show that human pheromones do exist and to a certain degree effect our behavior, their composition is not understood. Neither is it understood how the pheromones are transferred from one organism to another. Scientists seek to understand this better in the hope, among numerous other things, to help infertile couples by inducing ovulation, or to stop ovulation as a means of birth control (7).

Smell and Memory

The scent of cedar perhaps brings one back to a camping trip or an old house, the scent of wood-smoke reminds us of winters past, and the scent of hyacinth brings us to spring. Smell evokes memory in a way that no other sense can. Memory and smell are closely linked; in order to identify a smell, we must first remember it, and then place the object that it comes from into our vision. When the temporal cortical region of the brain (the site of memory) is damaged, the ability to identify smells is damaged (1). Memories, and the emotions associated with those memories are prompted by strong smells (1). Studies have even shown that recall can be enhanced if learning was done in the presence of odor, and that same odor was present at the time of recall. Although the accuracy of the memory is not affected by the type of sensory cue (olfactory or auditory), the intensity and vividness of the memory are increased when the cue is olfactory (8).

The actual ability to smell is closely tied to memory and experience. Although infants are able to smell at the time of birth, they appear to be unable to distinguish between a pleasant and an unpleasant odor (1). This implies that the response to an odor must be learned. Humans tie scents to experiences with emotions. Perhaps this is why young children tend to put objects that adults find revolting into their mouths-there is no previous experience with the object to label it as a good thing or a bad thing. The ability to smell certain odors is actually a combination of experience and genetics (7). People who cannot detect certain smells are said to have "specific anosmia". These people can be induced to perceive the odor if they are exposed to it repeatedly (7).

We tend to think of the loss of sight or hearing as a tragic occurrence, whereas the loss of scent is laughed off or tied to aging. The fact is that people who lose their sense of smell are deprived of taste and important aspects of functionality-there has even been research tying the loss of smell to Alzheimer's disease and memory loss. Anosmia is the condition in which the sense of smell is lost (1). Two to three percent of the population is anosmic, and about fifteen percent have some form of "odor blindness" (9). This can be caused by aging, traumatic head injury, a virus, or exposure to damaging chemicals (1). Anosmia is not life threatening, and is often not treated or taken seriously.

Several pharmaceutical companies are working on developing artificial noses-both for those afflicted by anosmia, and for medical diagnoses. They are seeking to find a way to detect the odor of dangerous bacteria in food or in viruses (9). There is even a company attempting to factor scent into the Internet experience (10).

Scientists are discovering important ways to use smell as a therapeutic tool. Simply smelling something during or before a negative experience often ties that smell to the experience, thus making it an unpleasant smell. The reaction also goes the other way, a smell is often associated with a positive experience. This could be very advantageous in medical treatments and psychiatry. A very exciting study was done recently-healthy males were injected with insulin once per day for four days. Their glucose was measured (it fell). During the injections, they were exposed to a smell. On the fifth day of research, the subjects were exposed to the smell without the insulin-their glucose fell (1).

Interestingly, odor evokes activity in the same cortical system where epilepsy often starts. Dr. Tim Betts of Birmingham University has conducted some studies using aromatherapy with epileptic patients. Almost all patients were able to reduce seizure frequency through associating a certain odor with relaxation (often this odor was Ylang Ylang, as opposed to rosemary which increased the frequency of the seizures) (1).

So the answers really are in our noses! How exciting! There is so much information yet to be tapped through studying the olfactory system. Maybe our sixth sense is really just a secret part of our sense of smell yet to be discovered. The olfactory system is related to so many mysterious parts of the brain-such as memory, recognition, and emotion. Wherever we go, our nose goes first, leading us, in a sense. Science is increasingly showing us that there is a reason for this.

WWW Sources

2) Newton's Apple: Taste and Smell.

3) Brain Briefings: Smell and the Olfactory System, Summer 1995.

4) Science News: The sweet smell of serum.(identifying odors circulate in the blood)(Article Briefing), Issue: March 13, 1999.

5) New Scientist: The Scent of Fear, Issue: May 1, 1999.

6) New Scientist: Heavenly Scent, July 3, 1999.

7) The Chicago Maroon: The Science of Sex and.Smell, April 3, 1998.

8) The Monell Chemical Senses Center.

9) Science Friday: Smell. March 10, 2000.

10) DigiScents: A Revolution of the Senses.