In this paper, I will discuss the anatomy and physiology, in other words the structure and function, of the olfactory system, which is responsible for the sense of smell. I will focus on why and how certain odors are perceived by humans and their connection to feelings and emotions. Finally, from the discussion on feelings, I will conclude with the applications and "power" that smell has in the field of an alternative medicine known as aromatherapy and why some people benefit and feel better after using essential oils, the compounds used in the therapy. Questions that come to mind include: How do specific odors help make a person heal and feel better, i.e. how is mood positively enhanced? What studies support this claim? Are there strong connections between emotions, the sense of smell, and healing? How is the immune system involved, if it is at all? Or is it basically the limbic system, the part of the brain which influences our emotions and memory (3), that is the prime factor in helping a patient, who is undergoing aromatherapy, feel better? It is my goal to understand and be able to answer these questions.
Though the nose is commonly referred to as the organ of smell, this is only superficially true. Olfaction, which is the sense of smell, does not begin at the nose, although odor molecules do enter the two nasal passages of the nose. Gordon Shepherd, a professor of neuroscience at Yale University, states, "In fact, the part of the nose we can see from the outside serves only to take in and channel the air containing odorous molecules." The neurons that sense these molecules lie deep within the nasal cavity, in a patch of cells called the olfactory epithelium (4). Each of the two nasal passages in humans has a 2.5 square centimeter patch containing about 50 million sensory receptor cells (5). These receptor cells send their axons into the olfactory bulb, a projection of the brain that lies over the nasal cavity in most primates, in the anterior region of the brain (6). The olfactory cilia of the sensory receptor cells (about 10-20 cilia for every olfactory neuron) are hair like projections (4). It is within these cilia, that the specific odor molecules bind to their respective chemoreceptors. This binding causes a change in permeability of the sensory neuron, which creates a slow electric potential, that travels to the olfactory bulb. From the olfactory bulb, the signal is transmitted to the limbic system in the brain, where memory is used to recognize the odor (3).
Through this understanding of how odors are perceived, it is important to know why the limbic system allows us to behave differently when experiencing different odors. Knowing that the limbic system is an area where memory is utilized, we can partly understand the connection between past experiences and the same odor that we are smelling in the present. The limbic system is not only a memory storage area, but it also regulates mood and emotion (7). Since odors, emotions, and memories meet here, this is why smell elicits distinct memories and emotions. The perception of a certain odor that is associated with a past memory allows for a recognition of the odor in the olfactory system and a corresponding past memory (3). A question arises as to how the limbic system is able to remember past smells, while the fact is that the olfactory neurons are continually replaced after about every sixty days. The reason is, when the olfactory neurons die, a new set of neurons generates beneath them. These axons of neurons that express the same receptor always lead to the same destination (4). This is how memories survive in the limbic system.
Research done by Dr. Barry Ache of the University of Florida indicates that odors can both excite and inhibit the receptor cells that detect and send information to the brain. Though his studies are done on lobsters, which rely heavily on their sense of smell and have well-developed olfactory organs, the evidence has possibilities. Ache states, "Masking bad odors, for example, is an important aspect of flavor and fragrance research." (8) . This can be applied to aromatherapy, which relies on fragrance research.
Aromatherapy is a term coined in the 1920's, by a French chemist named Rene Maurice Gattefosse, to describe the practice of using essential oils taken from plants, flowers, roots, seeds, etc., in healing. The use of plant oils, including essential oils, is meant for psychological and physical well being (7). Gattefosse conducted experiments with essential oils on wounded soldiers during World War 1. He found that essential oils were antiseptics that detoxified better than the chemical compounds that were currently being used (7).
One way in which essential oils are used is by inhalation through the nasal passages. The aroma of the oil is perceived through the sense of smell. A very important aspect of smell is that olfactory neurons make up the only sensory pathway that is in direct contact with the brain. Olfactory neurons, as mentioned before, are in contact with the limbic center of the brain. Because of the close connection to the limbic center, feelings and emotions can be regulated, as well as memory (7).
Though there has not been much research in the United States on aromatherapy, more research has been done in Europe indicating that the use of scent can affect physical and psychological changes in humans (9). One study done in the United States found that inhalation of certain aromas appeared to be able to induce sustained weight loss over a six-month period. The research was published in the Journal of Neurological and Orthopedic Medicine and Surgery (1995). The study involved over 3,000 subjects and Alan R. Hirsch, M.D., neurologist and director of the Smell and Taste Treatment and Research Foundation in Chicago, Illinois conducted it. Dr. Hirsch wanted to see if smelling pleasant food aromas would help reduce appetite and food cravings in people, thereby helping them lose weight by reducing their food intake. Subjects were given plastic nasal inhalers scented with banana, green apple and peppermint. They sniffed the inhalers before, during and between meals. They recorded how often they sniffed. It was found that those who sniffed more frequently lost weight faster than those who did not. Dr. Hirsch explained that it worked because "Odors have a direct effect on the satiety center in the brain, which is the area that tells your body when you've had enough to eat." Continual exposure to pleasant food aromas indicates to the brain that the body is satisfied and does not need to eat. It was shown in the study that there was successful weight loss (9).
Another study by several hospitals in the United States, including New York's Memorial Sloan-Kettering Cancer Center, have successfully reduced anxiety before and during MRI scans, using scents such as vanilla, lavender and heliotropin (9).
Aromachology, the study of the effects of odors on behavior, is a developing science and it is relatively new (3). Though aromatherapy has been practiced for thousands of years, dating back to the ancient Egyptians and the Vedic culture of India, where plant extracts and medicinal plants were widely used, there is much more scientific research being done for both fields today (1). It is desired that the unanswered questions that were posed in this paper will soon be answered through continual advances in science and technology that will lead to a clearer understanding of healing and its connection with the sense of smell.
2) Newton's Apple Teacher's Guide: Taste and Smell , A site explaining why food loses its flavor when you have a cold.
3) The Olfactory Process and its Effect on Human Behavior , A paper on the sense of smell and its connection to emotion
4) Seeing, Hearing and Smelling the World , A report from the Howard Hughes Medical Institute on new findings of senses
5) Smelling: It's More Than Meets the Olfactory Epithelium , A paper on the sense of smell and the ability of the brain to recognize odors.
6) Smell and the Olfactory System , A Society for Neuroscience site on olfactory neurons and how they link with other neurons in the brain. This site has some images that describe the olfactory system.
7) Aromatherapy Articles And A Brief History of Aromatherapy , A site on aromatherapy and smell.
8) SF News - April 2, 1998 - Researcher studies lobster sense of smell , An article about the spiny lobster as a model for understanding the processes involved in olfaction.
9) Aromatherapy: Health Benefits of the Science of Scent , An article on recent research on aromatherapy.
The Seasonal Smell of Silver Bells , An article on synesthesia, a human condition in which senses, that are usually separate, combine.
TIME: Following Our Noses , An article on pheromones and their possible usefulness in humans.
Sixth Sense: The Vomeronasal Organ , A paper on the possible existence of a "sixth sense" organ to detect the presence of odorless human pheromones.