Unraveling the Sense of Smell

The subject of my lecture is the sense of smell, one of the five senses through which we perceive the world. Through the sense of smell, humans and other mammals can perceive a vast number and variety of chemicals in the external world. It is estimated that humans can sense as many as 10,000 to 100,000 chemicals as having a distinct odor. All of these “odorants” are small, volatile molecules. However, they have diverse structures and somehow those different structures are perceived as having different odors (Figure 1). The sense of smell is mediated by the olfactory system, a system that is characterized by exquisite sensitivity and discriminatory power. Even a slight change in the structure of an odorant can change its perceived odor. For example, the close relative of a chemical that is perceived as pear can have the scent of an apple. In addition to odorants, the olfactory system detects pheromones, chemicals that are released from animals and act on members of the same species, stimulating hormonal changes or instinctive behaviors, such as mating or aggression. The olfactory system also detects predator odors, which can elicit innate fear responses. Over the past 16 years, our work has focused on two questions. First, how do mammals detect so many different environmental chemicals? And second, how does the brain translate those chemicals into diverse odor perceptions and behaviors? Odorants are initially detected by olfactory sensory neurons, which are located in the olfactory epithelium lining the nasal cavity (Figure 2). These neurons transmit signals to the olfactory bulb of the brain, which then relays those signals to the olfactory cortex. From there, olfactory information is sent to a number of other brain areas. These include higher cortical areas thought to be involved in odor discrimination as well as deep limbic areas of the brain, which are thought to mediate the emotional and physiological effects of odors. In contrast to odorants, pheromones are detected primarily in the vomeronasal organ, or VNO, a separate olfactory structure in the nasal septum.