The primary purpose of the olfactory system in humans is to detect and perceive volatile airborne chemicals and thus provide information about our environment and food quality that is critical to our health, a nutritious diet, and psychological well-being. It is believed that humans are capable of detecting and discriminating thousands of different odorant molecules, many at extremely low concentrations (i.e., ppb or ppt levels).
Smell or olfaction occurs when volatile chemicals stimulate olfactory receptor neurons located on a relatively small patch of specialized epithelial tissue high in the nasal cavity. These SENSORY neurons are bipolar with a ciliated dendrite facing the interior space of the nasal cavity where interaction with odorant molecules occurs , and an axon that travels as the olfactory nerve (Cranial Nerve I) to terminate in the olfactory bulb. In turn, the olfactory bulb projects more centrally and contributes inputs for higher cortical processing that results in olfactory perception. Odorants reach the olfactory receptors in two ways; they can enter the nostrils during normal inhalation (orthonasal route) or travel from the back of the oral cavity toward the roof of the mouth via the nasal pharynx (retronasal route). The perception of food flavor involves a combination of olfactory activation caused by odorous compounds released into the retronasal route through chewing or drinking, and the blending of taste (salty, sour, bitter, sweet) and other oral somatosensory sensations (texture, heat, cold). Nasal blockage and swelling can prevent odors from entering the retronasal stream, resulting in a shift of flavor towards blandness. Reported taste loss is more typically a loss of food flavor due to the blockage of this retronasal route than a decrement in taste perception.
The ability to detect odors, to recognize and discriminate odor quality, and to identify the source of odors are primary functions of the olfactory system. These abilities can be assessed both orthonasally (i.e., odors are inhaled) and retronasally (e.g., odors are placed inside the oral cavity when the mouth is closed). Odor detection and discrimination can be measured and quantified using a variety of techniques, including behavioral, psychophysical and electrophysiological measures. There are a variety of sophisticated psychophysical tests and testing procedures that are available for assessing olfactory capabilities and olfactory loss in humans. Odor detection is evaluated by measuring the lowest concentration of an odorant at which an individual (a) can just detect the odor’s presence or (b) can discriminate it from a sample of odorless air. The concentration at which this occurs is considered the detection threshold for that compound and the detection value can be compared with normative data gathered on other individuals to determine whether an individuals’ olfactory sensitivity falls within normal ranges. Odor identification is evaluated by presenting individuals with a variety of recognizable odorants at supra-threshold concentrations (i.e., above the detection level for most subjects) and requiring them to choose the correct identity of the odor from a set of possible names or pictures. Performance can be compared with age- and gender-adjusted norms. The quality or hedonic valence of an odor is another important element of olfactory perception and, particularly among children, can be measured using scales of hedonic judgments of odors.