Unravel the Secret of Olfaction

Abstract

As a special sense, olfaction is an inherited biologic function to monitor our environment. This sense alerts us to dangerous situa-tions such as gas leaks or fires, and provides cues to the freshness of food. Smell is tightly associated with flavor and taste, and there-fore smell and taste loss frequently occur simultaneously. Olfacto-ry stimuli evoke emotions and memories that are related to the neighbouring brain circuits of the amygdala and hypocampus [1,2]. Animals use this sense to avoid predators, while searching for food and evaluating potential sex-partners. Therefore, sense of smell is the primitive brain function and the first door to under-stand brain sciences. The olfactory epithelium is located on the most superior struc-tures of the nasal cavity at the cribriform plate, superior turbinate, and superior septum [3]. Olfactory epithelium consists of multiple functional cells such as 1) olfactory receptor cells (chemoreceptors and bipolar neuron cells) transmitting signal to the brain, 2) sup-porting cells (sustentacular cells) that provide metabolic and me-chanical support, 3) basal cells which provide a stem cell niche for olfactory tract neurogenesis, and 4) microvillar brush cells that are part of the afferent trigeminal tract. In addition, the Bowman's glands that produce mucus which contains odorant-binding pro-teins [4,5]. This important sensing area is susceptible to inflamma-tion, occupational toxins, and trauma, and therefore, various mech-anisms are involved in the pathogenesis of olfactory dysfunction. Humans have about 800 gene segments analogous for odorant receptor (OR) and about half of which seem to be non-coding pseu-do-genes [6]. Genomic drift, the random process of gene duplica-tions or deletions, plays a key role for olfactory evolution. The ge-nomic drift shapes the OR repertories of different species and also accounts for part of the interpersonal differences of functioning OR in human. Therefore, interestingly, each person detects envi-ronmental odors in a unique fashion using a different set of OR. Olfactory mechanisms have been largely a mystery until the re-cent discovery of G protein coupled receptors (GPCR) by Linda Buck and Richard Axel in 1991, a breakthrough for which the two researchers were awarded the 2004 Nobel Prize for Physiology and Medicine. Their discovery showed 1) the existence of a novel fami-ly of OR genes, 2) the organization of odorant receptors in the ol-factory epithelium, 3) a topographic map of the olfactory bulb, 4) combinational receptor codes for odors, and 5) mechanisms of signal conveyance in the olfactory cortex (stereotypy, divergence, and convergence). Surprisingly, vision and taste also use G-protein in their signal transduction, and G-proteins play key roles in many biologic functions such as gene expression, immunity, metabolism, and sleep. Park will discuss the mechanism of olfactory signal trans-duction in detail [7]. Olfactory dysfunction is a relatively common disorder caused by three etiologies such as sinonasal diseases, upper respiratory infection, and head trauma. Quantitative (hyposmia and anosmia) and qualitative (parosmia and phantosmia) olfactory dysfunction significantly decrease patients' quality of life and may affect emo-tion and memory. A detailed history and appropriate physical ex-amination will be helpful to identify the underlying causes of ol-factory dysfunction. In addition, simple psychophysical test (olfac-tory test) are essential to quantify the residual function which is the most important factor for prognosis. Psychophysiologic tests such as electroencephalography and odorant-event related poten-tials, and imaging study such as functional MRI are sometimes This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecom mons.org/licenses/by-nc/3.0) which permits un restricted non-commercial use, distribution, and reproduction in any medium, provided the origi nal work is properly cited.