Olfactory receptors and the mechanism of odor perception

Abstract

The olfactory system plays one of the key roles in the lives of humans and animals. It can detect thousands of different odor molecules through a large family of olfactory receptors (ORs), of a diverse protein sequence, which are located in olfactory sensory neurons (OSNs) in the olfactory epithelium in the nose of humans, and in the vomeronasal organ in animals. The OR family is comprised of 172 subfamilies, whose members have related protein sequences and are encoded by a single chromosomal locus. The human receptor gene family includes 339 intact receptor genes and 297 receptor pseudogenes, unevenly distributed among 51 different loci on 21 human chromosomes. Different parts of the genome may be involved in the detection of different types of structural odorants. Odor detection is mediated by odorant receptors. Signals generated in OSNs in response to odorants are transmitted to the olfactory bulb (OB) of the brain, i.e., the first relay station in the control of the olfactory system in mammals. Then, signals are transmitted to the olfactory brain cortex, which has a cortical structure with distinct layers and numerous glomerular modules, and forms a map of olfactory axon terminals. The axonal projection of OSNs is precisely organized with a few topographically fixed glomeruli. The purpose of this paper is to present the recent literature in the field of the undertaken subject. The review of articles is devoted to olfactory receptors and the mechanism of odor perception. In the first part, this review summarizes the mammalian protein ORs that are encoded by genes, as well as the location and structure of these receptors. Extensive studies reveal that mammals can have up to 1000 different OR genes, which constitute approximately 1% of the genomic complement of genes. The analysis of the entire receptor family has shown that it is comprised of 172 subfamilies, whose members are 60% identical in protein amino acid sequence and can recognize odorants with related structures. The second part of this review presents the opinions of many authors concerning the olfactory perception that initiates in the olfactory epithelium. Odor signals are further transmitted to the OB, i.e., the first relay station of the central olfactory system in the mammalian brain. Then such signals ultimately reach higher cortical areas involved in the conscious perception of odor. At the review's conclusion some diseases associated with smell disorders are discussed. The olfactory cortex still remains an unexplored and unexplained area in terms of the processing of odorant information. This subject requires further study.