The Olfactory System: Basic Anatomy and Physiology for General Otorhinolaryngologists

Abstract

Olfaction is one of the five basic human senses, and it is known to be one of the most primitive senses. The sense of olfaction may have been critical for human survival in prehistoric society, and although many believe its importance has diminished over time, its impact on human interaction, bonding, and propagation of the species remains. Even if we are unaware of it, the sense of smell greatly affects our lives. Sense of smell is closely related to overall quality of life and health. However, olfaction has been neglected from a scientific perspective compared to other senses. Olfaction has recently received substantial attention since the loss of smell and taste has been noted as a key symptom of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Studies investigating olfaction loss in association with coronavirus disease 2019 (COVID-19) have revealed that olfactory dysfunction can be both conductive and sensorineural, possibly causing structural changes in the brain. Olfactory training is one of the effective treatments for olfactory dysfunction, suggesting the reorganization of neural associations. Reduced ability to smell may also alert suspicion for neurodegenerative or psychiatric disorders. Here, we summarize the basic knowledge that we, as otorhinolaryngologists, should have about the sense of smell and the peripheral and central olfactory pathway for managing and helping patients with olfactory dysfunction.

A Comprehensive Review of COVID-19-Related Olfactory Deficiency: Unraveling Associations with Neurocognitive Disorders and Magnetic Resonance Imaging Findings

“…Additional articles related to this field were also included if appropriate. The olfactory system is responsible for perceiving volatile chemical substances and gases present in the air (odorants) and is phylogenetically one of the most primitive sensory systems (Figure 1) [14,15]. The olfactory organ consists of the olfactory epithelium (OE), OB, OT, and the piriform olfactory cortex [14].…”

“…The olfactory system is responsible for perceiving volatile chemical substances and gases present in the air (odorants) and is phylogenetically one of the most primitive sensory systems (Figure 1) [14,15]. The olfactory organ consists of the olfactory epithelium (OE), OB, OT, and the piriform olfactory cortex [14]. The transformation of olfactory signals into electrical signals occurs at the level of the olfactory epithelium that lines the apical region of the central mucosa, where three main types of cells can be distinguished: olfactory sensory neurons, responsible for transducing odor stimuli; support cells, similar to glial cells, which protect the neurons and help produce the mucus in which odor molecules are trapped; and basal cells, which are the progenitor stem cells of olfactory neurons [16].…”

“…The axons of the OSNs pass through the cribriform plate of the ethmoid bone to reach the OBs, where they form synaptic glomeruli. In the glomeruli of the OB, communication occurs between afferent olfactory neurons and second-order neurons, which transmit signals to the primary olfactory cortex [14]. The olfactory system uses a combinatorial receptor coding scheme to discriminate and identify odor molecules [18].…”

“…The orbitofrontal cortex, amygdala, and hippocampus participate in the translation of olfactory stimulation and its incorporation into thought processes, emotional reaction perception, memory formation, and learning processes. It is believed that the multiregional involvement of olfactory stimuli, consisting of the amygdala, prepyriform cortex, entorhinal cortex, and hippocampus, explains the relationship between odor, emotions, and memory formation [14,19]. constituting the fundamental interface for odor perception in the central nervous system to the OB.…”