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, it continues to have an impact on human interaction, bonding, and propagation of the species. Even if we are unaware of it, the sense of smell greatly affects our lives and is closely related to overall quality of life and health. Nonetheless, olfaction has been neglected from a scientific perspective compared to other senses. However, 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 an effective treatment for olfactory dysfunction, suggesting the reorganization of neural associations. A 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 pathways for managing and helping patients with olfactory dysfunction.

Keywords: Olfactory Impairment; Olfactory Pathway; Olfactory Perception; Olfactory Tract; Smell; Taste.

Fig. 1.

The path of olfactory perception from the olfactory epithelium to the olfactory bulb. An odorant molecule binds to an olfactory receptor neuron, generating an action potential that is relayed to the olfactory bulb and then to the olfactory cortex through the olfactory tract.

Fig. 2.

Schematic drawing of the olfactory mucosa. Olfactory receptor neurons (ORNs), globose and horizontal basal cells, supporting cells, and Bowman’s gland make up the olfactory epithelium.

Fig. 3.

Central pathways of olfaction. An olfactory signal is relayed from the olfactory epithelium to the olfactory bulb, primary olfactory cortex, and then to the secondary olfactory cortex.

Fig. 4.

Schematic diagram of the olfactory, gustatory pathway to the orbitofrontal cortex to form flavor. OFC, orbitofrontal cortex; Thal, thalamus; OB, olfactory bulb; PC, piriform cortex; Amyg, amygdala; Hipp, hippocampus; Ento, entorhinal cortex; NS, nucleus of solitary tract.