Olfactory Receptors: Location and Structure

Olfactory receptors are located on the roof of the nasal cavity, covering the superior nasal concha on each side of the nasal septum. This region, called the olfactory epithelium, spans approximately five square centimeters and contains olfactory sensory neurons rich in receptor proteins. The olfactory epithelium is composed of connective tissue called lamina propria and specialized sensory cells.

Each olfactory neuron is a bipolar neuron with apical dendrites that extend into a bulb and form several nonmotile olfactory cilia containing receptor proteins. These cilia are surrounded by mucus produced by olfactory glands, which helps capture and dissolve airborne odorants for efficient detection by the receptor proteins.

Airborne odorant molecules dissolve into the mucus lining the nasal cavity and bind to specific receptor proteins within the olfactory cilia's cellular membrane. These receptor proteins are G protein-coupled and generate a graded membrane potential in the olfactory neurons, initiating the smell sensation and connecting to the physiology of smell and olfactory pathway.

Olfactory neurons are susceptible to damage from noxious airborne substances and environmental exposure. Olfactory stem cells located at the base of the olfactory epithelium differentiate every 30 to 60 days to replace old olfactory neurons, ensuring continuous regeneration and maintenance of olfactory function.

Anosmia is the degradation or complete loss of the sense of smell. It can result from severe facial trauma that severs olfactory tract axons, certain medications like antibiotics that eliminate olfactory neurons, inflammation from respiratory infections or allergies, or age-related decline in olfactory neuron regenerative capacity.

Anosmia can diminish the gustatory experience by rendering food tasteless, as smell is essential to flavor perception. Individuals with compromised olfactory capacity may require augmented levels of spices and seasonings. There is a potential link between anosmia and mild depressive states due to diminished pleasure from food.

Olfaction is the singular sensory modality that bypasses a synapse in the thalamus before interfacing with the cerebral cortex. Olfactory axons project directly to the olfactory bulb and then to the primary olfactory cortex and limbic system, enabling direct linkage of odors with memory and emotional reactions.