Why Anosmia Occurs in the Common Cold — 3 Mark Answer

1. Mechanism of odor detection requires access of the odorant to the olfactory membrane. The olfactory receptor cells are bipolar neurons whose cilia project into a layer of mucus (secreted by Bowman's glands) coating the olfactory membrane high in the nasal cavity. For smell to occur, an odorant substance must first diffuse into this mucus and then bind to receptor proteins on the olfactory cilia before it can trigger the G-protein/cAMP/Na⁺ channel cascade that generates an action potential in the olfactory nerve (Guyton and Hall Textbook of Medical Physiology, "Stimulation of the Olfactory Cells").
2. In a common cold, the nasal and olfactory mucosa become inflamed and hypersecretory. Viral rhinitis causes swelling (edema) of the nasal mucosa and excessive, often thickened, mucus production. This has two effects: it physically obstructs airflow so that inspired air carrying odorant molecules cannot even reach the olfactory membrane in the superior nasal cavity, and it thickens the mucus layer so that odorants that do arrive cannot diffuse through it fast enough to reach and bind the olfactory cilia.
3. Because odorant access to the cilia is essential and slightly water-soluble diffusion through mucus is a prerequisite step for stimulation, blocking or altering this mucus layer (congestion, edema, excess/altered secretion) prevents odorant-receptor binding and cell excitation, even though the olfactory neurons themselves are structurally intact. Hence anosmia in a common cold is largely a conductive problem (mechanical/mucosal blockage) rather than a sensorineural one (destruction of receptor cells or olfactory pathway), and it typically resolves once the inflammation and congestion subside.

• Guyton and Hall Textbook of Medical Physiology, "Olfactory Membrane" and "Stimulation of the Olfactory Cells"