Odorant molecules and human olfactory anatomy: from nasal airflow to piriform cortex

Scope. Volumes 1–3 describe olfaction at the systems level and at the molecular–emotional level. This monograph addresses the two foundational levels still missing: what happens to an odour molecule before it reaches a receptor, and the quantitative anatomy of the olfactory system in humans — from the epithelium inside the nose to the cortex.

Methods. Four independent collection strands worked to a common protocol: every number must have a traceable source together with species, method and sample size; human data are kept separate from animal data; where sources conflict, every value is recorded with its likely cause instead of choosing one. The result is 259 data points, about 228 of them quantitative.

Main findings. (1) The figure “10–15% of inhaled airflow reaches the olfactory region” originates from one physical model and one simulation of a single nasal cavity; simulations on individual nasal geometries give roughly 1% to 15%, and lower in older adults. (2) Human olfactory mucus chemically transforms odorants: in vitro, some compounds are metabolised by up to 99%, the extent of transformation varies about ninefold between individuals, and metabolites appear in the breath within only a few seconds. (3) Apart from OR51E2 — a receptor for short-chain fatty acids — every published human odorant receptor structure is a consensus or engineered receptor; there is not yet a structure of a “typical” odorant receptor in its native form. (4) At group level, people could discriminate only 3 of the 10 enantiomer pairs tested. (5) The human olfactory bulb has about 5,000–8,000 glomeruli per side — roughly four to seven times the textbook figure — and shows almost no neuronal turnover in adulthood (under 1% after a hundred years). (6) True three-layered piriform cortex makes up only a small part of the region that MRI studies call piriform; published volumes differ up to about sevenfold depending on the definition. (7) Limited evidence (about 10 cases) suggests that the thalamic route serves attention rather than perception; and the “right-hemisphere dominance” hypothesis is clearest in the orbitofrontal cortex, not a general property.

Conclusion. Many foundational numbers of human olfaction — epithelial area, total neuron count, number of olfactory nerve axons, mucus thickness — have never been properly measured in humans. At the same time, some numbers that have been measured differ sharply from what textbooks still repeat. For the fragrance industry the most important implication is this: what customers smell has already been transformed by the airflow, mucus and enzymes in their own noses before the brain receives the signal — and these transformation steps differ from person to person, and possibly with age.