- Function
- Airborne odorants (~400 receptor types)
- Quantity
- 12–30 million neurons; ~390–400 intact OR genes
- Density
- Packed into ~5 cm² of olfactory epithelium
- Distribution
- Roof of nasal cavity, both sides; each type intermingled, not patched
One receptor type per neuron. Combinatorial activation lets humans discriminate thousands of odors from a few hundred receptor genes.