Individual olfactory sensory neurons express only one of the thousand receptor genes, such that neurons are functionally distinct. Cells expressing a given receptor in the olfactory epithelium are randomly dispersed within 1 of 4 broad zones. Sensory neurons extend a single unbranched axon to the olfactory bulb such that the projections from neurons expressing a specific receptor converge on 2 out of the 1800 glomeruli. The pattern of convergence is absolute and invariant in all individuals in a species. The bulb therefore provides a spatial map that identifies which of the numerous receptors have been activated within the sensory epithelium so that the quality of an olfactory stimulus is encoded by specific combination of glomeruli activated by a given odorant (combinatorial code).