Molecular physiology of olfaction.

Olfactory reception is mediated by olfactory receptor cells located in the olfactory epithelium. These cells are bipolar neurons that extend a dendrite toward the nasal lumen and an axon toward the olfactory bulb in the brain. The dendrite possesses a group of apical cilia embedded in mucus. Odorant recognition and signal transduction are initiated at the membranes of these chemosensory cilia and culminate in excitation of the olfactory receptor cell. Differential activation by odorants of distinct groups of olfactory receptor cells generates patterns of neuronal activity that encode odor quality and concentration. The identities of primary odorant recognition sites at the ciliary membrane remain to be established. However, a significant body of information has become available with respect to olfactory transduction mechanisms. It is now becoming clear that olfactory transduction involves the interplay of several second messenger systems to control the responses of these exquisitely sensitive chemosensory neurons.