Follow your nose: Mechanisms of signal transduction

The human olfactory system can recognize and discriminate a large number (> 10(4)) of different odorant molecules. Details about the molecular mechanis ms underlying chemo-electrical transduction have increased considerably. Th e initial events are thought to involve the interaction of an odor molecule with the specific receptor protein and the activation of a stimulatory G-p rotein, which in its turn acts on adenylyl cyclase. The following increase in cyclic adenosine monophospate (cAMP) leads to an invert current flux via openings cAMP directly gated cation channels. The depolarization of the ce ll generates an electrical output signal (action potential) that is propaga ted to the olfactory bulb along the nerve fibre of the olfactory sensory ne uron. The zonal organization detected by the expression of the odorant rece ptor genes seems to be conserved also at the level of the glomerula sheet a t the surface of the main olfactory bulb. The axons of olfactory receptor n eurons expressing the same olfactory protein project typically to only a pa ir of glomeruli which reside at stereotyped and symmetrical positions in ea ch bulb. These observations define a glomerulus as a convertioned site of a xonal projections from olfactory sensory neurons that express a given olfac tory receptor protein and also as a functional unit for integrating olfacto ry information.