Mechanism of Excitation of the Olfactory Cells. The portion of each olfactory cell that responds to the olfactory chemical stimuli is the olfactory cilia. The odorant substance, on coming in contact with the olfactory membrane surface, first diffuses into the mucus that covers the cilia. Then it binds with receptor proteins in the membrane of each cilium. Each receptor protein is actually a long molecule that threads its way through the membrane about seven times, folding inward and outward. The odorant binds with the portion of the receptor protein that folds to the outside. The inside of the folding protein, however, is coupled to a so-called G-protein, itself a combination of three sub-units. On excitation of the receptor protein, an alpha subunit breaks away from the G-protein and immediately activates adenylyl cyclase, which is attached to the inside of the ciliary membrane near the receptor cell body. The activated cyclase, in turn, converts many molecules of intracellular adenosine triphosphate into cyclic adenosine monophosphate (cAMP). Finally, this cAMP activates another nearby membrane protein, a gated sodium ion channel, that opens its "gate" and allows large numbers of sodium ions to pour through the membrane into the receptor cell cytoplasm. The sodium ions increase the electrical potential in the positive direction inside the cell membrane, thus exciting the olfactory neuron and transmitting action potentials into the central nervous system by way of the olfactory nerve.
The importance of this mechanism for activating olfactory nerves is that it greatly multiplies the excitatory effect of even the weakest odorant.To summarize: (1) Activation of the receptor protein by the odorant substance activates the G-protein complex. (2) This, in turn, activates multiple molecules of adenylyl cyclase inside the olfactory cell membrane. (3) This causes the formation of many times more molecules of cAMP. (4) Finally, the cAMP opens still many times more sodium ion channels. Therefore, even the most minute concentration of a specific odorant initiates a cascading effect that opens extremely large numbers of sodium channels. This accounts for the exquisite sensitivity of the olfactory neurons to even the slightest amount of odorant.
In addition to the basic chemical mechanism by which the olfactory cells are stimulated, several physical factors affect the degree of stimulation. First, only volatile substances that can be sniffed into the nostrils can be smelled. Second, the stimulating substance must be at least slightly water soluble so that it can pass through the mucus to reach the olfactory cilia. Third, it is helpful for the substance to be at least slightly lipid soluble, presumably because lipid constituents of the cilium itself are a weak barrier to non-lipid-soluble odorants.
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