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the channel for the activating cyclic nucleotide.44 In this way it acts as a regulator of adaptation, allowing the system to detect odorants over a very wide dose range. Ca2 also acts to terminate signals through the activation of cAMP phosphodiesterase. On the other hand, Ca2 can enhance the signal generated by cAMP by opening a Cl channel present in the apical membranes of the olfactory neurons.
Rather unusually, these cells maintain a rather high concentration of Cl ions so that when the channels are opened, and the Cl ions flow out, the cells tend to depolarize. This mechanism is thought to act as a fail-safe, in case the availability of Na in the nasal mucus, a very uncontrolled environment, is insufficient to generate a sufficient inward current of positive charges through the CNG to cause membrane depolarization.
cGMP is also elevated in olfactory neurons following receptor activation, though this occurs on a slower time scale than the elevation of cAMP and then over a more sustained time course. It is thought to act as a determinant of adaptation and desensitization, suppressing the gain of the cAMP pathway and helping to prevent saturation of the olfactory response to high and repeated stimuli.45
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