Electric Signals Amplify Weak Olfactory Stimuli in the Nose

Microscopic image of an olfactory receptor cell which was isolated from the olfactory epithelium. The sensory cilia that detect odorants in the inhaled air are visible at the top. At the bottom end, an axon emanates and conveys the sensory information to the brain. The size of the cell is 50 micrometer.

The human olfactory system possesses a special electric amplification mechanism that enables olfactory cells to respond even to extremely weak stimuli. Scientists at Heidelberg University headed by physiologist Prof. Dr. Stephan Frings have now established how this mechanism works. Crucial is the role played by chloride ions stored in the sensory cilia of the nose. As soon as the olfactory receptors in the sensory cilia detect odorants, the chloride ions are immediately discharged. This process generates strong electric signals that pass on the relevant olfactory information to the brain.

Our noses detect a huge variety of odorants in the air we breathe. The olfactory system is confronted with an immense diversity of chemical compounds. The air in a room where a coffee machine is making coffee, where there are plants on the window-sill and people walking in and out contains thousands of different odorants. But our olfactory system finds this apparent chaos easy to deal with. It unerringly identifies the smell of coffee, although that smell alone is made up of over 800 different odorants. For this purpose the olfactory cells in the nose are equipped with olfactory receptors, proteins presented to the inhaled air on fine sensory cilia by the olfactory cells.

Up to now, research on olfactory cells and their receptors has been dogged by one unanswered question. The concentration of individual odorants in the nose – i.e. the number of molecules of a given odorant per cubic centimetre of ambient air – is very low. In addition, olfactory receptors have proved to be relatively insensitive, only responding very weakly to low odorant concentrations. So how can the key function of our highly sensitive olfactory system be performed by receptors that are themselves remarkably insensitive? The answer lies in the electric amplification mechanism for the olfactory cells deciphered by Prof. Frings and his team at Heidelberg University’s Centre for Organismal Studies.

The sensory cilia of the olfactory cells prepare themselves for the job in a special way. A protein complex pumps chloride ions into the interior of the sensory cilia, thus making them into well-filled chloride stores. When an olfactory stimulus occurs, another protein swings into action, a chloride channel that the sensory cilia possess many copies of in their external membranes. These chloride channels remain closed as long as the olfactory cell is at rest. When an olfactory stimulus is registered, the weak response of the olfactory receptors immediately opens all the channels. The release of negatively charged chloride ions causes a loading inversion in the olfactory cell. This in its turn produces strong electric signals that are conveyed to the brain with the olfactory information.

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Original publication:
T. Hengl, H. Kaneko, K. Dauner, K. Vocke, S. Frings, F. Möhrlen: Molecular Components of Signal Amplification in Olfactory Sensory Cilia. PNAS (30 March 2010) 107:6052-6057, doi: 10.1073/pnas.0909032107


Contact:
Prof. Dr. Stephan Frings
Centre for Organismal Studies
Department of Molecular Animal Physiology
phone: +49 6221 545661
s.frings@zoo.uni-heidelberg.de

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