Electrochemical circuits for detecting nanovolt potentials with ion channels

The ampullae of Lorenzini are organs in some fish used for sensing very weak electric and thermal fields. Both fields generate a voltage gradient across ionic gel-filled ampullary canals, which is detected by voltage-gated calcium and potassium channels in basal hair cells. While the sensitivity of these ion channels is of the order of 10 millivolts, some fish species can detect voltage differences on the order of nanovolts across the ampullary canal. How can this system achieve millionfold amplification? Previous work has shown that sensory systems can exploit proximity to a critical/bifurcation point in their ion channel mediated electrical dynamics to robustly amplify weak signals. In the pit organ such dynamics could achieve 1000-fold amplification from microscopic channels properties to action potential rate. But the specific scheme proposed there would not be able to simply achieve this millionfold amplification. Here we propose a mechanism for nanovolt sensing by hair cells in the ampullae of Lorenzini. In our model the very small intracellular calcium concentration can be used as an integrator of channel currents. If outward calcium pumps are zeroth-order and not limited by calcium concentration, this integrator becomes ultrasensitive to tiny changes in currents.