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Slide 17

Adaptation in Vitro is Associated With a Large Afterhyperpolarization That is Sensitive to Na+ but not Ca2+

Adaptation in vitro is associated with the activation of a prolonged afterhyperpolarization that does not decrease in amplitude with lowering extracellular Ca2+, but does decrease when lowering the concentration of extracellular Na+. In Control, the cell was injected with 20 seconds of sinewave current. The firing rate adapted to the injected current (bottom graph) and generated a prolonged afterhyperpolarization. Following removal of Ca2+ from the bathing medium and inclusion of Mn2+, the cell still generated a large afterhyperpolarization, although the adaptation of spike firing rate was markedly reduced. Reduction of extracellular concentration of Na+ greatly reduced the adaptation of firing rate and decreased the amplitude of the AHP. Reinstatement of extracellular Na+ again resulted in marked adaptation.

These results suggest that adaptation is mediated by both Ca2+ and Na+ dependent currents, with the long lasting afterhyperpolarization being mediated largely by Na+ dependent currents.