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I am struggling to understand negative conductance shown on I-V curves on ion channels. Mechanistically, negative conductance means that inward (or outward) current increases when voltage across membrane decreases. I-V curves of such a channel have both positive and negative slopes. How it is achieved by ion-channels?

For example, this is in addition to the example posted in the answer by aandreev. enter image description here

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  • $\begingroup$ Thanks for the question. I am afraid I do not entirely get your question. How is what achieved? Or specifically, where does "it" refer to in your concluding sentence? $\endgroup$ – AliceD May 9 '15 at 4:04
  • $\begingroup$ @AliceD. Sorry for the confusion. I've made some edits. A figure added to question shows one mechanism by which negative conductance is obtained. $\endgroup$ – Dilawar May 9 '15 at 19:27
  • $\begingroup$ With this figure you cannot really say with surety that slope reverses direction. More replicates and data points are required to prove that. $\endgroup$ – WYSIWYG Jun 1 '15 at 8:34
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This is the figure OP talks about:

enter image description here

Negative Conductance Caused by Entry of Sodium and Cesium Ions into the Potassium Channels of Squid Axons, Francisco Bezanill, Clay M. Armstrong (1972).

Short answer is that in non-linear mode of ion channel operation other ions start passing through it (channel loses specificity). Cumulative effect (because of different concentrations of different ions) is negative total conductance, if you don't measure currents of specific ions.

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The experiment described in the linked article Bazanilla & Armstrong (1972) is about a voltage clamp experiment in squid axon. Voltage clamping basically means that the potential difference across the axonal membrane can be set at will by an external artificial electronic power source.

NMDA receptors are channels that conduct positively charged ions (mainly Na+). If the voltage is clamped at negative potentials under physiological conditions, positive current will be drawn inward. This is generally plotted as a negative current, as shown in your picture in your post. If, however, the voltage is made more positive, current will reverse at a certain point, and positive currents will be measured by the electrodes. This positive current is characterized by an outward flow of positive ions through the NMDA receptor.

The sign of the current is arbitrary, but the most important point to make is that most channels allow current to pass both ways. There are rectifier channels, however, that will only allow one-way traffic. Rectifiers are more exceptions than the rule.

Linked reference
- Bazanilla & Armstrong, J Gen Physiol (1972); 60: 588-608

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