Question

You are recording from a neuron that has without current injection a tonic firing pattern of action potentials. While you inject a long pulse of a depolarizing current, the spikes that the cell produces are first quite large, but become wider over time, their amplitude decreases, and eventually there is no response any more. It looks like of like this:

wwwu inject positive current to derpolarize

For a moment you then hyperpolarize the cell. Then you depolarize the cell again, and you see the same thing happening all over again. What is most likely going on?

*NOTE: MORE THAN ONE ANSWER COULD BE CORRECT, PLEASE SELECT ALL CORRECT ANSWERS*

Answer 1

The concentration of sodium inside the cell becomes depleted.

For sodium channels to reactivate the membrane potential needs to repolarize below a certain level which is not happening during the depolarizing current injection.

Both the height and the rising rate of action potential decreases when the concentration of extracellular sodium was reduced,

An action potential is a transient, electrical signal, which is caused by a rapid change in resting membrane potential (-70 mV). This occurs when the threshold potential (-55 mV) is reached, this causes a rapid opening in the voltage-gated sodium channels leading to an influx of sodium ions into the cell.