action potential?

Question

why an action potential is triggering by injecting positive current into a neuron, but not by injecting negative current?

Answer 1

Injecting positive current into a neuron causes the membrane potential to rise. By raising this membrane potential we bring it closer to threshold (0) and therefore once it passes this it activates the all or nothing potential, or an action potential. This activates the sodium channels which causes the quick rise of the potential to a positive number.

If we inject negative current, we bring the cell further away from threshold and can not activate the sodium channels. So if you inject negative current, you bring it further away from threshold.

Good luck and hope this helps!

Answer 2

That is because the action potential is dependent upon voltage sensitive ion channels. If the voltage is negative, the channels do not allow ions to pass, and no action potential gets propagated.