$A$ neuron becomes an electrically charged cell primarily due to the differential permeability and diffusion of which ion?

Given is the diagrammatic representation of impulse conduction through an axon (at points $A$ and $B$). View the diagram and arrange the steps of impulse conduction.
$I.$ The polarity of the membrane at site $A$ is reversed and depolarized,i.e.,the outer surface becomes negatively charged and the inner side becomes positively charged,generating a nerve impulse.
$II.$ $A$ stimulus causes a disturbance to the membrane at site $A$ of the nerve fibre,resulting in the leakage of $Na^+$ ions inside the nerve fibre.
$III.$ On the outer surface,current flows from site $B$ to site $A$ to complete the circuit of current flow. Hence,the polarity at the site is reversed,and an action potential is generated at site $B$. The impulse (action potential) generated at site $A$ arrives at site $B$. The sequence is repeated along the length of the axon and consequently,the impulse is conducted.
$IV.$ Immediately ahead,the axon (e.g.,site $B$) membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result,a current flows on the inner surface from site $A$ to site $B$.

To start the conduction of an impulse,the value of the action potential must not be

For the propagation of a nerve impulse along a nerve fiber,what is the sequence of change in the electrical potential difference across the axolemma?

The ionic gradients across the resting membrane are maintained by the

When a neuron is in a resting state,i.e.,not conducting any impulse,the axonal membrane is: