$A$ nerve impulse will travel through a nerve fibre only if the membrane suddenly becomes more permeable to

Assertion: The imbalance in concentration of $Na^+$,$K^+$ and proteins generates resting potential.
Reason: To maintain the unequal distribution of $Na^+$ and $K^+$,the neurons use electrical energy.

For visual sense,the nerve impulse is generated by

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 $B$.

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

During the propagation of a nerve impulse,the action potential results from the movement of