Q.Explain the biological role of $\text{Na}^+$ and $\text{K}^+$ ions in the transmission of a nerve impulse.

Sodium and potassium ions are distributed very unevenly across the membrane of a nerve cell: $\text{Na}^+$ is the principal cation of the fluid outside the cell (the extracellular fluid), while $\text{K}^+$ is the principal cation of the fluid inside the cell (the intracellular fluid). This concentration gradient does not arise passively -- it is continuously, actively maintained against the natural tendency of ions to diffuse and equalize, by a membrane-bound enzyme called the $\text{Na}^+$/$\text{K}^+$-ATPase (the sodium-potassium pump), which uses the chemical energy of ATP hydrolysis to pump three $\text{Na}^+$ ions out of the cell for every two $\text{K}^+$ ions it pumps back in.

This carefully maintained ion gradient is exactly what makes nerve-impulse transmission possible. At rest, the nerve cell membrane is polarized, with the inside slightly negative relative to the outside. When a stimulus arrives, voltage-gated $\text{Na}^+$ channels in the membrane briefly open, and $\text{Na}^+$ ions rush down their concentration gradient into the cell, sharply reversing (depolarizing) the local membrane potential -- this rapid, propagating change in potential is the electrical signal called the action potential, and it travels rapidly along the length of the nerve fibre. …

Describe the resting distribution of Na+/K+ maintained by the ATPase pump, then walk through the sequence of channel openings (Na+ in = depolarization/action potent …

Reversing which ion is extracellular vs intracellular (Na+ outside, K+ inside -- a commonly swapped fact); omitting the active role of th …