The correct option is 3 Na+ out, 2 K+ in.
Explanation
The sodium-potassium pump ($Na^+$-$K^+$ ATPase) is a vital transmembrane protein found in the plasma membrane of neurons. It functions as a primary active transport mechanism, utilizing energy from ATP hydrolysis to move ions against their concentration gradients.
Mechanism of Action:
- Sodium Transport: The pump binds intracellular sodium ions. Upon the hydrolysis of one ATP molecule, the pump undergoes a conformational change and transports 3 Sodium ions ($Na^+$) out of the cell.
- Potassium Transport: Subsequently, the pump binds extracellular potassium ions and transports 2 Potassium ions ($K^+$) into the cell as it returns to its original conformation.
This stoichiometry (3 positive charges exiting for every 2 positive charges entering) creates a net deficit of positive charge inside the neuron. This electrogenic nature is fundamental in establishing and maintaining the resting membrane potential (typically -70 mV) required for nerve impulse transmission.
Key Takeaway: The specific stoichiometry of the sodium-potassium pump is 3 $Na^+$ OUT and 2 $K^+$ IN per ATP molecule.