Final answer:
An increase in extracellular sodium concentration would make the equilibrium potential for sodium more positive, because the ions would drive a greater influx into the cell, increasing the cell's interior positive charge relative to the exterior.
Step-by-step explanation:
If the concentration of sodium in the extracellular fluid increased, then the equilibrium potential for sodium would become more positive.
This outcome is based on the principles of the Nernst equation, which predicts the equilibrium potential of a particular ion based on its concentration gradient across a membrane. In the resting state, because sodium (Na+) is at higher concentrations outside the cell, it will tend to move into the cell if channels are available. An increase in extracellular sodium concentration will enhance the concentration gradient, thereby driving more sodium into the cell when channels open, and creating a more positive potential inside the cell as sodium ions are positively charged.
For example, during an action potential, the influx of sodium through voltage-gated channels significantly increases the membrane potential from a resting negative value towards and above zero, often reaching around +30 mV. This process is critical to the initiation and propagation of electrical signals in nerves and muscles. Thus, if the sodium concentration outside the cell were to increase, it would result in a more positive equilibrium potential, assuming all other factors remain constant.