Final answer:
Increasing the sodium ion permeability (PNa) to 0.4 would typically lead to depolarization of the cell membrane potential (Vm) due to the influx of positively charged sodium ions.
Step-by-step explanation:
When considering what happens to membrane potential (Vm) when the permeability (PNa) of the membrane to sodium ions is changed, we turn to the Goldman-Hodgkin-Katz (GHK) voltage equation. Although the complete equation is not provided in the question, this equation relates membrane permeabilities of various ions to the membrane potential. If PNa is increased to 0.4, it suggests that sodium permeability has increased, which would typically result in a depolarization of the membrane potential. This is due to sodium ions having a positive charge and the intracellular environment typically being negatively charged compared to the extracellular environment. With more positive charge entering the cell, the inside becomes less negative, hence the depolarization.