Final answer:
Changing the internal potassium concentration [K]+ to 5mM would result in a less positive (more negative) Nernst potential for potassium (EK) due to the decreased ratio of [K]out/[K]in as per the Nernst equation.
Step-by-step explanation:
If the concentration of the internal potassium [K]+ is changed to 5mM, this will have an impact on the Nernst potential for potassium, often referred to as EK. The equation to calculate the Nernst potential for a monovalent ion such as potassium (K+) is given by the Nernst equation: EK = (RT/F) * ln([K]out/[K]in), where R is the gas constant, T is the temperature in Kelvin, F is the Faraday constant, and [K]out/[K]in are the external and internal concentrations of potassium, respectively. If [K]+in is increased, this would decrease the ratio of [K]out/[K]in, leading to a less positive (more negative) EK, since ln(x) decreases as x decreases. Conversely, if [K]+in is decreased, EK becomes more positive. This relationship shows the direct impact of changes in ion concentrations across the membrane on the membrane potential, and hence how neurons and muscle cells might react to alterations in ion gradients.