The resting membrane potential (RMP) is like the baseline electrical charge of a cell. It mainly depends on how certain tiny particles, called ions, are spread out across the cell's outer layer, called the membrane. Here’s a simple breakdown of how it works:

1. Main Ions: Three important ions play a big role in creating the RMP:

   • Potassium ions (K⁺): These ions are super important because the cell membrane lets them move in and out more easily than other ions. This makes them crucial for setting the RMP.
   • Sodium ions (Na⁺): There aren’t as many Na⁺ ions moving around during resting conditions, but they still have an effect on the cell's charge.
   • Chloride ions (Cl⁻): These ions help steady the cell's charge, but they aren’t as critical as K⁺ and Na⁺.

2. Ion Differences: The RMP is affected by the different amounts of these ions inside and outside the cell:

   • There is a high level of K⁺ inside the cell.
   • There is a lower level of Na⁺ inside the cell compared to the outside.

   This difference is kept up by a special pump in the cell membrane that pushes out 3 Na⁺ ions for every 2 K⁺ ions it brings in. This pump is called the sodium-potassium pump.

3. Understanding Ions: To see how these ions affect the RMP, we can use a formula called the Nernst equation. It helps us see the balance of charges:

   • For K⁺, the level is around -90 mV (millivolts), and for Na⁺, it’s about +60 mV.

4. Looking at Everything Together: To get a full picture, we can use another formula called the Goldman equation. This one counts how easily different ions can move across the membrane:

   • The equation takes into account the movement of both Na⁺ and K⁺ ions to find the overall RMP.

In short, the RMP is usually around -70 mV because K⁺ ions can move freely and there are specific differences in ion levels across the membrane. This resting charge is very important as it sets up all the electrical activities in the cell that happen afterward!