Resting Membrane Potential

Our body is made up of 60% of water. The water present in our body is divided into two main compartments:

1. Intracellular fluid compartment
2. Extracellular fluid compartment

The Intracellular fluid compartment consists of fluid which is present inside the cell.

The Extracellular fluid compartment consists of fluid which is present outside the cell but inside the body.

Intra and Extracellular fluid compartments

The concentration of ions in intra and extracellular fluid compartments varies. 

The Intracellular fluid compartment consists of more concentration of Potassium ions and nondiffusible protein molecules

The Extracellular fluid compartment consists of greater concentration of Sodium and Chloride ions.

The varying distribution ions on either side of the cell membrane plays a major role in generation of electrical potential which form the basis for cell integration and communication.

Resting Membrane Potential (RMP)

It is the potential difference across the cell membrane when the cell is at rest. It is more negative inside and keeps the cell in an excitable state. 

• RMP of a motor nerve is about -79 mV
• RMP of a muscle ranges from -80 to -90 mV

Molecular basis of Resting Membrane Potential

The concentration of ions inside the cell and outside the cell is varies. 

• Sodium ion concentration outside the cell is 150 mmol/L
• Sodium ion concentration inside the cell is 15 mmol/L
• Potassium ion concentration outside the cell is 5.5 mmol/L
• Potassium ion concentration inside the cell is 150 mmol/L 

Sodium ion concentration is greater outside as compared to inside and Potassium ion concentration is greater inside as compared to outside the cell. 

This causes a natural concentration gradient for sodium ions to move inward and potassium ions to move outward the cell.

• When the cell is at rest the permeability of cell membrane is 50 to 75 greater for Potassium ions due to which Potassium ions move out of the cell taking away the positive charges from within the cell making it more negative.
• The Sodium-Potassium ATPase pump present in almost all the cells of the body pumps 3 sodium ions to the outside in exchange for 2 Potassium ions which move inside. The difference in pumping of 3 sodium and 2 potassium ions makes the inside of the cell more negative
• The large nondiffusible negatively charged protein molecules present in the cell add negativity to the inside of the cell.