Resting potential

The resting membrane potential (RMP) refers to the steady electrical potential difference across the plasma membrane when the cell is in an unstimulated state. In most neurons, this potential is approximately –70 millivolts (mV), indicating that the interior of the cell is negatively charged relative to the extracellular fluid.

Ionic Basis of the Resting Membrane Potential

The RMP arises from the combined effects of ionic concentration gradients and selective permeability of the cell membrane.

Distribution of Ions

The intracellular and extracellular fluids differ markedly in their ionic composition:

• Potassium ions (K⁺): High concentration inside the cell
• Sodium ions (Na⁺): High concentration outside the cell
• Chloride ions (Cl⁻): Predominantly outside the cell
• Negatively charged proteins (A⁻): Confined within the cell

This unequal distribution is essential for the development of electrical potential.

Selective Permeability of the Membrane

The cell membrane is selectively permeable due to the presence of ion channels:

• It is highly permeable to K⁺ via potassium leak channels
• It is slightly permeable to Na⁺
• It is largely impermeable to intracellular proteins

Because of this, K⁺ diffuses out of the cell more readily than Na⁺ enters, leaving behind negatively charged proteins and contributing to the negative intracellular environment.

Sodium–Potassium Pump

The Na⁺/K⁺ ATPase pump is an active transport mechanism that plays a critical role in maintaining the RMP.

• Pumps 3 Na⁺ ions out of the cell
• Pumps 2 K⁺ ions into the cell
• Requires energy in the form of ATP

This pump:

1. Maintains the concentration gradients of Na⁺ and K⁺
2. Contributes slightly to the negativity of the resting potential (electrogenic effect)

Electrochemical Gradients

The movement of ions is governed by two forces:

1. Chemical gradient – drives ions from high to low concentration
2. Electrical gradient – attracts or repels ions based on charge

For example:

• K⁺ tends to diffuse out of the cell due to its concentration gradient
• However, the negative charge inside the cell attracts K⁺ back in

At equilibrium, these opposing forces balance.

Equilibrium Potential

The equilibrium (Nernst) potential is the membrane potential at which there is no net movement of a particular ion.

• For K⁺: approximately –90 mV
• For Na⁺: approximately +60 mV

Since the membrane is more permeable to K⁺, the RMP is closer to the potassium equilibrium potential.

Factors Influencing the Resting Membrane Potential

The value of the RMP can be altered by several factors:

• Changes in extracellular potassium concentration
• Alterations in membrane permeability
• Activity of the Na⁺/K⁺ pump
• Type of cell (neurons, muscle cells, etc.)

Functional Significance

The resting membrane potential is essential for:

• Maintaining cellular homeostasis
• Enabling the generation of action potentials
• Facilitating nerve impulse transmission and muscle contraction