Membrane Potential and Resting Membrane Potential in Neurons

Membrane Potential

Definition and Overview

The membrane potential refers to the voltage resulting from charge separation between the interior and exterior of a cell. This electrical potential difference is fundamental to the function of excitable cells such as neurons and muscle cells.

• Resting Potential: The membrane potential of a cell when it is not stimulated. The resting membrane potential is typically negative, meaning the inside of the cell is more negative than the outside.

• At resting potential, the cell interior is more negative than the exterior.

Factors Contributing to Resting Membrane Potential

The resting membrane potential is established and maintained by several key factors:

• Ionic Composition: Differences in the concentration of ions (such as Na+, K+, Cl-) between the intracellular and extracellular fluids.

• Plasma Membrane Permeability: The selective permeability of the cell membrane to different ions, especially sodium (Na+) and potassium (K+).

• Na+/K+-ATPase Pump: This active transport protein maintains concentration gradients by pumping 3 Na+ ions out of the cell and 2 K+ ions into the cell, using ATP as an energy source.

Key Terms

• Resting Membrane Potential: The steady-state voltage across the cell membrane when the cell is not actively sending signals.

• Na+/K+-ATPase: An enzyme that uses energy from ATP to transport sodium and potassium ions against their concentration gradients.

• Leak Channels: Membrane proteins that allow passive movement of ions (especially K+ and Na+) down their concentration gradients.

Mechanism of Resting Membrane Potential Generation

• Potassium ions (K+) tend to leak out of the cell through K+ leak channels, making the inside of the cell more negative.

• Sodium ions (Na+) leak into the cell through Na+ leak channels, but to a lesser extent than K+ leaks out.

• The Na+/K+-ATPase pump actively transports Na+ out and K+ in, maintaining the concentration gradients necessary for the resting potential.

Relevant Equation

The Nernst equation can be used to calculate the equilibrium potential for a particular ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Where: $E_{ion}$ = equilibrium potential for the ion $R$ = universal gas constant $T$ = temperature in Kelvin $z$ = charge of the ion $F$ = Faraday's constant $[ion]_{outside}$ = concentration of the ion outside the cell $[ion]_{inside}$ = concentration of the ion inside the cell

Example Application

Question: Which of the following is the MOST accurate regarding the sodium potassium pump?

• The sodium potassium pump creates the resting potential by ejecting 3 Na+ ions and transporting 2 K+ ions into the cell.

• The sodium potassium pump is the main factor in creating resting potential.

• The sodium potassium pump directly impacts resting potential by allowing more negatively charged ions into the cell.

• The sodium potassium pump stabilizes resting potential in a neuron.

Correct Answer: The sodium potassium pump stabilizes resting potential in a neuron by maintaining the necessary ion gradients.

Practice Question

Which of the following is the MOST important factor in generating resting membrane potential?

Table: Comparison of Key Contributors to Resting Membrane Potential

Factor	Role
Ionic Composition	Creates concentration gradients for Na+, K+, and other ions
Membrane Permeability	Allows selective movement of ions via leak channels
Na+/K+-ATPase Pump	Maintains ion gradients by active transport

Additional info: The resting membrane potential in neurons is typically around -70 mV, as shown in the diagram. The sodium-potassium pump is essential for long-term maintenance of this potential, but the immediate value is primarily determined by the permeability of the membrane to potassium ions.