Overview
This lecture provides an overview of membrane potentials, focusing on the differences between diffusion and equilibrium potential, and introduces how ionic gradients generate electrical potentials in nerve and muscle physiology.
Diffusion Potential vs. Equilibrium Potential
- Diffusion potential is the voltage generated when an ion moves down its concentration gradient across a permeable membrane.
- An ion must have membrane permeability to create a diffusion potential; no permeability means no potential.
- Movement of charged ions results in electric current, creating a potential difference.
- Equilibrium potential is the membrane voltage when the ion's net movement stops (equilibrium is reached).
- The equilibrium potential is determined by the ion concentrations and membrane permeability and can be calculated using the Nernst equation.
Ionic Gradients and Membrane Potential
- Potassium (K⁺) is more concentrated inside the cell; sodium (Na⁺) is more concentrated outside.
- K⁺ tends to move out of the cell, making the inside more negative (negative diffusion potential).
- At K⁺ equilibrium, the membrane potential is about -94 mV.
- Na⁺ tends to move into the cell, making the inside more positive.
- At Na⁺ equilibrium, the membrane potential is about +61 mV.
- Both chemical (concentration) and electrical gradients affect ion movement direction and strength.
Membrane Potential Generation
- Membrane potential arises from a difference in electric charge across the membrane.
- Sources include passive ion diffusion (e.g., open Na⁺ or K⁺ channels) and active transport (e.g., sodium-potassium ATPase).
- Sodium-potassium ATPase pumps out 3 Na⁺ and brings in 2 K⁺, contributing to net negative charge inside.
- The cell membrane is more permeable to K⁺ due to potassium leak channels, making K⁺ essential for the resting membrane potential.
Key Terms & Definitions
- Diffusion Potential — Voltage from ion movement down its concentration gradient if the membrane is permeable.
- Equilibrium Potential — The membrane voltage when net ion movement stops (calculated by Nernst equation).
- Sodium-Potassium ATPase — An enzyme pumping 3 Na⁺ out and 2 K⁺ in, making the inside of the cell more negative.
- Potassium Leak Channels — Membrane channels allowing K⁺ to move freely, increasing K⁺ permeability at rest.
Action Items / Next Steps
- Review the difference between diffusion and equilibrium potentials.
- Understand how the Nernst equation is used to calculate equilibrium potentials.
- Prepare for a deeper discussion of resting membrane potential and action potentials in the next lecture.