뒤로Ch 5c Membrane Potential and Dynamics: Study Notes for Anatomy & Physiology
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Membrane Potential and Dynamics
Introduction
The membrane potential is a fundamental concept in human physiology, describing the electrical potential difference across a cell's plasma membrane. It is essential for the function of excitable cells such as neurons and muscle cells. This section explores the basis of membrane potential, its generation, and its physiological significance.
The Resting Membrane Potential
The resting membrane potential refers to the steady-state electrical charge difference across the cell membrane when the cell is not actively sending signals.
Electrically Neutral Body: The human body as a whole is electrically neutral, but there is a chemical disequilibrium between the intracellular fluid (ICF) and extracellular fluid (ECF).
Charge Distribution: The ICF has a net negative charge, while the ECF has a net positive charge.
Illustration: In diagrams, this is shown as excess positive charge (cations) outside the cell and excess negative charge (anions) inside the cell.
Key Terms:
ICF: Intracellular fluid, the fluid inside the cell.
ECF: Extracellular fluid, the fluid outside the cell.
Cation: Positively charged ion (e.g., Na+, K+).
Anion: Negatively charged ion (e.g., Cl-, A-).
Electricity Review
Understanding membrane potential requires basic knowledge of electrical principles:
Law of Conservation of Electrical Charges: The total amount of electrical charge in the universe is constant.
Opposite Charges Attract: Positive and negative charges attract each other; like charges repel.
Energy Requirement: Separating positive and negative charges requires energy.
Conductors vs. Insulators: Conductors allow free movement of charges (e.g., body fluids), while insulators restrict movement (e.g., cell membranes).
Cell Membrane Enables Separation of Electrical Charge
The cell membrane, composed of a phospholipid bilayer, acts as a barrier that separates charges between the ICF and ECF.
K+ Movement: Potassium ions move out of the cell down their concentration gradient.
Anion Impermeability: Large anions cannot follow K+ out because the membrane is impermeable to anions.
Electrical Gradient: This selective movement establishes an electrical gradient across the membrane.
Electrochemical Gradient and Equilibrium Potential
The electrochemical gradient is the combined effect of electrical and concentration gradients on ion movement.
Equilibrium Potential (Eion): The membrane potential at which the electrical force exactly opposes the concentration gradient for a particular ion.
Nernst Equation: Used to calculate the equilibrium potential for any ion:
Where z is the charge of the ion, [ion]out is the extracellular concentration, and [ion]in is the intracellular concentration.
Ion Distribution in a Neutral Cell
At the start, when the cell has no membrane potential, the distribution of ions is balanced:
Ion | Location | Charge |
|---|---|---|
Sodium (Na+) | ECF | Positive |
Potassium (K+) | ICF | Positive |
Chloride (Cl-) | ECF | Negative |
Large Anions (A-) | ICF | Negative |
Additional info: This table summarizes the main ions contributing to membrane potential in a typical cell.
Summary of Key Concepts
Resting Membrane Potential: The steady-state voltage across the cell membrane, typically -70 mV in neurons.
Ion Gradients: Maintained by active transport (e.g., Na+/K+ ATPase pump).
Electrochemical Gradient: Drives ion movement and is essential for cell signaling.
Equilibrium Potential: Calculated for each ion using the Nernst equation.
Example: In neurons, the resting membrane potential is crucial for the generation and propagation of action potentials, which underlie nerve signaling.
