Membrane and Equilibrium Potentials

Membrane Potential: Definition and Concepts

The membrane potential is the voltage difference across a cell's plasma membrane, resulting from the unequal distribution of ions between the intracellular fluid (ICF) and extracellular fluid (ECF). It is fundamental to the function of excitable cells such as neurons and muscle cells.

• Definition: The electrical potential difference (voltage) across the cell membrane.

• Units: Millivolts (mV).

• Polarized Membrane: A membrane is said to be polarized when there is a difference in electrical charge across it (typically, the inside is negative relative to the outside).

• Depolarization: A decrease in membrane potential (the inside becomes less negative).

• Hyperpolarization: An increase in membrane potential (the inside becomes more negative).

• Repolarization: The return of the membrane potential to its resting value after depolarization.

Example: If a cell's membrane potential changes from -70 mV to -50 mV, this is depolarization.

Ion Concentrations and Membrane Potential

The membrane potential is influenced by the relative concentrations of ions such as Na+, K+, Cl-, and A- (anionic proteins) inside and outside the cell.

• Typical Ion Distribution: High K+ inside, high Na+ and Cl- outside.

• Resting Membrane Potential: Usually negative (e.g., -70 mV in neurons).

• ICF vs. ECF: The ICF is typically more negative than the ECF.

Example: In a healthy human cell, the K+ concentration is higher inside the cell, while Na+ is higher outside.

Movement of Ions Across the Membrane

Ions move across the membrane via channels, driven by two forces:

• Concentration Gradient: Ions move from high to low concentration.

• Electrical Gradient: Ions move toward the opposite charge.

Example: K+ tends to move out of the cell (down its concentration gradient) but is pulled back in by the negative charge inside the cell.

Equilibrium Potential

The equilibrium potential for an ion is the membrane potential at which the net flow of that ion across the membrane is zero (electrochemical equilibrium).

• Nernst Equation: Used to calculate the equilibrium potential for a specific ion:

• Where R is the gas constant, T is temperature in Kelvin, z is the charge of the ion, F is Faraday's constant, and [ion]out and [ion]in are the ion concentrations outside and inside the cell, respectively.

• Sign of Equilibrium Potential: Determined by the direction of the concentration gradient and the charge of the ion.

Example: For K+, the equilibrium potential is typically around -90 mV.

Goldman-Hodgkin-Katz (GHK) Equation

The GHK equation calculates the membrane potential considering the permeability of the membrane to multiple ions:

• P represents the permeability of the membrane to each ion.

• The resting membrane potential is most influenced by the ion with the highest permeability (usually K+).

Neuronal Physiology

Structure and Function of Neurons

Neurons are specialized cells for communication in the nervous system. They have three primary functions:

• Input: Receiving signals (dendrites).

• Integration: Processing signals (cell body).

• Output: Sending signals (axon).

Example: Sensory neurons receive input from the environment, integrate the information, and send output to the central nervous system.

Flow of Information in Neurons

• Information flows from dendrites (input) → cell body (integration) → axon (output).

• Neurons communicate via electrical and chemical signals.

Action Potential

Generation and Propagation

An action potential is a rapid, temporary change in membrane potential that travels along the axon of a neuron.

• Phases: Depolarization, repolarization, and hyperpolarization.

• Voltage-Gated Channels: Na+ and K+ channels open and close in response to changes in membrane potential.

• Directionality: Action potentials travel from the axon hillock to the axon terminal.

Example: During depolarization, Na+ channels open, allowing Na+ to enter the cell, making the inside more positive.

Key Points for Understanding Action Potentials

• Understand the se in unidirectional propagation.

Osmolarity and Tonicity

Definitions and Concepts

Osmolarity is the total concentration of solute particles in a solution, measured in osmoles per liter (Osm/L or mOsm/L). Tonicity refers to the effect of a solution on cell volume, determined by the concentration of non-penetrating solutes.

• Isotonic: No net movement of water; cell volume remains constant.

• Hypotonic: Water enters the cell; cell swells.

• Hypertonic: Water leaves the cell; cell shrinks.

Example: Placing a red blood cell in a hypertonic solution causes it to shrink.

Calculating Osmolarity and Tonicity

• Convert molarity to osmolarity for each solute (if necessary).

• Calculate total osmolarity for each compartment (ICF and ECF).

• Move only penetrating solutes until equilibrium is reached.

• Recount osmolarities after equilibrium to determine water movement.

Example: If ECF is hypertonic relative to ICF, water will move out of the cell.

Common Solutes

• Penetrating solutes: Urea, glucose (in some contexts), hormones.

• Non-penetrating solutes: NaCl, KCl (do not cross the membrane easily).

Typical values: Intracellular osmolarity of healthy human cells is about 300 mOsm.

Summary Table: Osmolarity and Tonicity

Additional Info

• Remember that the Na+-K+-ATPase pump maintains the resting membrane potential by moving 3 Na+ out and 2 K+ in, consuming ATP.

• Organic solutes like glucose and hormones may be considered penetrating or non-penetrating depending on context.

• Be able to apply these concepts to laboratory scenarios, such as red blood cell experiments.