Cell Physiology II: Ions, Action Potential, & Electrolyte Imbalance

Cell Physiology II: Ions, Action Potential, & Electrolyte Imbalance

Objectives

  • Contrast Extracellular and Intracellular Fluid
    • Understand the differences between the two types of body fluids.
  • Describe the steps of an action potential
    • Detail the sequential events that culminate in an action potential.

Cell Membrane and the Fluids

  • Extracellular Fluid (ECF)
    • Fluid located outside of cells.
  • Intracellular Fluid (ICF)
    • Fluid contained within cells.

Fluid Distribution

  • Total Body Water Composition
    • Comprises approximately 60% of total body weight.
    • 60-40-20 Rule
    • 40% of total body weight is ICF (24 liters).
    • 20% of total body weight is ECF (16 liters - interstitial fluid + 4 liters - plasma).
    • Fluid Compartment Distribution
    • Body fluid is continuously exchanged among three compartments:
      1. Intracellular Fluid (ICF)
      2. Interstitial Fluid (IF)
      3. Plasma (intravascular fluid)

Ionic Composition of ICF and ECF

  • Sodium and Potassium Ions
    • Intracellular Fluid (ICF): High concentrations of Potassium ions (K+), low concentrations of Sodium ions (Na+).
    • Extracellular Fluid (ECF): High concentrations of Sodium ions (Na+), low concentrations of Potassium ions (K+).
  • Magnesium (Mg2+) and Calcium (Ca2+)
    • Present in both fluid compartments but vary in concentration:
    • K+ is often referred to metaphorically as “K-ing” the inside of the cell.

Transmembrane Potential Review

  • Reference Electrode Positioning
  • Measurement of Transmembrane Potential
    • Typical Values:
    • Resting potential: -70 mV (inside cell is negative relative to outside)
    • Potential can rise or fall to +70 mV during action potentials considering measuring across the membrane.

3 Requirements for Transmembrane Potential

  1. Concentration Gradient of Ions (Na+, K+)
  2. Selective Permeability through Channels
  3. Maintained Charge Difference Across Membrane
    • Resting potential is approximately -70 mV in neurons.

Mechanisms Affecting Transmembrane Potential

  • Response to Changes in Membrane Permeability
    • Changes in permeability can induce fluctuations in transmembrane potential.
  • Role of Membrane Channels
    • Sodium and potassium channels can either be passive (leak) or active (gated).

Passive Channels (Leak Channels)

  • Characteristics
    • Always open and allow continuous flow of ions.
    • Permeability may vary based on environmental conditions.

Na+ and K+ Leak Channels

  • Mechanism
    • K+ ion moves out of the cell, whereas Na+ ions move in.
    • The balance of both ions influences resting membrane potential, varying across different tissue types.

Active Channels (Gated Channels)

  • Definition
    • Open and close in response to specific stimuli.
    • Typically closed at resting potential, becoming active when triggered.

3 Conditions of Gated Channels

  1. Closed but Capable of Opening
  2. Open (Activated)
  3. Closed, Not Capable of Opening (Inactivated)

Types of Gated Channels

  • (a) Chemically Regulated Channels
    • Open upon binding with specific chemicals (e.g. Acetylcholine, ACh).
    • Primarily located on neuron cell bodies and dendrites.
    • Ligands are substances that induce change in the receptor.
  • (b) Voltage-Regulated Channels
    • Respond to changes in transmembrane potential, featuring both activation and inactivation gates.
    • Present in neural axons, skeletal muscle, and cardiac muscle.
  • (c) Mechanically Regulated Channels
    • Open in response to mechanical distortion.
    • Found in sensory receptors that detect touch, pressure, or vibration.

Action Potentials

  • Definition
    • Propagated, short-lasting changes in transmembrane potential signal conduction, affecting the entire excitable membrane.

Action Potential in Nerve Cells

  • Transmission
    • Signals sent from the cell body through the axon via changes in ion concentrations between ECF and ICF.

Propagation of Action Potential

  • Directionality
    • Action potentials propagate in one direction through the axon.

4 Steps in the Generation of Action Potentials

  1. Depolarization to Threshold
    • Threshold is roughly -55 mV.
    • Membrane potential undergoes rapid depolarization upon reaching threshold.
  2. Activation of Na+ Channels
    • At threshold, Na+ channels open leading to an influx of Na+ ions, causing further depolarization (inner membrane changes from negative to positive).
  3. Inactivation of Na+ Channels, Activation of K+ Channels
    • At +30 mV, inactivation gates on Na+ channels close while K+ channels open to initiate repolarization.
  4. Return to Normal Permeability
    • K+ channels close when returning to resting potential (-70 mV), hyperpolarizing the membrane to -90 mV briefly before stabilizing at resting levels.

All-or-None Principle

  • Definition
    • If a stimulus exceeds the threshold, action potential is triggered at a consistent magnitude regardless of stimulus strength.

The Refractory Period

  • Definition
    • A phase where a neuron cannot be re-stimulated immediately after an action potential.
    • Consists of two phases:
    1. Absolute Refractory Period
      • No stimulation can trigger another action potential.
    2. Relative Refractory Period
      • A stronger stimulus is required to initiate another action potential as the membrane potential edges back to normal.

Sodium-Potassium Exchange Pump

  • Functionality
    • Maintains concentration gradients of Na+ and K+ over time through active transport requiring ATP:
    • 1 ATP is used for each 2 K+ ions exchanged for 3 Na+ ions.
    • In the absence of ATP, neuronal function ceases.

Electrolyte Balance & Action Potentials

  • Balance Maintenance
    • Electrolyte balance is crucial for ideal action potential functioning, reflecting incoming versus outgoing electrolytes.
  • Implications of Imbalance
    • Imbalance can destabilize action potentials and response efficacy.

Electrolyte Imbalance Terminology

  1. Hypernatremia – High sodium levels.
  2. Hyponatremia – Low sodium levels.
  3. Hyperkalemia – High potassium levels.
  4. Hypokalemia – Low potassium levels.

Conclusion

  • Understanding the principles of ion movement and action potentials is critical for appreciating cellular communication and overall physiological function.

End of Notes