action potential, Na+/K+

Neuron Development of an Electrical Impulse (Action Potential)

  • Discussion Overview
    • Focuses on neuron capabilities to generate electrical impulses known as action potentials.
    • Initial explanations include background on cellular properties and charge differences across the neuron membrane.

Key Concepts

Neurons as Electrical Cells
  • Neurons are specialized cells that can send electrical impulses due to charge differences between their interior and exterior.
  • There exist charged ions both inside and outside the neuron membrane.
    • Outside: Higher concentration of positive ions.
    • Inside: Higher concentration of negatively charged ions.
  • Cell Membrane Potential
    • The inside of a neuron is more negative than the outside.
    • This difference is crucial as it provides the potential to trigger an electrical impulse.
Understanding Ion Charges
  • Ions: Charged atoms or molecules.
    • Cations: Carry a positive charge (e.g., sodium, potassium, calcium).
    • Anions: Carry a negative charge (e.g., chloride).
Cell Membrane Functionality
  • Ion Channels: Proteins embedded in the cell membrane that control ion movement.
    • Selective Permeability: Ion channels allow specific ions to pass through while prohibiting others based on their type.
    • Types of Ion Channels:
    • Always Open: Allow continuous movement of ions (e.g., sodium channel).
    • Gated Channels: Usually closed but can open upon specific stimuli.
Resting Membrane Potential
  • Resting Membrane Potential of a neuron is approximately -70 mV.
    • Represents the voltage difference when a neuron is not actively sending impulses.
Ion Concentration Differences
  • Sodium (Na+):
    • More concentrated outside the cell, high drive to enter (diffusion).
  • Potassium (K+):
    • More concentrated inside the cell, high drive to exit (diffusion).
  • Permeability: The resting neuron is more permeable to potassium, leading to more potassium diffusing out than sodium diffusing in.
Sodium-Potassium Pump
  • Actively transports ions against their concentration gradient.
    • Pumps 3 Na+ ions out for every 2 K+ ions in.
    • This activity contributes to the internal negativity of the neuron.
  • Fixed Anions:
    • Anions inside the cell are unable to leave, contributing to negative charge.
Summary of Factors Contributing to Resting Potential
  • The combination of sodium entering the cell and more potassium leaving results in a net negative charge inside due to:
    • Sodium diffusion in (less than potassium diffusion out).
    • Unequal transport by the sodium-potassium pump.
    • Presence of immobile fixed anions.

Mechanism of Action Potential

  • Gated ion channels open in response to stimuli, causing a rapid influx of sodium ions into the cell, changing the internal charge.
    • This process is termed depolarization.
    • Changes resting potential from -70 mV to a peak around +30 mV.
Threshold Potential
  • Threshold is defined as -55 mV which must be reached for action potential to occur.
    • A stimulus must be strong enough to depolarize the neuron to this level for wave propagation (firing of action potential).
Phases of Action Potential
  1. Depolarization:
    • Sodium channels open, allowing Na+ to rapidly enter the cell (positive feedback loop).
    • Neuron becomes more positive.
  2. Repolarization:
    • Sodium channels close, potassium channels open.
    • K+ exits the cell, restoring the negative charge.
  3. Hyperpolarization:
    • Excess K+ may leave, making the neuron more negative than -70 mV.
    • Eventually, sodium-potassium pumps return ions to resting state, completing the cycle.
Action Potential Graphs and Timing
  • Action potentials occur rapidly, typically within 1 millisecond.
  • Depolarization, repolarization, and hyperpolarization phases are observable on a voltage-time graph.
  • Key landmarks in the graph include:
    • Resting Membrane Potential: -70 mV
    • Threshold Potential: -55 mV
    • Peak Action Potential: +30 mV
Significance of Action Potential
  • Action potentials enable neurons to transmit information and can be triggered by stimuli such as neurotransmitters, physical pressure, or light.
  • Comparable to a domino effect: reaching the threshold activates further ion channels, propagating the impulse.
Implications and Summary

  • A solid understanding of the ion dynamics through channels and pumps is essential to grasp the electrical signalling in neurons.

  • Action potentials are fundamental to communication within the nervous system and underpin complex processes such as movement and sensation.

  • Reinforcement of the link between stimulus and action potential generation is critical for various biological and physiological applications.