Neurology | Resting Membrane, Graded, Action Potentials

Introduction

This tutorial provides a comprehensive overview of resting membrane potentials, graded potentials, and action potentials in neurons. Understanding these concepts is essential for anyone studying neurology or physiology, as they are fundamental to how neurons communicate and operate.

Step 1: Understand Resting Membrane Potential

• Definition: The resting membrane potential is the voltage difference across a neuronal cell membrane when the neuron is at rest, typically around -70 mV to -90 mV.

• Key Components:

  • Exists in all cells, not just neurons.
  • Maintained by ion concentration gradients and membrane permeability.

• Establishment Mechanisms:

  • Sodium-Potassium ATPases:
    • Pumps 3 sodium ions (Na+) out and 2 potassium ions (K+) into the cell.
    • Creates a negative charge inside the cell.
  • Leaky Potassium Channels:
    • Allow K+ to move out of the cell, making the inside more negative.
  • Leaky Sodium Channels:
    • Permit a small amount of Na+ to enter the cell, slightly depolarizing it.

Step 2: Graded Potentials

• Definition: Graded potentials are changes in membrane potential that can either depolarize (make more positive) or hyperpolarize (make more negative) the neuron.

• Types:

  • Excitatory Postsynaptic Potential (EPSP):

    • Caused by excitatory neurotransmitters (e.g., glutamate) binding to receptors, allowing cations like Na+ to enter.
    • Moves the membrane potential closer to the threshold (around -55 mV).

  • Inhibitory Postsynaptic Potential (IPSP):

    • Caused by inhibitory neurotransmitters (e.g., GABA) binding to receptors, allowing Cl- to enter or K+ to exit.
    • Moves the membrane potential further away from the threshold.

• Summation:

  • Temporal Summation: Multiple EPSPs from one presynaptic neuron add together over time to reach the threshold.
  • Spatial Summation: EPSPs from multiple presynaptic neurons occur simultaneously, contributing to reaching the threshold.

Step 3: Action Potentials

• Definition: An action potential is a rapid, transient change in the membrane potential that propagates along the axon.
• Phases:
  1. Threshold Reached: When the membrane potential reaches -55 mV, voltage-gated sodium channels open.
  2. Depolarization: Na+ rushes in, making the inside of the cell positive (approximately +30 mV).
  3. Repolarization: At +30 mV, sodium channels close and voltage-gated potassium channels open, allowing K+ to exit the cell, returning the membrane potential to negative.
  4. Hyperpolarization: The cell becomes more negative than resting potential (often around -90 mV) due to slow closing of K+ channels.
  5. Return to Resting Potential: The membrane potential stabilizes back to -70 mV through the action of sodium-potassium pumps and leak channels.

Step 4: Refractory Periods

• Absolute Refractory Period: From the peak of the action potential until the cell returns to resting potential, the neuron cannot be stimulated again.
• Relative Refractory Period: Following the absolute period, a stronger-than-usual stimulus can initiate another action potential, but it requires more effort due to hyperpolarization.

Conclusion

In this tutorial, we've covered the essential concepts of resting membrane potentials, graded potentials, and action potentials, alongside their mechanisms and significance in neuronal communication. Understanding these processes is crucial for further studies in neurobiology and related fields. To deepen your knowledge, consider exploring the methods for measuring these potentials in lab settings or their implications in neurological diseases.