Resting Membrane Potential, Graded Potentials, and Action Potentials

Jun 20, 2024

Resting Membrane Potential, Graded Potentials, and Action Potentials

Overview

  • This lecture covers resting membrane potentials, graded potentials, and action potentials of neurons.
  • Importance of hitting like, subscribing, and checking social media links.

Resting Membrane Potential

  • Definition: Voltage difference across the cell membrane at rest.
    • Applies to all cells, but focused on neurons here.
    • Typical range: -70 mV to -90 mV (average: -70 mV).

How is Resting Membrane Potential Achieved?

  1. Sodium-Potassium ATPases
    • Pumps 3 Na⁺ out and 2 K⁺ in.
    • Creates a small negative charge inside.
    • Establishes concentration gradients (high Na⁺ outside, high K⁺ inside).
  2. Leaky Potassium Channels
    • Always open, allowing K⁺ to move out passively, making the inside more negative.
    • Potassium bound to anions (phosphates and proteins) inside the cell.
  3. Leaky Sodium Channels
    • Allow Na⁺ to move into the cell passively but less permeable than K⁺.

Important Concepts

  • Nernst Potential Calculation
    • Applies when ion movement due to electrostatic and concentration gradients are balanced.
    • Equation: $$ E = \frac{61.5}{z} \log_{10} \left(\frac{[Ion]{outside}}{[Ion]{inside}} \right) $$
    • Example: Equilibrium potentials for K⁺ (-90 mV) and Na⁺ (+70 mV).

Graded Potentials

  • Purpose: Alter the resting membrane potential to move closer to or away from threshold potential.
    • Excitatory (depolarizing, EPSP) vs. Inhibitory (hyperpolarizing, IPSP).
  • Mechanism: Involves neurotransmitters binding to ligand-gated ion channels.
    • EPSP: Typically involves Na⁺ or Ca²⁺ moving into the cell.
    • IPSP: Typically involves Cl⁻ moving in or K⁺ moving out.
  • Temporal Summation: Repeated stimuli from one neuron.
  • Spatial Summation: Multiple simultaneous stimuli from different neurons.

Action Potentials

Key Steps

  1. Initial Conditions: Resting membrane potential at -70 mV.
  2. Threshold Potential: Requires depolarization to -55 mV via EPSP.
  3. Depolarization
    • Voltage-gated Na⁺ channels open, Na⁺ rushes in, membrane potential shifts to +30 mV.
    • Channel States:
      • Rest: Activation gates closed, inactivation gates open.
      • Depolarizing: Both gates transition (activation opens, inactivation starts closing).
      • Peak: Activation gates open, inactivation gates closed.
  4. Repolarization
    • Voltage-gated K⁺ channels open, K⁺ leaves, potential drops back to -90 mV.
    • Channels are slow to close, slight hyperpolarization occurs.
    • Resting membrane potential restored by Na⁺/K⁺ ATPases and leaky channels.
  5. Refractory Periods:
    • Absolute: No new APs can be generated (from peak to resting).
    • Relative: Requires stronger than normal stimulus (hyperpolarization to resting).

Summary

  • Resting membrane potential: Negative inside, essential for neuron function.
  • Graded potentials: Small changes, can sum to affect action potentials.
  • Action potentials: All-or-none, travel along axon, essential for neuron signaling.