CBNS 106 Lecture 3 Neuronal Membrane

Membrane Potential

Overall

• Membrane potential (Vm) = difference in electrical charge across the neuronal membrane

• Measured in millivolts (mV)

• Resting membrane potential ≈ -65 mV (inside negative)

💡 Why it matters:

• This electrical gradient is required for action potentials and signaling

Requirements for Resting Membrane Potential

1. Charge separation

• Membrane separates ions → creates voltage

2. Selective permeability

• Ion channels allow specific ions through

3. Concentration gradients

• Created + maintained by ion pumps

**Electrical vs Concentration gradient

a) driven by diff forces

b) At rest (-65mV) they are competing

• Resting membrane potential is created because K⁺ freely leaks out down its concentration gradient, leaving behind negative charges inside the cell, which then pull K⁺ back in via the electrical gradient; at rest these opposing forces are nearly balanced (with minimal Na⁺ leak), so the membrane stays at a stable negative voltage (~–70 mV)

Nature of the membrane

Chemical Environment (Water + Ions)

• Water = polar solvent

• Ions = charged particles:

  • Cations (+): Na⁺, K⁺

  • Anions (−): Cl⁻

💡 Key detail:

• Ions are hydrophilic (charged + surrounded by water)
  → Cannot cross lipid membrane alone

Phospholipid Bilayer + Proteins

• Lipid bilayer = hydrophobic barrier

• Prevents ion movement

👉 Movement requires proteins:

• Ion channels

• Ion pumps

• Receptors

💡 These proteins control all electrical signaling

**Because the lipid bilayer has a hydrophobic (nonpolar) core that repels charged, water-associated ions, ions cannot cross the membrane on their own and therefore require ion channels to provide a selective pathway for movement

How membrane protein create and control electrical activity

Protein Structure

a) made of amino acids

b) Types

1. Primary

2. Secondary

3. Tertiary

4. Quaternary

**structure is what allows to become channels, pumps, receptors

Ion channels/how ions move

a) proteins form pores (channels)

Types:

• Leak channels

  • always open

  • set resting membrane potential (VERY important)

  • Always open

  • Found throughout neuron

  • Cause high K⁺ permeability

    • K⁺ leaves cell → makes inside negative
      → Major driver of resting potential (Vm)

• Voltage-gated channels

  • open when voltage changes

  • action potentials

• Ligand-gated channels

  • open when neurotransmitters bind

  • synaptic signaling

Ion Pumps

• Na/K pump uses atp

• 3 Na in 2 K out

How Leak channels, VG channels, and Na/K pump all relate

• K over time degrades the gradient with constant leak

• VG channels only work when they detect change in electrical gradient

• The Na⁺/K⁺ pump maintains long-term Na⁺ and K⁺ concentration gradients by using ATP to counteract passive leak, ensuring the resting membrane potential and electrical excitability can be sustained. (WORKS CONSTANTLY AT REST)

Key Terms

1. Equilibrium Potential (Eion)

• The membrane voltage where no net movement of a specific ion occurs

• At this point:

  • Electrical force = Diffusion force

• Each ion has its own Eion (depends on gradient + charge)

💡 Think: “where the ion is perfectly balanced”

2. Driving Force

• The force that actually pushes ions across the membrane

• Formula:

  • Driving force = Vm − Eion

💡 Interpretation:

• Large value → strong ion movement

• Zero → no movement

3. Ion Flux

• The actual movement of ions across the membrane

• Happens only if:

  • Channels are open (permeability exists)

  • AND driving force is present

💡 Ion flux = what physically changes Vm

4. Membrane Permeability

• How easily an ion can cross the membrane

• Depends on:

  • Number of open channels

  • Type of channels (leak vs gated)

At rest:

• K⁺ >>> Cl⁻ > Na⁺ > Ca²⁺

💡 Means:

• K⁺ dominates membrane potential

5. Law of Permeability (CRITICAL RULE)
The membrane potential always moves toward the Eion of the ion with the highest permeability

💡 This is why K⁺ dominates resting potential

K+ and Na+ importance

1. Potassium (K⁺) – The Main Player

• High inside, low outside
  → Diffusion pushes K⁺ OUT

• Inside is negative
  → Electrical force pulls K⁺ IN

👉 Balance occurs at:

• EK ≈ -80 mV

2. Sodium (Na⁺) Comparison

• High outside, low inside
  → Diffusion pushes Na⁺ IN

👉 Equilibrium:

• ENa ≈ +60 mV

Important Table- Ion concentrations for a “typical” neuron

Plug into Nernst equations

Nernst Equations

Resting Membrane Permeability