Action Potential & Nerve Conduction

1. Resting Membrane Potential (Background)

• The Na⁺-K⁺ pump (3 Na⁺ out, 2 K⁺ in per cycle)
• K⁺ leak channels (K⁺ leaks out, making inside more negative)
• Na⁺ is high outside, K⁺ is high inside

2. What is an Action Potential?

3. Phases of the Action Potential

Phase 1 - Resting Stage (-70 mV)

• Membrane is polarized
• Activation gates of Na⁺ channels: CLOSED
• K⁺ channels: mostly closed

Phase 2 - Depolarization

• A stimulus brings the membrane potential to threshold (~-55 mV, roughly 15 mV above resting)
• At threshold, voltage-gated Na⁺ channels snap open - sodium permeability increases 500 to 5000-fold
• Na⁺ rushes IN (because it is both electrically and chemically attracted inward)
• Membrane potential shoots up rapidly to +35 mV (overshoot)

Phase 3 - Repolarization

• Within a fraction of a millisecond, the inactivation gate of Na⁺ channels closes (slower gate)
• Simultaneously, voltage-gated K⁺ channels open (delayed)
• K⁺ rushes OUT, pulling the potential back toward -70 mV
• The membrane repolarizes

Phase 4 - Hyperpolarization (Undershoot)

• K⁺ channels may stay open slightly too long
• Membrane dips briefly below -70 mV (to about -80 mV)
• Returns to resting once K⁺ channels close

4. The Voltage-Gated Channels (Key Mechanism)

Sodium Channel - Two Gates

Potassium Channel - One Gate

• Opens delayed compared to Na⁺ channel
• Stays open during repolarization
• Responsible for driving the membrane back to negative and causing hyperpolarization

5. Refractory Periods

6. All-or-Nothing Law

• Subthreshold stimulus → no AP
• Threshold stimulus → full AP (always the same amplitude and duration)
• Superthreshold → same full AP (not bigger)

7. Propagation of the Action Potential (Conduction)

1. Na⁺ rushes in, and positive charge spreads inside the axon to the adjacent segment
2. That segment reaches threshold → its Na⁺ channels open → another AP fires there
3. This continues down the axon to the axon terminal

8. Myelination & Saltatory Conduction

The Problem with Thick Unmyelinated Axons

• Thick axons conduct faster BUT take up enormous space
• To achieve the same speeds via size, the human brain would be impossibly large

The Solution: Myelin

• Acts as electrical insulation
• Reduces membrane capacitance 50-fold
• Forces ionic current to flow down the inside of the axon rather than leaking out

Saltatory Conduction (AP "jumps" node to node)

• Action potentials only occur AT the nodes
• Electrical current flows down the axoplasm from one node, skipping the myelinated internodes (which can be 0.2-2.0 mm long)
• The AP jumps from node to node ("saltare" = to leap in Latin)

1. Speed - conduction velocity increases 5 to 50-fold compared to unmyelinated fibers
2. Energy efficiency - only nodes depolarize, so ~100x fewer ions cross the membrane → much less work for the Na⁺-K⁺ pump to restore gradients

9. Conduction Velocity - Quick Reference

10. Clinical Connections

Summary Flow Chart

Stimulus → Threshold (-55 mV) reached
         ↓
Na⁺ channels OPEN → Na⁺ rushes IN → Depolarization (+35 mV)
         ↓
Na⁺ inactivation gate CLOSES + K⁺ channels OPEN
         ↓
K⁺ rushes OUT → Repolarization → slight Hyperpolarization
         ↓
Channels close → Returns to Resting (-70 mV)
         ↓
Positive charge spreads ahead → Next patch reaches threshold
         ↓
In myelinated fiber → jumps node to node (SALTATORY CONDUCTION)