Conduction Modes: Continuous vs Saltatory

Continuous Conduction (unmyelinated)

• Definition: Action potential propagates along the entire axon membrane without skipping due to absence of myelin.
• Mechanism: local currents depolarize adjacent segments; each segment regenerates the AP so the signal maintains its strength along the full length.
• Ion channels: distributed along the entire membrane.
• Speed: slower conduction; typically around v_{\text{continuous}} \approx 2 \ \text{m/s}.

Saltatory Conduction (myelinated)

• Definition: AP leaps between nodes of Ranvier; myelin sheath prevents ion flow under myelin, concentrating ion channels at nodes.
• Mechanism: current travels a longer distance inside the axon between nodes and triggers AP at the next node; skips myelinated regions.
• Ion channels: primarily at nodes of Ranvier (gaps in myelin).
• Speed: much faster than continuous; conduction velocities can exceed v_{\text{saltatory}} \gtrsim 100 \ \text{m/s}.

Conduction Velocity: Key Factors

• Degree of myelination: more myelin -> faster conduction.
• Axon diameter: larger diameter -> lower internal resistance and faster conduction.
• Summary rule: fastest axons are large and heavily myelinated.

Mechanism and Practical Takeaways

• Action potential strength is maintained as it propagates; it does not decay along the axon.
• In unmyelinated axons (continuous): AP is regenerated along the entire membrane.
• In myelinated axons (saltatory): AP is regenerated only at nodes; signal effectively jumps from node to node.

Quick Recap

• Two modes: Continuous vs Saltatory.
• Continuous: full-length membrane propagation; slower; speeds around v_{\text{continuous}} \approx 2 \ \text{m/s}.
• Saltatory: node-to-node propagation; faster; speeds around v_{\text{saltatory}} \gtrsim 100 \ \text{m/s}.
• Key determinants: myelination degree and axon diameter.