Conduction Velocity & Axonal Properties

Conduction Velocity – Overview

• Conduction velocity = speed at which an action potential (AP) travels down the length of an axon.

• Determined largely by two anatomical/physiological variables:

  1. Axon diameter.

  2. Degree of myelination.

• Fast conduction is crucial for rapid communication within the nervous system and for synchronizing responses (e.g.
  reflexes, coordinated movement, cognition).

Key Axonal Features Influencing Conduction Velocity

• 1. Axon Diameter

  • The thicker (larger‐diameter) the axon, the faster the AP propagates.

  • Mechanistic explanation:

  • A larger circumference provides space for more voltage-gated sodium ($\mathrm{Na}^+$) channels per unit length.

  • Lower internal (axoplasmic) resistance yields quicker passive current flow along the interior of the fiber.

  • Practical significance: motor neurons innervating skeletal muscle often have large diameters to minimize latency.

• 2. Myelination

  • Myelin is produced by:

  • Oligodendrocytes (CNS).

  • Schwann cells (PNS).

  • Acts as an electrical insulator, dramatically increasing conduction velocity.

  • Generates the phenomenon of saltatory conduction:

  • APs “jump” from one exposed membrane segment (node of Ranvier) to the next.

  • Only at the nodes are voltage-gated $\mathrm{Na}^+$ and $\mathrm{K}^+$ channels clustered.

  • Between nodes, the myelin sheath eliminates leakage channels and sodium-potassium pumps, allowing passive spread of current with minimal dissipation.

  • Reduces metabolic load because fewer membrane areas require ionic restoration after each AP.

Structural Scenarios Shown in the Source Image

1. Bare Plasma Membrane (e.g., dendrites, soma)

   • No voltage-gated channels.

   • Contains:

     • Ligand-gated (chemical) channels → initiate graded potentials.

     • Leakage channels for $\mathrm{Na}^+$ and $\mathrm{K}^+$.

     • $\mathrm{Na}^+/\mathrm{K}^+$ ATPase pump.

   • Result: Graded potentials decay quickly; cannot propagate as APs.

2. Unmyelinated Axon

   • Membrane includes extensive voltage-gated channels in addition to leakage channels and pumps.

   • AP must be regenerated sequentially at every point along the membrane.

   • Conduction is continuous but slower due to time-consuming channel cycling at each segment.

3. Myelinated Axon

   • Myelin wraps repeatedly around axonal membrane.

   • Voltage-gated channels, leakage channels, and $\mathrm{Na}^+/\mathrm{K}^+$ pumps are absent under the myelin.

   • Present only at nodes of Ranvier (exposed membrane gaps).

   • Enables rapid saltatory conduction; passive spread beneath myelin + active boosting at nodes.

Detailed Membrane Structures & Channel Distribution

Dendritic/Somatic Membrane (Bare)

• Ligand-gated channel = “receptor” → opens when neurotransmitter binds.

• Leakage channels continuously allow small ion fluxes (resting membrane permeability).

• $\mathrm{Na}^+/\mathrm{K}^+$ pump maintain Na⁺ out K⁺ in gradients using ATP.

• Consequence: Graded potentials are short-distance; amplitude decays because:

  1. No voltage-gated amplification.

  2. Ions leak out or are pumped back quickly.

Unmyelinated Axon Membrane

• Voltage-gated $\mathrm{Na}^+$ channels densely spaced along entire length.

• Each AP opens adjacent channels by local current flow → sequential regeneration.

• Conduction velocity limited by:

  1. Channel opening/closing kinetics.

  2. Membrane capacitance and axial resistance.

Myelinated Axon Membrane

• Myelin = multiple layers of glial cell membrane with high lipid content → excellent insulator.

• Eliminates leakage currents; reduces capacitance.

• Nodes of Ranvier (~1 µm long, spaced ~1 mm apart):

  • Very high density of voltage-gated $\mathrm{Na}^+$ channels (≈2000–12 000 per $\mu\text{m}^2$ vs. ~100 in unmyelinated regions).

• Current generated at one node travels internally to the next node with minimal loss; depolarizes it to threshold → AP “jumps.”

Functional & Physiological Implications

• Large-diameter, heavily myelinated fibers (Aα) can conduct at $\approx 120\ \text{m·s}^{-1}$.

• Small-diameter, lightly myelinated (Aδ) or unmyelinated (C) fibers conduct more slowly (as low as $\approx 0.5\ \text{m·s}^{-1}$) → explains delayed, dull pain vs. sharp pain.

• Diseases such as multiple sclerosis (CNS) or Guillain-Barré (PNS) degrade myelin → conduction blocks, slowed reflexes, motor deficits.

Key Terminology Recap

• Conduction velocity: rate of AP propagation.

• Axon diameter: cross-sectional width; larger = faster.

• Myelination: wrapping by glial membranes.

• Saltatory conduction: nodal jumping of APs.

• Node of Ranvier: bare axonal membrane between myelin segments rich in voltage-gated channels.

• Graded potential: local, decremental change in membrane potential; occurs on dendrites/soma.

Concept Links to Previous Material

• Graded potentials (EPSPs/IPSPs) initiate at dendrites/soma and, if summed to threshold at the axon hillock, trigger an AP.

• The action potential itself then travels along the axon at a velocity determined by diameter + myelin.

• The $\mathrm{Na}^+/\mathrm{K}^+$ pump’s restoration of ion gradients is energetically cheaper in myelinated fibers due to reduced active surface area.

Ethical / Clinical Considerations

• Enhancing remyelination or protecting myelin is a major therapeutic goal for demyelinating disease.

• Nerve fiber classification guides anesthetic choice: local anesthetics preferentially block small, unmyelinated pain fibers first.