myelin sheath facts bme202

What is the approximate relationship between conduction speed and fiber diameter in myelinated axons?

Speed (m/s) ≈ 6 × fiber outer diameter (µm). A 10 µm fiber conducts at about 60 m/s.

How does conduction speed scale with diameter in unmyelinated fibers?

It increases with the square root of diameter (√d), not linearly as in myelinated fibers.

What is the maximum conduction velocity in myelinated axons?

About 120 m/s or higher (seen in cats).

What fraction of fiber diameter is typically taken up by the axon (α)?

About 60% in large fibers; smaller fibers have relatively thinner myelin.

Why are myelinated axons below 1 µm in diameter rare?

Below this size, myelin provides little speed advantage due to the very small axon core.

What is the relationship between internodal distance and fiber diameter?

Internodal distance ≈ 100 × fiber diameter, so a 10 µm fiber has ~1 mm internode length.

What does a “safety factor of 5” mean for myelinated fibers?

If Na⁺ channels at four nodes fail, the current from node 0 can still depolarize node 5 to threshold.

How does action potential shape compare across myelinated fibers?

It’s remarkably similar; rise time and duration are nearly constant across fiber diameters.

Why are AP shapes congruent across myelinated fibers?

Because action currents and depolarization spread depend on internodal capacitance and myelin thickness.

For a 10 µm myelinated fiber, summarize its parameters.

60% axon core, 40% myelin, 60 m/s velocity, 1 mm internode, ~3 nA Na⁺ current, safety factor ~5.

What do normal mammalian fibers optimize?

Their geometry and properties maximize conduction velocity for their diameter given sodium kinetics, myelin capacitance, and axoplasmic resistivity.

What causes myelin thickness and internodal length to match axon diameter during development?

Trophic interactions between axons and Schwann cells regulate myelin formation and scaling.

What happens in peripheral nerve regeneration?

Axon diameters and myelin thickness recover nearly normally, but internodal lengths are shorter, reducing conduction velocity.

Which species has the best-optimized axons?

Cats—they reach up to 20 µm diameter and 120 m/s conduction velocity, faster than humans (~80 m/s max).

Why do humans have slower conduction than cats?

Human axons are less optimized despite needing to transmit signals over longer distances.

How do cold-blooded animals differ in axon optimization?

They show different relationships due to variable temperature conditions limiting optimization.

What happens when myelin is degraded (demyelination)?

Action potentials may fail to propagate because node N cannot sufficiently charge node N+1.

Why does demyelination prevent propagation?

Demyelination increases capacitance between nodes, making it too slow (large τ = Ri·C) for node N+1 to reach threshold before node N repolarizes.

What does exponential decay of voltage signals mean for APs?

Time-varying (fast) signals like APs decay even more steeply than steady-state voltages, making rapid conduction dependent on low capacitance from intact myelin.