Relation between myelin sheath thickness, internode geometry, and sheath resistance.

Area of Science:

• Neuroscience
• Biophysics

Background:

• Myelin sheath thickness was traditionally linked solely to axon caliber.
• The role of internode length variation on myelin sheath thickness is a recent area of study.

Purpose of the Study:

• To develop an equation predicting myelin lamellae number based on axon caliber and internode length.
• To investigate the biophysical basis of myelin sheath resistance in relation to its geometry.

Main Methods:

• Precise measurements of myelin sheath thickness and internode geometry.
• Analysis of normal, regenerated, and hypomyelinated nerve fibers.
• Comparison of observed relationships with biophysical models of myelin resistance.

Main Results:

• An equation was defined to predict lamellae number from axon caliber and internode length.
• Myelin sheath thickness does not adjust for radial current flow.
• Sheath thickness variation precisely matches the spiral path resistance to axonal current density.

Conclusions:

• Myelin sheath geometry is finely tuned to axonal current density.
• This tuning reconciles low overall sheath resistance with high myelin leaflet resistance.
• A new biophysical concept for myelin sheath resistance is proposed.