Could Sex Differences in White Matter be Explained by g ratio?

Area of Science:

• Neuroscience
• Developmental Neuroscience
• Neuroimaging

Background:

• White matter development during male adolescence involves changes in axon and myelin growth.
• Magnetic resonance imaging (MRI) studies suggest an increasing g ratio (axon diameter to outer diameter) in this period.
• The g ratio is critical for nerve impulse conduction velocity.

Purpose of the Study:

• To review the Rushton (1951) model of optimal g ratio for conduction velocity.
• To examine evidence for higher g ratios in large-diameter fibers.
• To explore implications of altered g ratios in adolescent white matter for neurological disorders.

Main Methods:

• Review of existing literature on white matter development and the g ratio.
• Analysis of the Rushton (1951) model and its relevance to current findings.
• Estimation of myelin thickness relative to axon diameter based on fiber size.

Main Results:

• An increasing g ratio, where axon growth outpaces myelin thickness, is observed in adolescent male white matter.
• Large-diameter fibers (>9.6 µm) are estimated to have relatively thinner myelin sheaths.
• Brains with a higher proportion of large-diameter fibers may exhibit lower overall myelin concentration.

Conclusions:

• A "suboptimal" g ratio, deviating from the optimal 0.6, may occur in adolescent white matter.
• These changes could have implications for the development of "disconnection" disorders.
• Schizophrenia, emerging in late adolescence, is a potential disorder linked to these white matter alterations.