Myelin and Modeling: Bootstrapping Cortical Microcircuits.

Robert Turner

doi:10.3389/fncir.2019.00034

Abstract

Histological studies of myelin-stained sectioned cadaver brain and in vivo myelin-weighted magnetic resonance imaging (MRI) show that the cerebral cortex is organized into cortical areas with generally well-defined boundaries, which have consistent internal patterns of myelination. The process of myelination is largely driven by neural experience, in which the axonal passage of action potentials stimulates neighboring oligodendrocytes to perform their task. This bootstrapping process, such that the traffic of action potentials facilitates increased traffic, suggests the hypothesis that the specific pattern of myelination (myeloarchitecture) in each cortical area reveals the principal cortical microcircuits required for the function of that area. If this idea is correct, the observable sequential maturation of specific brain areas can provide evidence for models of the stages of cognitive development.

Highlights

Mammalian cerebral cortex invariably shows localized functional specialization, such that the performance of specific actions is associated with specific and reproducible patterns of increased and decreased neural activity on a spatial scale of a few millimetres, often organized in larger scale networks across the brain
The fundamental points are: (a) myelination is generally driven by a neuron’s experience; (b) it is only once the relevant axons have reached an appropriate level of myelination that cortical microcircuits can be considered to be consolidated, and available for routine later use; and (c) the distribution of myelin in the white matter and cerebral cortex is observable in vivo with a spatial resolution of about 300 microns using magnetic resonance imaging (MRI) techniques at high magnetic field
IN VIVO VISUALIZATION OF MYELOARCHITECTURE. It was recognized in the initial development of MRI that much of the contrast observed in brain tissue arose from the presence of myelin

Summary

INTRODUCTION

Mammalian cerebral cortex invariably shows localized functional specialization, such that the performance of specific actions is associated with specific and reproducible patterns of increased and decreased neural activity on a spatial scale of a few millimetres, often organized in larger scale networks across the brain. For some simple tasks, such as visual (Bridge et al, 2005) and auditory (Dick et al, 2012; De Martino et al, 2015) perception, good congruence has been observed between the anatomically and functionally defined boundaries of the relevant cortical areas, and these boundaries often coincide with chemoarchitectonic boundaries In describing their hybrid cortical parcellation scheme, Glasser and Van Essen (2011) assert that the boundaries they observe, using a crude magnetic resonance imaging (MRI) measure of cortical myelination, coincide with those they derive from maps of functional connectivity in the so-called ‘‘resting state,’’ in those cortical regions where. The fundamental points are: (a) myelination is generally driven by a neuron’s experience; (b) it is only once the relevant axons have reached an appropriate level of myelination that cortical microcircuits can be considered to be consolidated, and available for routine later use; and (c) the distribution of myelin in the white matter and cerebral cortex is observable in vivo with a spatial resolution of about 300 microns using MRI techniques at high magnetic field

BENEFITS OF MYELINATION

THE PROCESS OF MYELINATION

IN VIVO VISUALIZATION OF MYELOARCHITECTURE

MYELOARCHITECTONIC AREAS

Findings

CORTICAL MICROCIRCUITS