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Abstract:
Axon diameters in the cortical white matter are of interest for various reasons: a) to get an impression of the range of conduction velocities, b) to estimate number of fibres in various fibre bundles, c) for comparison with fiber studies in vivo by way of diffusion weighted imaging. We investigated therefore the distribution of diameters of (myelinated) axons in the cortical white matter of human brains and of a macaque with the electron microscope. We took samples a) from the region of the superior longitudinal fascicle, b) from the transition of the white matter between temporal and frontal lobe where the uncinate and the inferior occipitofrontal fascicle merge, and c) from the corpus callosum. All bundles showed similar distributions: the bulk of fibres was relatively thin; the (inner) diameters of myelinated fibers had an average size between 0.5 and 0.8 mm in most samples. The total range of diameters of myelinated fibers was between 0.2 and 9 mm. Thus, a wide range of conduction velocities exists in the various bundles of the cortical white matter. The figure shows an electron microscopic section from the depth of the cortical white matter in the monkey.
