Distribution of axon diameters in cortical white matter: an electron-microscopic study on three human brains and a macaque

Abstract

The aim of this study was to obtain information on the axonal diameters of cortico-cortical fibres in the human brain, connecting distant regions of the same hemisphere via the white matter. Samples for electron microscopy were taken from the region of the superior longitudinal fascicle and from the transitional white matter between temporal and frontal lobe where the uncinate and inferior occipitofrontal fascicle merge. We measured the inner diameter of cross sections of myelinated axons. For comparison with data from the literature on the human corpus callosum, we also took samples from that region. For comparison with well-fixed material, we also included samples from corresponding regions of a monkey brain (Macaca mulatta). Fibre diameters in human brains ranged from 0.16 to 9 . Distributions of diameters were similar in the three systems of cortico-cortical fibres investigated, both in humans and the monkey, with most of the average values below 1 m diameter and a small population of much thicker fibres. Within individual human brains, the averages were larger in the superior longitudinal fascicle than in the transitional zone between temporal and frontal lobe. An asymmetry between left and right could be found in one of the human brains, as well as in the monkey brain. A correlation was also found between the thickness of the myelin sheath and the inner axon diameter for axons whose calibre was greater than about 0.6 . The results are compared to white matter data in other mammals and are discussed with respect to conduction velocity, brain size, cognition, as well as diffusion weighted imaging studies.

Quantifying axonal responses in patient-specific models of subthalamic deep brain stimulation

“…The vast majority of axons in the CNS have fiber diameters below 4 µm [Liewald et al, 2014; Aboitiz et al, 1992], except pyramidal axons whose diameters, although predominantly between 2 µm and 4 µm, can range up to 12 µm [Firmin et al, 2014]. Nonetheless, it is the largest diameter axons that dominate the response to electrical stimulation due to their low stimulation thresholds [McNeal, 1976].…”

How Klingler’s dissection permits exploration of brain structural connectivity? An electron microscopy study of human white matter

“…Indeed, since the study of Aboitiz et al (1992), who described the fiber composition of the human corpus callosum using both light microscopy and EM, the few studies that have used EM on human cerebral white matter mostly investigated axonal diameters (Zikopoulos and Barbas 2010;Liu and Schumann 2014;Liewald et al 2014) to compare their results to those provided by diffusion MRI (Assaf et al 2008;Barazany et al 2009;Alexander et al 2010). Another interesting point is that little is known about the impact of fixation on white matter ultrastructure observed under EM.…”

Are there optical communication channels in the brain

“…The ratio of the radius of the axon, r to the outer radius of the myelin sheath r (g-ratio) is taken equal to 0.6 for the most of the simulations, close to the experimental values (64). In reality, the radius of the myelinated axons in the brain changes from 0.2 microns to close to 10 microns (65). For the purpose of guiding light inside the myelin sheath, Ref.…”