Brain Crosstalk in Puberty and Adolescence by Jean-Pierre Bourguignon Jean-Claude Carel & Yves Christen

Author:Jean-Pierre Bourguignon, Jean-Claude Carel & Yves Christen
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

White Matter

Whether brain tissue is classified as GM or WM depends largely on the amount of myelinated axons. Myelination is the wrapping of oligodendrocytes around axons, which acts as an electrical insulator, increasing the speed of neuronal signal transmission. The insulating properties of myelin allow signals to travel at speeds up to 100× faster than in unmyelinated axons. Additionally, myelination allows ion pumps to reset the ion gradients only at the nodes between sections of myelin (called nodes of Ranvier), instead of along the entire expanse of the axons, resulting in up to a 30-fold increase in the frequency with which a given neuron can transmit information. The combination of increased speed (100×) and quicker recovery time (30×) can yield a 3,000-fold increase in the amount of information transmitted per second. Myelination does not simply maximize speed of transmission but also modulates the timing and synchrony of neuronal firing patterns that convey meaning in the brain (Fields 2002). Signal processing plays a central role in developmental changes in the brain’s ability to adapt in response to its environment by inhibiting axon sprouting and the creation of new synapses (Fields 2008). These non-subtle impacts of myelin on the brain’s ability to process information may underlie many of the cognitive abilities associated with our species.

WM volumes increase steadily throughout childhood and adolescence (Fig. 1c), with similar rates across the major lobes (i.e., the frontal, temporal, and parietal lobes). However, for smaller regions, the growth rates can be quite dynamic, with as much as a 50 % change over a 2-year period (Thompson et al. 2000). The most prominent WM structure is the corpus callosum. It is comprised of approximately 200 million mostly myelinated axons connecting homologous areas of the left and right cerebral hemispheres. Its developmental trajectory reflects the general increases in total WM volume.

Increasing WM volumes on anatomic MRI and greater coherence on fMRI, EEG, and MEG converge to identify a pattern of increased connectivity among spatially disparate brain regions as a hallmark of adolescent development. Diffusion tensor imaging studies, which assess the directionality of WM, also demonstrate an increase in WM organization during adolescence. Changes in WM organization in specific regions correlate with improvements in language (Nagy et al. 2004), reading (Deutsch et al. 2005), response inhibition ability (Liston et al. 2006) and memory (Nagy et al. 2004). Characterizing developing neural circuitry and the changing relationships among disparate brain components is one of the most active areas of neuroimaging research, utilizing graph theory to quantify such things as the small world network properties of the brain (Hagmann et al. 2010).

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