White Matter
White Matter
- Description
White matter refers to the neurons with myelinated axons. Myelination increases transmission speed of action potentials. White matter is involved in both short and long range connections.1
White Matter and ASD
Neuroimaging studies have seen that there are increases in white matter volume in outer zones of the brain, but not inner zones. This suggests that those with ASD have a greater number of short to medium intrahemispheric connections and a lower number of long range interhemispheric connections.2 Both gray and white matter have overall significant growth differences between typically developing children and those with autism, but this may not be indicative of dysfunction because gray and white matter growth tends to differ depending on the brain region and gender.3
White matter tracts were examined using diffusion tensor imaging in those with autism and their siblings and compared to age and IQ matched controls. One study found no correlations between autism symptomatology and white matter FA or axial diffusivity. However, they did find that both autism and sibling groups had widespread, significantly reduced white matter FA values. This suggests that white matter structure may be a marker of genetic risk for ASD.4
Investigations into the characteristics of individual axons underneath the anterior cingulate cortex(ACC), the orbitofrontal cortex(OFC), and the lateral prefrontal cortex(PFC) suggests mechanisms for the under-connectivity between areas far from each other and over-connectivity between areas close to one another. One study found that there were significantly decreased diameters in long range axons only in area 32 under the ACC. Additionally, they found evidence that the increased density of short range white matter under the ACC may be due to increased branching rather than more specific neurons. Furthermore, there was increased GAP-43, a protein released during rapid axon growth. In the tissue under the OFC, myelin was significantly thinnner in neurons regardless of axon diameter.5
Citations
1. Geschwind DH. Advances in autism.Annu Rev Med. 2009;60:367-80. PMID 19630577
2. Raichle ME et. al. A default mode of brain function: a brief history of an evolving idea. Neuroimage. 2007 Oct 1;37(4):1083-90; discussion 1097-9 PMID 17719799
3. Schumann CM et. al. Longitudinal magnetic resonance imaging study of cortical development through early childhood in autism. J Neurosci. 2010 Mar 24;30(12):4419-27. PMID 20335478
4. Barnea-Goraly N et. al. Similar white matter aberrations in children with autism and their unaffected siblings: a diffusion tensor imaging study using tract-based spatial statistics.Arch Gen Psychiatry. 2010 Oct;67(10):1052-60. PMID 20921121
5. Zikopoulos B et. al. Changes in prefrontal axons may disrupt the network in autism.J Neurosci. 2010 Nov 3;30(44):14595-609. PMID 21048117
