Myelin content is inversely correlated with intracortical circuit complexity - in general, more myelin content means simpler and perhaps less dynamic intracortical circuits. Using existing PET data and functional network parcellations, we examine metabolic differences in the differently myelinated cortical functional networks. Lightly myelinated cognitive association networks tend to have higher aerobic glycolysis than heavily myelinated early sensory-motor ones, perhaps reflecting greater ongoing dynamic anabolic cortical processes. This finding is consistent with the hypothesis that intracortical myelination may stabilize intracortical circuits and inhibit synaptic plasticity.
Trends and properties of human cerebral cortex: correlations with cortical myelin content
So the question is - how is the human defined has having "successful" plasticity in terms of adaptation?
Todd Preuss of Yerkes Primate Center gives a brief video lecture
Human Brains are bizarrely large - more than three times compared to other primates for body size.
Intercortical Myelin - is found in white and gray matter.
The human cortex is mainly associative cortex or Unmyelinated
Especially the primary Auditory Cortex.
In humans, primary auditory cortex (A1) is located on a well-developed Hechl’s gyrus (HG), located in the lateral sulcus. Like the adjacent planum temporale, volume measurements of HG have often found a left>right asymmetry, which may correspond to the human hemispheric specialization of language. In many studies, the left HG was in the range of 10–30% larger than the right
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3851973/
One possibility is that the advanced age of brain donors in sample B (mean age 74 years, compared to 55 in sample A) caused gray matter loss preferentially in the right hemisphere. In whole cortex, neuron loss between these ages could be 10% or more, at least in male brains that were the subject of our study (Pelvig et al., 2008). There is precedence for a hemispheric selectivity of gray matter loss with aging (Raz et al., 2004), but we are unaware of studies that evaluated auditory areas in individuals over 70.
Todd Preuss:
trace fiber pathways through the white matter between areas. It's based on the propensity of water molecules to diffuse along rather than across fiber tracts. Because the myelin sheaths that surround fibers are fatty and thus hydrophobic. And we can use diffusion imaging to reconstruct the several homologous major fiber bundles that connect distant areas in humans and also in apes....
Now long before the development of neuroimaging we knew from gross dissections that Broca's Area in the frontal lobe was connected with Wernicke's Area in the Temporal Lobe by a fiber bundle called the arcuate fasciculus
One would think with the evolution of human language there would be some difference in the areas in the arcuate in humans and apes. And comparative diffusion imaging studies indicate that indeed there are. In humans the arcuate fasciculus has much stronger connections with the middle temporal gyrus than in chimpanzees or in macques for that matter.
That's interesting because the middle temporal gyrus is known to harbor semantic representations including representations of word meanings. Now that we know what part of human cortex expanded in human evolution we can take advantage of our new found knowledge of evolutionary changes in gene structure and gene expression, to see how the two are related....
White matter plasticity [myelinated] - to take an example, learning to make alternating left and right hand movements requires communication between the two hemispheres which results in changes in the fiber tract that connects them - namely the Corpus Callosum. How do you modify signal strength in the white matter - in part in least by modifying the amount of myelin that surrounds the axons.
CELIBACY!!
This is orchestrated by a symphony of molecular signals passing between neurons and the cells that make myelin shealths...
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