The process of myelin elaboration that covers, insulates and promotes rapid signal conduction in axons and which is principally a postnatal event in humans. In the human brain, myelination continues at least through the first decade of life, with some phylogenetically recent structures like the frontal cortex only appearing fully myelinated in the second decade. The work of Paul Ivan Yakovlev (1894-1983) and André Roch Lecours (1936-2005), who studied many pre- and post-term human brains indicates the following times for myelination of some familiar pathways:
* the visual system begins to myelinate about the 40th week in utero, and its myelination is nearly complete a few months after birth.
* the medial lemniscus in the brain stem begins to myelinate about the 30th week in utero, and its myelination continues past the first year of life
* the inferior and superior cerebellar peduncles begin to myelinate in utero; myelination in the middle cerebellar peduncle gets underway only a few months after birth (parallel with the frontopontine pathways)
* the fornix, a C-shaped bundle of axons under the corpus callosum connecting the hippocampus with the hypothalumus, only starts to acquire myelin at 3 to 4 months postnatal. (So much for theories of memory going back to the womb!)
* the pyramidal tracts start to acquire myelin just prior to birth, and their myelination continues (in a rostral caudal direction) into the second year
* cortico-cortico connections involving ‘higher’ intellectual functions apparently continue myelination throughout life
Source: Yakovlev, P.I., & Lecours, A-R. (1967). The myelogentic cycles of regional maturation of the brain. In: Minkowski, A. (Ed.), Regional development of the brain in early life. Oxford: Blackwell, pp. 3-70.
See Brain stem, Cerebellum (anatomy), Corpus callosum, Corticobulbar tract (CBT), Corticospinal tracts (CST), Diffusion anisotropy, Diffusion tensor imaging, Direct corticomotoneural connections (or tracts), Fatty acid, Glial cells, Hippocampus, Hypothalamus, Myelin, Oligodendrocytes, Purines, White matter