According to Yakovlev and Lecours (1967), myelination is a synchronized and orderly process of maturation of functionally allied systems of fibers. Several general rules governing the chronological and topographical sequences of the myelination process have been described. No single rule acts alone; rather, the interplay is complex, and one rule may supersede another to dictate the sequence of myelination within or across a particular axonal system.
As summarized by Kinney and colleagues (1994), these general rules are as follows: (1) Proximal pathways myelinate earlier and faster than do distal pathways; (2) sensory pathways myelinate before motor pathways; (3) projection pathways myelinate before associative pathways; (4) central telencephalic sites myelinate before poles; and (5) occipital poles myelinate before frontotemporal poles.
The more proximal the components of the fiber system, the shorter its myelination interval, as exemplified in (a) the visual system, in which the optic tract myelinates faster than the optic radiations which myelinate faster than the subcortical association fibers of the visual cortex; (b) the auditory system, in which the proximal auditory radiation myelinates faster than Heschl's gyrus (thalamic projections into the auditory cortex); and (c) the pyramidal system, in which the posterior limb of the internal capsule myelinates faster than the bulbar pyramid which myelinates faster than the lateral corticospinal tracts.
Axonal myelination also proceeds from proximal to distal. Considering trans-synaptic systems, myelination proceeds from the neuron along the direction of impulse conduction. Thus, the proximal components of a fiber system myelinate earlier than do distal components. Moreover, these proximal components myelinate faster. Corollary to this, however, are the principles of central region before pole and occipital before temporal. For example, in the telencephalon, Heschl's gyrus, the distal auditory thalamic projection to the temporal lobe, myelinates after the distal optic radiation to the occipital pole. Proximal motor components also myelinate faster than do distal sensory components. Thus, the posterior limb of the internal capsule, which contains the proximal portion of the pyramidal system, myelinates earlier and faster than Heschl's gyrus, the distal portion of the auditory system (Kinney et al., 1994).
Myelination of fiber systems mediating sensory input to the thalamus and cerebral cortex precedes myelination of those fiber systems carrying output relating to movement, and these latter fibers myelinate before the association fibers. Thus, the fiber systems mediating vestibular and acoustic input myelinate early and rapidly before birth, the optic radiation and the pre- and postcentral cortical thalamic projections myelinate rapidly during the first year after birth, and these sensory systems anticipate the myelination of the pyramidal systems. Postnatally, the visual and auditory systems tend to have shorter myelination intervals than do pyramidal systems (Kinney et al., 1994; Yakovlev and Lecours, 1967).
Primary projection systems begin to myelinate earlier and have shorter myelination intervals than do associative systems. The thalamocorticopyramidal fiber systems have a relatively short cycle of myelination that is completed during the first postnatal year. The nonspecific thalamocorticopontine fiber systems show a long myelination cycle, prolonging into early childhood. In general, the association fibers in the supratentorial brain and the reticular formation in the brainstem continue to myelinate to at least the third decade (Kinney et al., 1994; Yakovlev and Lecours, 1967).
Overall, myelination progresses from caudal to ceph-alad and from dorsal to ventral. Within any particular region of the brain, the posterior region tends to myelinate first. Myelination in the central white matter of the cerebral hemispheres proceeds from the region around the central sulcus toward the poles, with the occipital pole myelinating before the frontal pole, which in turn myelinates before the temporal pole. In addition, the posterior poles have shorter myelination intervals than the anterior frontotemporal regions. The posterior limb of the internal capsule myelinates earlier and faster than the anterior limb. The body and sple-nium of the corpus callosum, interconnecting the posterior frontal and parietooccipital hemispheres, myelinate earlier and faster than the genu and rostrum, interconnecting the frontal poles.
There are exceptions to these rules, however. In the spinal and cranial nerves, motor roots myelinate earliest and rapidly, and anticipate myelination of the sensory roots. Maturation of myelin occurs in the optic tract before it does in the optic chiasm. Similarly, myelination is completed in the auditory radiation before it is in the brachium of the inferior colliculus. In addition, myelination of the callosal rostrum, genu, body, and splenium occurs before the respective subcortical and central white matter origins (Kinney et al., 1994).
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