How are neurons myelinated

Myelination

Myelination w, myelinogenesis, myelination, myelogenesis, E.myelination, during the development of nerve cells, the covering of their axons by membranes of glial cells (myelin sheath). The number of myelin lamellae mostly correlates with the axon diameter (caliber); thicker axons show up to 200. At the beginning of myelination, the glial cell encloses the axon; the contact of their membranes forms that Mesaxon (see fig.). The outer lamella of the mesaxon grows spirally around the inner one, whereby myelin proteins and lipids are increasingly incorporated and compacting takes place. In the course of the ontogenesis of the nervous system, the axons are generally myelinated in the order of their caliber growth (i.e. mostly their ontogenetic disposition or their phylogenetic age): In the spinal cord, the motor fibers of the anterior root myelinate in front of the sensitive ones of the posterior root and these in front of the segmental interneuron axons. In the brain stem there is a comparable development process of the motor before the sensitive parts of the cranial nerves. The "wave of myelination" rises from the brain stem via the mid- and diencephalon to the cerebral cortex and begins in the primary motor cortex in front of the sensory cortex and then occurs in the visual and auditory cortex. The myelination reaches the adult state in the individual sensory systems at different times; in the frontal lobe it does not come to an end for life.



Myelination

Myelination of two axons in the central nervous system by an oligodendroglial cell. The right axon shows an early and the left a later stage of the myelination process.