As already noted, glial cells and neurons develop from the same populations of immature cells as neurons. Glial cells continue to be added to the nervous system throughout life. (Sometimes, however, the process becomes aberrant, resulting in glial tumors, or gliomas, of the brain.) In fact, the most intense phase of glial cell proliferation in many animals occurs after birth, when glial cells are added from immature cells located in the ventricular zone.
The development of sheaths around axons—the process of myelination (illustrated in Figure 1)—greatly changes the rate at which axons conduct messages (see Figure 3.8 in the textbook). Myelination has a strong impact on behavior because it allows large networks of cells to communicate rapidly. Multiple sclerosis is a disorder in which myelin is destroyed, probably by the person’s own immune system, in random distinct patches (Manova and Kostadinova, 2000). The resultant desynchronization of activity in these locations can cause devastating disruptions of sensory and motor function.
In humans, the earliest myelination in the peripheral nervous system is evident in cranial and spinal nerves about 24 weeks after conception. But the most intense phase of myelination occurs shortly after birth. Furthermore, some investigators believe that myelin can be added to axons throughout life. The first nerve tracts in the human nervous system to become myelinated are in the spinal cord. Myelination then spreads successively into the hindbrain, midbrain, and forebrain. Within the cerebral cortex, sensory zones are myelinated before motor zones; correspondingly, sensory functions mature before motor functions.
Manova, M. G., and Kostadinova, I. I. (2000). Some aspects of the immunotherapy of multiple sclerosis. Folia Medica 42(1): 5–9.