mieliin

Mieliin, known in English as myelin, is the lipid-rich insulating sheath surrounding many axons in the nervous system. In the central nervous system mieliin is produced by oligodendrocytes; in the peripheral nervous system by Schwann cells. The sheath wraps around axons in compact layers, with gaps called nodes of Ranvier that enable saltatory conduction and speed up signal transmission. The myelin membrane is high in lipids and contains important proteins such as myelin basic protein (MBP) and proteolipid protein (PLP) in the CNS, and peripheral myelin protein 22 (PMP22) in the PNS. These components help stabilize the sheath and maintain electrical insulation.

The thickness of myelin relative to axon diameter is described by the g-ratio, with typical values around 0.6 to 0.7. Myelination begins during fetal development and continues after birth, influenced by neuronal activity, hormones, and growth factors. In adulthood, myelin can be damaged by disease or injury, but remyelination can occur through oligodendrocyte precursor cells and Schwann cells, though it is less efficient with age.

Dysmyelination or demyelination underlies several neurological disorders. Multiple sclerosis is a common CNS demyelinating disease; leukodystrophies are genetic disorders affecting myelin; peripheral neuropathies such as Charcot-Marie-Tooth disease involve peripheral myelin defects. Diagnosis relies on clinical assessment, imaging, and sometimes histology, which shows myelin loss and changes in axonal integrity.