Demyelinating diseases introduction

Demyelinating disorders of the central nervous system affect myelin and/or oligodendroglia with relative sparing of axons.

The central nervous system is composed of neurons with neuroectodermal and mesodermal supporting cells.

The neuroectodermal cells comprise:

astrocytes ependymal cells oligodendrocytes.

The oligodendrocytes, like Schwann cells in the peripheral nervous system, are responsible for the formation of myelin around central nervous system axons.

One Schwann cell myelinates one axon but one oligodendrocyte may myelinate several contiguous axons, and the close proximity of cell to axon may not be obvious by light microscopy.

Oligodendrocytes are present in grey matter near neuronal cell bodies and in white matter near axons.

Myelin is composed of protein and lipids. Protein accounts for 20% of total content. The lipid fraction may be divided into: cholesterol glycophosphatides

(lecithins) sphingolipids

(sphingomyelins).

The laying down of myelin in the central nervous system commences at the fourth month of fetal life in the median longitudinal bundle, then in frontal and parietal lobes at birth. Most of the cerebrum is myelinated by the end of the 2nd year. Myelination continues until the 10th year of life.

Myelin disorders may be classified as diseases in which:

1. Myelin is inherently abnormal or was never properly formed - these disorders generally present in infancy and early childhood and have a biochemical basis, e.g. leukodystrophy.

2. Myelin which was normal when formed breaks down as a consequence of pathological insult, e.g. multiple sclerosis.

Astrocyte

Cell body Oligodendrocyte

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