Oligodendrocyte · Grey Matter

An oligodendrocyte is the glial cell that wraps axons of the brain and spinal cord in myelin, making them conduct fast, and it is what turns grey matter's local wiring into the white matter's long-range cables.

Oligodendrocyte. An oligodendrocyte is the glial cell that wraps axons of the brain and spinal cord in myelin, making them conduct fast, and it is what turns grey matter's local wiring into the white matter's long-range cables.

Each oligodendrocyte sends out a number of flat processes, and each one winds around a stretch of a different axon to form one internode of myelin; a single cell can myelinate a few dozen axons at once (estimates run from about 20 to 60, up to about 50 internodes in the optic nerve). In the peripheral nerves the same job is done by Schwann cells, which wrap one segment of one axon each. Myelin gives saltatory conduction, and it is the fat of these sheaths that makes bundles of myelinated axons look white.

Myelin is more than insulation. Oligodendrocytes pass lactate and other metabolites to the axon under the sheath, and an axon cut off from that support degenerates even if it still conducts.

Myelination continues long after birth. In humans it proceeds into early adulthood, with the frontal lobes among the last regions to finish, and precursor cells (OPCs) remain throughout life, able to make new oligodendrocytes and new myelin in response to activity and learning.

Losing it has a cost in time. When oligodendrocytes or their myelin are damaged (multiple sclerosis is the classic case), signals slow or fail; repair by new myelin is possible but often incomplete.

Oligodendrocytes decide the brain's timing.

By setting where and how thickly axons are wrapped, they set how long each signal takes to arrive, and arrival time is part of what a signal means.

Questions: What makes grey matter grey and white matter white at the level of cells? Grey matter is where neurons do their local work: it holds the cell bodies, dendrites, synapses and short local axons, together with astrocytes, microglia and a dense web of capillaries, and it looks pinkish grey in living tissue. White matter is made of bundles of long axons that connect distant regions, most of them wrapped in myelin by oligodendrocytes, and the high lipid content of the myelin makes it look white. In the cerebrum the grey matter forms the outer cortex and the deep nuclei, with the white matter between them; in the spinal cord the arrangement is reversed, with grey matter in a butterfly-shaped core. The division is functional as much as visual: grey matter computes, white matter carries. How does one oligodendrocyte myelinate many axons at once? An oligodendrocyte sends out several long processes, and the tip of each one flattens into a sheet that winds around a segment of a different axon, layer after layer, forming one internode of myelin. Depending on the region a single cell makes roughly 20 to 60 such segments, up to about 50 in the optic nerve, so one cell serves many axons and each axon is served by many cells along its length. In the peripheral nerves Schwann cells do the same job one segment at a time. The economy has a cost: losing one oligodendrocyte strips myelin from many axons at once. What happens to conduction when an axon loses its myelin? Without the insulation, current from each node leaks out along the bare stretch before it reaches the next node, and the sodium channels it needs are concentrated at the nodes, with few under the old sheath. The spike may slow, fail to cross the gap, or get through only at low firing rates, and signals that should arrive together lose their timing. In multiple sclerosis the immune system damages myelin in the brain and spinal cord, and the symptoms (blurred vision, weakness, numbness) depend on which tracts are hit. Partial recovery comes from redistributing sodium channels along the bare axon and from new myelin made by precursor cells, which is often thinner than the original.