Glial

http://med.stanford.edu/ism/2009/september/glia-0921.html



Alzheimer’s disease is characterized by massive synapse loss.

As one ascends the scale of evolutionary complexity, an increasing proportion of the brain’s cells are glial. In the simple nematode worm, they’re sparse; in a fruit fly, they’re up to 25 percent; in a mouse, about 65 percent. In a human brain, behind every great neuron stand nine great glial cells.

http://med.stanford.edu/ism/images/featureStories/glia-illo-092109.jpg

There are three main types of glial cells. Oligodendrocytes (1) send projections that wrap axons (2) – long, signal-carrying portions of neurons (3) – in sheathes of a fatty substance called myelin (4), speeding signal conduction. Microglia (5) are, essentially, the brain’s immune cells, but they also monitor neighboring brain cells for damage and gobble up debris, and they probably have other functions, too. Astrocytes (6) carry on a host of activities. Their long extensions can monitor levels of neuronal activity either along axons at synapses (7) – junctions that relay signals from one neuron to the next – and, when those activity levels are high, signal to local blood vessels (8) to dilate, increasing blood supply to hard-working neurons. Astrocytes also produce and secrete substances that have a major influence on the formation and elimination of synapses.