Oligodendrocyte and microglia action in the brain. At top, the axon (output process, orange) of a neuron (nerve cell) is seen. It is being enclosed by processes originating from a neighbouring oligodendrocyte cell (red). This provides a sheath of the fatty chemical myelin, which improves the conduction of the nerve signal. At bottom, microglia cells (green) are seen around a blood vessel. Microglia are the immune cells of the brain, and they are inspecting damage in the blood vessel wall. They can engulf any foreign bodies they detect, preventing them from damaging the brain cells.