We have the central nervous system so that's the brain and the spinal cord and we also have our peripheral nervous system. And both of these categories contain their own glial cells. So schwann cells are the cells found in the peripheral nervous system while oligodendrocytes are those glial cells found in the central nervous system. And what these cells do is they move around the axon of the neuron and they basically cover that neuron at specific sections with a layer of insulating myelin. Now, because the myelin is insulating, what that means is no action potential can actually be generated on the cell membrane where it is covered with that myelin material. Now, just because the cell membrane is actually covered with that myelin material, that does not mean that no electric signal can travel through that axon.