Nerve cells are called neurons and are able to conduct an electrical impulse. They may be interneurons or anaxonic, unipolar, bipolar or multipolar neurons. All neurons (except for anaxonic ones) have an axon that carries an impulse away from the cell body and one, two or many dendrites that carry an impulse towards the cell body. Other cells, collectively called neuroglia, are support cells for neurons but do not carry impulses. Within the CNS they are ependymal cells, astrocytes, microglia and oligodendrocytes, while in the PNS they are satellite cells and Schwann cells. Oligodendrocytes and Schwann cells form the myelin sheath around axons.