One cannot discuss the neuron without mentioning glial cells or neuroglia. It was once thought that these cells simply held everything together (gloios means glue, in Greek), but we now know that neuroglia are highly specialized cells. For example, neuroglia are responsible for physical support, protection against infection (through phagocytosis), and the connection of nerve cells to blood vessels. The Schwann cell (or neurolemmocyte) is a common type of glial cell found in peripheral nerve axons. Schwann cells wrap "jelly roll style" around the axon, forming a whitish phospholipid (fatty) protective and insulating cover known as the myelin sheath. The myelin sheath of the axon of peripheral nerves has interruptions or exposed gaps known as the Nodes of Ranvier. Schwann cells also make up the neurolemma, a continuous sheath that covers both the myelin sheath and the axon at the Nodes of Ranvier. Action potentials traveling down the axon occur only at the Nodes of Ranvier, jumping rapidly from gap to gap (saltatory conduction), which conducts impulses significantly faster than in nonmyelinated nerves. The neurolemma is found only in peripheral nerve fibers and plays a crucial part in nerve fiber regeneration. Damaged axons will regenerate; damaged cell bodies will not. The myelinated sheaths of the axons of neuron in the brain and spinal cord (the central nervous system) are made from different glial cells (oligodendrocytes), which lack a neurolemma, so making the regeneration of their axons impossible. Multiple sclerosis is a serious demyelinating disease of the central nervous system. Not all axons are myelinated; the presence of myelin is one difference between white matter (which has myelinated axons) and gray matter (which does not).