The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system, as it is derived from an outpouching of the diencephalon during embryonic development. As a consequence, the fibres are covered with myelin produced by oligodendrocytes, rather than Schwann cells, which are found in the peripheral nervous system, and are encased within the meninges. The name "optic nerve" is, in the technical sense, a misnomer, as the optic system lies within the central nervous system and therefore should be named the "optic tract," as nerves exist only, by definition, within the peripheral nervous system. Therefore, peripheral neuropathies like Guillain-Barré syndrome do not affect the optic nerve.