Myelin sheath around an axon is formed by

1. Axons are of two types, myelinated and non myelinated. 2. The myelinated nerve fibres are enveloped with Schwann cells. 3. Those Schwann cells form the myelin sheath around the axon. 4. An unmyelinated fibre is still enclosed by a Schwann cell, but no sheath is formed. _Source: syllabus Biology Ch18 'Neural Control and Coordination', section 18.3_