The myelinated and non myelinated neurons or nerve fibers are found in the peripheral nervous system and autonomic nervous system.
The cranial and the spinal nerves are made up of myelinated neurons. The myelinated neurons are those in which the axons are surrounded by a sheath or covering laid by the Schwann cells. There are gaps between the adjoining myelinated sheath and they are known as the nodes of Ranvier. Due to the presence of myelin the myelinated neurons are thicker and carry the impulse faster than non myelinated neurons.
As the name suggests the non myelinated neurons are those whose axons are surrounded by the Schwann cells but they do not form the myelin sheath. These type of nerve fibers are found in parasympathetic and sympathetic nervous system.
Like the meninges, the myelin sheath acts as an insulator thus protecting the axon from injury. But in certain neurological disorders probably due to viral infection or autoimmunity the myelin sheath is destroyed, these diseases are termed as the demyelinating diseases (Multiple sclerosis and perivenous encephalomyelitis).