Myelinated nerve fibers are primarily found in spinal cord and cranial nerves. These neurons are surrounded by Schwann cells that form a myelin sheath around the axon, which helps in the faster conduction of electrical signals. Unmyelinated nerve fibers, on the other hand, are found in peripheral nerves, somatic and autonomous neural systems. Myelination of neurons is a crucial process in the development and function of the nervous system.