Action potential attenuation in unmyelinated axons vs demyelinated axons
Myelinated axons have schwann cells around them, electrically insulating the neuron axons except for gaps in the sheath which are called the Nodes of Ranvier. The insulation increases the speed of transmission of action potentials.

Without myelination conduction signals along the nerve can be impaired or lost.
There are some diseases E.g. Acute disseminated encephalomyelitis where myelination is lost.