Abnormal nervous system physiology demyelination a partial absence or patchy, incomplete area of a myelin sheath over all or part of an axon or fiber tract due to degeneration or destruction of the myelin sheath MPheno.ontology Demyelination is a term used to describe the destruction of the myelin sheath that surrounds axonal fibres. This is caused by diseases that damage the myelin sheath or the cells that form it. MP:0000921 MP:0010049 myelin destruction myelin degeneration