 The myelin sheath is a multi-layered membrane, unique to the nervous system, that functions as an insulator to greatly increase the efficiency of axonal impulse conduction.In humans, point mutations in PLP are the cause of Pelizaeus–Merzbacher disease (PMD), a neurologic disorder of myelin metabolism.In animals demyelinating diseases such as mouse 'jimpy' or dog 'shaking pup' are also caused by mutations in PLP.