Vertebrate Nervous System

All nerve fibres found within the peripheral nervous system contain a thin membrane called the neurilemma,which surrounds the axon. The neurilemma promotes the regeneration of damaged axons. This explains why feeling gradually returns to your finger following a paper cut—severed neurons can be rejoined. However, not all nerve cells contain a neurilemma and a myelin sheath.Nerves within the brain that contain myelinated fibres and a neurilemma are called white matter, because the myelinated axons are whitish in appearance. Other nerve cells within the brain and spinal cord, referred to as the grey matter, lack a myelin sheath and a neurilemma, and do not regenerate after injury. Damage to the grey matter is usually permanent.