Myelin

Myelin is composed of proteins and lipids, and its main function is to insulate nerve fibres, where it is named the myelin sheath.

This means that electrical impulses can go down the nerves more efficiently than nerves without myelin. 

Diseases associated with myelin breakdown

• Alzheimer's Disease 
• Multiple Sclerosis 

And many other demyelinating diseases, where the bodys own immune system attacks the myelin forming the myelin sheath and so nerves become uninsulated.