Loss of myelin. Myelin is a fat-like substance and is the major content of specialised cells (Schwann cells) that are wrapped around the long fibres (neurites) that transmit signals from nerve cells in the CNS. The myelin actually speeds up the signal transmission, but also protects the nerve cells. Patients with loss of myelin, as in multiple sclerosis, tire easily and movements slow down. Myelin is also the reason for the white appearance of the white brain matter.