In the spine , white matter is outside , and gray matter inside : the myelin sourrounds the nerve cells , and helps the transmission of nerve signals . White matter is composed of bundles of myelinated nerve cell processes ( or axons ) , which connect various gray matter areas ( the locations of nerve cell bodies ) of the brain to each other , and carry nerve impulses between neurons . Myelin acts as an insulator , increasing the speed of transmission of all nerve signals .