The dark surround is another cell type, the Schwann cell. The dark material effectively thickens the nerve cell membrane and changes the way signals propagate along the nerve cells. The presence of this myelin sheath speeds the propagation of signals markedly. Not all nerve cells have the myelin. An unmyelinated nerve cell appears in the lower right corner and is marked u .
Myelin brings about a marked increase in conduction velocity .
The Schwann cells forming the myelin sheath are under study in the Development and Regeneration Laboratory .