Kalmar "myelin" tana nufin wani abu mai kitse wanda ke samar da abin rufe fuska a kusa da zaruruwan jijiya a jiki. Kwayoyin halitta na musamman da ake kira oligodendrocytes ne ke samar da su a cikin tsarin juyayi na tsakiya da kuma kwayoyin Schwann a cikin tsarin juyayi na gefe. Myelin yana taimakawa wajen rufewa da kare zaruruwan jijiya, yana ba da damar saurin watsa siginar lantarki cikin sauri da inganci tsakanin ƙwayoyin jijiya. Lalacewa ga myelin na iya haifar da kewayon cututtukan jijiya, gami da mahara sclerosis.