In Mur boundary, what are Ex(imin), Ex(imax),...?

In Mur boundary, (in textbooks or papers) there are equations for tangential fields of electric field components at the boundaries i.e. there are equations for Ey(imin), Ez(imin), ... etc are available. But what about non-tangential field components at the boundaries i.e. Ex(imin), Ex(imax), .... etc? What values or equations should I use in Mur boundary implementation for Ex(imin), Ex(imax), Ey(jmin), Ey(jmax), Ez(kmin), Ez(kmax)? Some clarification to understand this would be highly appreciated. Thank you.