  Synchronized oscillations are of critical functional importance in many
biological systems. We show that such oscillations can arise without
centralized coordination in a disordered system of electrically coupled
excitable and passive cells. Increasing the coupling strength results in waves
that lead to coherent periodic activity, exhibiting cluster, local and global
synchronization under different conditions. Our results may explain the
self-organized transition in a pregnant uterus from transient, localized
activity initially to system-wide coherent excitations just before delivery.
