  Rhythmogenesis, which is critical for many biological functions, involves a
transition to coherent activity through cell-cell communication. In the absence
of centralized coordination by specialized cells (pacemakers), competing
oscillating clusters impede this global synchrony. We show that spatial
symmetry-breaking through a frequency gradient results in the emergence of
localized wave sources driving system-wide activity. Such gradients, arising
through heterogeneous inter-cellular coupling, may explain directed rhythmic
activity during labor in the uterus despite the absence of pacemakers.
