  In this paper, we study pairs of oscillators that are indirectly coupled via
active (excitable) cells. We introduce a scalar phase model for coupled
oscillators and excitable cells. We first show that one excitable and one
oscillatory cell will exhibit phase locking at a variety of $m:n$ patterns. We
next introduce a second oscillatory cell and show that the only attractor is
synchrony between the oscillators. We will also study the robustness to
heterogeneity when excitable cell fires or is quiescent. We next examine the
dynamics when the oscillators are coupled via two excitable cells. In this
case, the dynamics are very complicated with many forms of bistability and, in
some cases, chaotic behavior. We also apply weak coupling analysis to this case
and explain some of the degeneracies observed in the bifurcation diagram. We
also look at pairs of oscillators coupled via long chains of excitable cells
and show that small differences in the frequency of the oscillators makes their
locking more robust. Finally, we demonstrate many of the same phenomena seen in
the phase model for a gap-junction coupled system of Morris-Lecar neurons.
