Gap junctions.

These are not chemical synapses but electrical in nature. They produce a current proportional to the difference between pre- and postsynaptic potential. No transmitter or action potential is involved:

I_gap=G(V_post-V_pre)

Sometimes these act as rectifiers so that the positive and negative current flow is not equal for potential differences of the same magnitude

 

Figure 4: Rectifying gap junction

Many non-neural cells are coupled in this manner; muscle, glia, and most humoral cells.