It is thought that the ribbons help guide the vesicles to the release cites in the cell membrane and that such a synapse is capable of high neurotransmitter release rates. This type of synapse involves an axon which makes contact with the pre-synaptic neuron just beyond a cleft of the dendrite. Typically there are other axons which do make contact within the dendritic cleft.
Contact is at the deepest point in the cleft.
Contact is not at the deepest point in the cleft. Typically there are other axons which do make contact at the deepest point within the same cleft. When these junctions are open, electric potential flows readily between the two cells.