Reciprocal synapses between cholinergic axons and small granule-containing cells in the rat cardiac ganglion.

Electron microscopy of the rat cardiac ganglion shows occurrence of small granule-containing cells that form reciprocal synaptic junctions with cholinergic terminals. At the synaptic junctions which are from axon to granule-containing cell, the intraaxonal vesicles are clustered against the junctional axolemma, but dense-cored vesicles in the postsynaptic cell do not cluster towards the membrane densities in these synapses. By contrast, the synaptic zone polarized in the opposite direction shows an absence of axonal vesicles in close proximity to the postsynaptic axolemma, but there is a marked aggregation of dense-cored vesicles towards the presynaptic specializations of granule-containing cells. The synaptic zones are multifocal rather than bifocal, and the minimal distance separating each synaptic zone is about 0.3 mu. These findings may indicate that cholinergic excitation of some or all granule-containing cells causes a reciprocal inhibition of one or more cholinergic terminals.