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The fine structure of intraganglionic blood vessels of rat superior cervical sympathetic ganglia is studied with the light microscope and with both conventional and ultrastructural histochemical methods. Two sets of small capillaries together with larger sinusoidal ones are identified. One set of capillaries is associated with the clustered (type II) small catecholamine-containing (CC) cells and exhibits features suggestive of fluid transport function (multiple wide fenestrae and active pinocytosis). The second set of capillaries is in direct relation to the sympathetic neurons (SN) and shows characteristics suggestive of absorptive function (microvilli and pinocytotic vesicles). The larger sinusoidal capillaries are observed in the vicinity of type II CC cells, extend parallel to the long axes of the perikarya of the neurons and occasionally form loops around them. The latter are assumed to be larger blood spaces connecting the two capillary sets and serve to slow the circulation around the neurons. A pattern of portal-like intraganglionic microcirculation through which type II CC cells participate in modulating the SN is postulated. Type II CC cells secrete a catecholamine modulator which, driven by concentration gradient, gains access to the circulation through the fenestrated capillaries. The sinusoidal capillaries serve to perfuse the SN with a slow stream of blood rich in the catecholamine modulator. The latter can be filtered through the microvilli and pinocytotic vesicles of the second set of capillaries to induce slow inhibitory postsynaptic potential on the SN.

The synapses of the rat superior cervical sympathetic ganglion were studied with both conventional and ultrastructural histochemical methods. Besides the cholinergic synapses polarized from preganglionic fibers to sympathetic ganglion neurons, two morphologically and functionally different types of synapses were observed in relation to the small granule-containing (catecholamine-containing) cells of the rat superior cervical ganglion. The first type is an efferent adrenergic synapse polarized from granule-containing cells to the dendrites of the sympathetic ganglion neurons. This type of synapse might mediate the inhibitory effects (slow inhibitory postsynaptic potentials) induced by catecholamines on the sympathetic neurons. The second type is a reciprocal type of synapse between the granule-containing cells and the cholinergic preganglionic fibers. Through such synapses, these cells could exert a modulating effect on the excitatory preganglionic fibers. Therefore, we propose that these cells, through their multiple synaptic connections, exhibit a local modulatory feedback system in the rat sympathetic ganglia and may serve as interneurons between the preganglionic and postganglionic sympathetic neurons.