ULTRASTRUCTURE OF SMALL NEURONS IN SUPERIOR CERVICAL GANGLION

The granule-containing cells of the rat superior cervical sympathetic ganglion were examined by light and electron microscopy. They are sparsely distributed in small cluster of about 2-5 cells. Each group has an incomplete sheath of satellite cells and lies in apposition to a blood capillary. The granule-containing cells contain numerous microvilli that project into the intercellular clefts, and prominent in their cytoplasm are dark-cored vesicles with 1400 Å average overall diameter. Available evidence suggests that these contain a catecholamine, probably noradrenalin or its close analogue dopamine. On their cell bodies and minor processes they receive synaptic boutons which, judged by the types of vesicles found in them, are pre-ganglionic. In addition, they posses longer processes: these form synaptic contacts with nerve elements that are likely to be dendrites of ganglionic neurons. The dark-cored vesicles that approach the pre-synaptic membrane differ from those described in other mammalian synapses. On the basis of this evidence, the granule-containing cells can be classified as interneurons which receive impulses from pre-ganglionic sympathetic fibers and transmit to small nerve processes within the ganglion. Although they are commonly referred to as chromaffin cells they should not be allowed membership of this class, since they reportedly do not give the chromaffin reaction. The function of the interneuron may be to provide negative feedback to ganglionic neurons, thus limiting their dependence on direct signals from the pre-ganglionic neuron. From the numbers of interneurons that are found in the ganglia, it may be inferred that their contacts are restricted to ganglion cells on a particular pathway. Information about the kind of sympathetic pathway that would require such an interneuron would be welcome. © 1969.