Functional Morphology of Monoamine-Storing Cells in the Rat Superior Cervical Ganglion

Small monoamine-containing, paraneuronal or paraganglionic cells in sympathetic ganglia are presumed to control synaptic transmission by modulating the outputs of postganglionic neurons to the effectors. Different mechanisms of action may account for this modulating effect (Williams et al. , 1976; Fig. 2-20): (1) There is electron-microscopic evidence that some paraganglionic cells are interposed between preganglionic axons and postganglionic nerve cells (Williams, 1967; Yokota, 1973). These interneurons presumably modulate transmission directly by inhibitory efferent synapses. (2) Other paraganglionic cells, forming clusters in intimate relationship with fenestrated capillaries, may demonstrate a functional association with the bloodstream, exerting their inhibitory effect indirectly on the postganglionic neuron by means of a neurosecretory mechanism (Siegrist et al. , 1968).