Neuronal morphology and the synaptic organisation of sympathetic ganglia

In this article, we provide a short review of the structure and synaptic or ganisation of the final motor neurons in the sympathetic ganglia of mammals . Combinations of pathway tracing, multiple-labelling immunofluorescence an d intracellular dye injection have shown that neurons in different function al pathways differ nor only in their patterns of neuropeptide expression, b ut also in the size of their cell bodies and dendritic fields. Thus, vasoco nstrictor neurons consistently are smaller than any other major functional class of neurons. Serial section ultrastructural analysis of dye filled neu rons, together with electron microscopic and confocal microscopic analysis of immunolabelled synaptic inputs to sympathetic final motor neurons indica te that synapses are rare and randomly distributed over the surface of the neurons. The total number of synapses is simply proportional to the total s urface area of the neurons. Many terminal boutons of peptide-containing pre ganglionic neurons do nor make conventional synapses with target neurons. F urthermore, there is a spatial mismatch in the distribution of peptide-cont aining terminals and neurons expressing receptors for the corresponding pep tides. Together, these results suggest that there are likely to be signific ant differences in the ways that the final sympathetic motor neurons in dis tinct functional pathways integrate their synaptic inputs, fn at least some pathways, heterosynaptic actions of neuropeptides probably contribute to s ubtle modulation of ganglionic transmission. (C) 2000 Elsevier Science B.V. All rights reserved.