Neuronal morphology and the synaptic organisation of sympathetic ganglia

In this article, we provide a short review of the structure and synaptic organisation of the final motor neurons in the sympathetic ganglia of mammals. Combinations of pathway tracing, multiple-labelling immunofluorescence and intracellular dye injection have shown that neurons in different functional pathways differ not only in their patterns of neuropeptide expression, but also in the size of their cell bodies and dendritic fields. Thus, vasoconstrictor neurons consistently are smaller than any other major functional class of neurons. Serial section ultrastructural analysis of dye filled neurons, together with electron microscopic and confocal microscopic analysis of immunolabelled synaptic inputs to sympathetic final motor neurons indicate that synapses are rare and randomly distributed over the surface of the neurons. The total number of synapses is simply proportional to the total surface area of the neurons. Many terminal boutons of peptide-containing preganglionic neurons do not make conventional synapses with target neurons. Furthermore, there is a spatial mismatch in the distribution of peptide-containing terminals and neurons expressing receptors for the corresponding peptides. Together, these results suggest that there are likely to be significant differences in the ways that the final sympathetic motor neurons in distinct functional pathways integrate their synaptic inputs. In at least some pathways, heterosynaptic actions of neuropeptides probably contribute to subtle modulation of ganglionic transmission.