Synaptic organisation of lumbar sympathetic ganglia of guinea pigs: Serialsection ultrastructural analysis of dye-filled sympathetic final motor neurons

The authors serially sectioned seven dye-filled neuronal somata and more th an 1.6 mm of their dendrites from the lumbar sympathetic ganglia of guinea pigs and examined them ultrastructurally to determine the distribution of p reganglionic synaptic inputs to their dendrites and cell bodies. Most of th e surface of the neurons was covered with Schwann cells. Apposing boutons w ere rare, with an average density of one axosomatic bouton per 125 mu m(2) of somatic membrane and one axodendritic bouton per 25 mu m of dendrite. Ma ny dendritic segments that were more than 50 mu m long completely lacked an y apposing boutons. Although the average density of apposing boutons was lo w, local densities could be high, so that clusters of up to four adjacent b outons occurred on cell bodies and dendrites alike. The spatial arrangement of the apposing boutons for each of the cells examined here was not signif icantly different from a random distribution. Consequently, the number of a pposing boutons observed for any neuron was simply proportional to the amou nt of neuronal surface sampled in the serial section run. About 50% of bout ons directly apposing the neurons lacked any detectable presynaptic special isations. When they were present, the presynaptic densities had a mean leng th of about 220 nm, with no difference between boutons that made axosomatic or axodendritic appositions. By applying these data to complete reconstruc tions of the dendritic trees of dye-filled sympathetic neurons at the light microscopic level, the authors estimated that few neurons in the lumbar sy mpathetic chain of guinea pigs would receive more than 200 synapses or appo sing boutons and that many of them would receive less than 100 synapses. Up to 50% of these boutons would be predicted to make axosomatic contacts. Th ese new observations provide a strong morphological framework for a better understanding of how sympathetic final motor neurons process their pregangl ionic synaptic inputs. J. Comp. Neural. 402:285-302, 1998. (C) 1998 Wiley-L iss, Inc.