Synaptic organisation of lumbar sympathetic ganglia of guinea pigs: Serialsection ultrastructural analysis of dye-filled sympathetic final motor neurons
Il. Gibbins et al., Synaptic organisation of lumbar sympathetic ganglia of guinea pigs: Serialsection ultrastructural analysis of dye-filled sympathetic final motor neurons, J COMP NEUR, 402(3), 1998, pp. 285-302
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.