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Incoming synapses and size of small granule‐containing cells in a rat sympathetic ganglion after post‐ganglionic axotomy.

A quantitative ultrastructural study has been made of the reaction of the incoming synapses of small granule‐containing cells after axotomy of the major post‐ganglionic branches of the superior cervical ganglion of the young adult rat. These cells are intrinsic and interneurone‐like in this ganglion, receiving a preganglionic input and giving outgoing synapses to principal post‐ganglionic neurones. Unlike their outgoing synapses, which are lost after post‐ganglionic axotomy (Case & Matthews, 1986), the incoming synapses of the small granule‐containing cells in axotomized ganglia increased in incidence post‐operatively. The increase first became clearly evident 5‐7 days post‐operatively and was greater, being both more sustained and progressive, after bilateral than after unilateral axotomy. After bilateral axotomy the incidence of incoming synapses rose to more than four times that of normal ganglia and was still elevated at 128 days post‐operatively, but was within normal limits at 390 days. After a unilateral lesion, increases of similar extent and time course to those in the axotomized ganglia were seen in the incoming synapses of small granule‐containing cells in the uninjured contralateral ganglia. The incoming synapses of the small granule‐containing cells are multifocal, i.e. show several points or active foci of synaptic specialization. The increase in synapses expressed itself both through an increased incidence of these synaptic active foci per nerve terminal and through an increase in the number of presynaptic nerve terminal profiles associated with the cells. Control observations indicated that the increase in synapses was not due to surgical stress, nor was it attributable solely to post‐operative ageing. The nerve terminals which were presynaptic to the small granule‐containing cells post‐operatively were all of preganglionic origin: no incoming synapses or presynaptic nerve terminals remained at 2 days after a preganglionic denervation of axotomized or contralateral ganglia, at whatever stage this was performed throughout the range of survival intervals. There was some evidence that the synapses had increased by sprouting, including terminal sprouting, of the preganglionic nerve fibres. In the shorter term there was an increase in the proportion of small nerve terminal profiles. In the longer term the mean size of the terminal profiles increased, and very large terminals of unusual form were seen. After post‐ganglionic axotomy, and in particular after a bilateral lesion, the small granule‐containing cells became hypertrophied.(ABSTRACT TRUNCATED AT 400 WORDS)