Density of somatic innervation on mammalian autonomic ganglion cells is inversely related to dendritic complexity and preganglionic convergence.

I have studied superior cervical ganglion cells in mouse, hamster, rat, guinea pig, and rabbit by electron microscopy to determine how the distribution of synapses on these neurons is affected by the systematic differences in dendritic morphology and preganglionic convergence that are evident in the superior cervical ganglia of these species (Purves, D., and J. W. Lichtman (1985) Science 228: 298-302). The density of dendritic innervation is approximately the same in these animals regardless of target cell geometry (and always greater than the density of synapses on the cell soma); however, more complex ganglion cells, which are innervated by commensurately more axons, receive progressively fewer cell body synapses than geometrically simpler ganglion cells. Evidently the somatic membrane becomes a less favorable site for synapse formation as dendritic complexity and the number of inputs increase. This paradoxical decrease in the density of somatic innervation as preganglionic convergence increases may reflect competitive interactions between the axons innervating individual ganglion cells.