Cross-excitation in dorsal root ganglia does not depend on close cell-to-cell apposition.

About 90% of neurons in dorsal root ganglia (DRGs) of rats 2-5 weeks of age are depolarized and excited by impulse activity in neighboring neurons that share the same DRG. Synaptic contacts are extremely rare in DRGs, but instances of close membrane apposition between pairs of neuronal somata are not uncommon, especially in prenatal rats. Close membrane apposition could permit electrotonic interactions among neighboring DRG neurons. We carried out an ultrastructural examination of DRGs taken from rats 2-5 weeks of age and found that by this age < 2% of cells remain in close apposition with neighbors. The remainder are separated by one or two layers of satellite glial cytoplasm. It is, therefore, unlikely that close apposition between adjacent neurons contributes significantly to functional cross-excitation in the DRG.