Frog sympathetic ganglion cells have local axon collaterals

Amphibian autonomic ganglia have been used as simple models for studies involving the physiology of synaptic transmission. These models assume an anatomical simplicity where the ganglion is a simple relay for central nervous system output to peripheral autonomic targets. Cholinergic preganglionic fibers innervate the soma and proximal axon of the unipolar ganglion cells, which were thought to relay the information to the periphery with little ganglionic processing. However, several different types of synaptic potentials occur in response to preganglionic stimulation. Also, a variety of neuropeptides are found in both preganglionic fibers and ganglion cells; at least one of the peptides found in preganglionic fibers is known to act as a neurotransmitter in the ganglion. Finally, there may be communication between ganglion cells. In the present study, we have explored the morphology of lumbar sympathetic chain ganglion cells by intracellular injection with horseradish peroxidase to determine whether an anatomical substrate exists for processing information within these ganglia. We have shown that 39% of these cells have axons that branch within the ganglion. While both major classes of ganglion cells (B cells and C cells) had intraganglionic axon collaterals, there was a marked difference in the frequency: 65% of the C cell axons had collaterals while only 19% of the B cell axons collateralized within the ganglion. Ultrastructural examination of labeled axon collaterals indicated that these collaterals receive synaptic input; whether the collaterals also make synapses has not been definitively established.