Ultrastructural organization of receptor cell axons in frog olfactory nerve

Repeated odorant or electrical stimulation of the frog olfactory nerve leads to long-lasting reduction of excitability of the receptor neurons. In the turtle olfactory nerve electrical stimulation causes an increase in extracellular potassium concentration due to the efflux of potassium from active axons. Elevated potassium concentration depolarizes an axon membrane and inactivates it. If axons travel parallel to each other in the nerve for extended distances, changes in ionic concentration due to activity in one axon will reduce excitability of its neighbors. This phenomenon may have effects like those of lateral inhibition among neighbors first described in Limulus and may act as a filter with a long time constant, for the nervous message. The amphibian olfactory nerve consists of densely packed, small-diameter, unmyelinated fibers. In this study we have examined the ultrastructure of the frog (Rana pipiens) olfactory nerve in longitudinal and cross-sections to determine whether axons follow a sinuous course and change neighbors often along the length of the olfactory nerve, or whether they follow parallel trajectories and thus tend to stay close to the same neighbors. We have found that axons have a very straight course within the nerve and that axons tend to remain adjacent to the same individual axons over long distances. We think that the anatomical substrate of the olfactory nerve favors strong inhibitory axon-axon interaction.