Variations of trigger zones in vestibular afferent fibers of fish

The ferric chloride-ferrocyanide cytochemical stain was used to selectively visualize spike trigger zones of 8th nerve fibers in 3 species of fish. Within the sensory epithelia of the otolithic organs, branches in arbors of large-diameter myelinated axons, but not in arbors of small-diameter myelinated axons, stained. Stained extraepithelial nodes of Ranvier were found in axons of all diameters. In the electron microscope, synaptic bodies in hair cells were adjacent to both stained and unstained postsynaptic membrane. The contrasting locations of excitable membrane suggest different modes of synaptic integration between large-and small-diameter axons. This difference may underlie the physiologically defined classes of afferent 8th nerve axons in fish.