Electron microscopic comparison of the terminals of two electrophysiologically distinct types of primary vestibular afferent fibers in the cat.

Two types of electrophysiologically distinct vestibular primary afferents (regular-type and irregular-type) were injected intraaxonally with HRP and their terminals were compared using the electron microscope. In general, regular-type terminals were small and two or more terminals contacted a single dendrite side by side, whereas irregular-type terminals were large and covered a wide surface of the dendrites. Both types of terminals contained spherical clear vesicles with almost the same diameter. Prominent postsynaptic membrane specializations were observed in both types. Mitochondria in both types had an elliptical profile with the same elongation index, but mitochondria in the regular-type terminals were thicker than those in the irregular-type ones.