Synaptic structures in the type II hair cell in the vestibular system of the guinea pig. A freeze-fracture and TEM study.

The synaptic contacts of the type II hair cell in the vestibular system of the guinea pig was described in thin-sectioned and freeze-fractured specimens. Synaptic bodies were present at the apposition with both large and small afferent terminals. About 20% of the synaptic bodies observed consisted of complexes of two or more adjacent synaptic discs. In freeze-fracture replicas, the cytoplasmic leaflet of the hair cell plamalemma beneath the synaptic body had a bar-shaped aggregate of large particles. The size and shape of the particle aggregate was the same as that of the synaptic body. Small plasmalemmal deformations, interpreted as sites of synaptic vesicle exocytosis, were found immediately adjacent to the particle aggregate. On the postsynaptic membrane, an aggregate of intramembrane particles was present at the synaptic junction. The type II hair cell had no gap junctions or close membrane appositions between it an the apposed afferent fiber. Efferent boutons ending on the type II hair cell had no intramembrane particle specialization on the postsynaptic membrane; however those efferent boutons ending on large and small afferent fibers had an aggregate of medium-sized particles on the external leaflet of the postsynaptic bouton beneath the presynaptic active zone.