Receptor synapses without synaptic ribbons in the cochlea of the cat

This report describes recepto-neural junctions thought to be chemical synapses without synaptic ribbons or vesicle aggregates. Such synapses occur in the cat between the outer hair cells of the cochlea and auditory nerve fibers. These synapses are formed by flat or indented junctions having symmetric membrane complexes associated with smooth endoplasmic reticulum and coated vesicles in the outer hair cell cytoplasm. Inner hair cells have receptoneural junctions with asymmetric membrane complexes; synaptic ribbons and vesicles gather at one type of junction, while the other type associates with endoplasmic reticulum. Gap junctions are not seen at cochlear synapses but do link supporting cells. The results suggest that inner and outer hair cell synapses have different properties and raise questions about the role of synaptic vesicles and endoplasmic reticulum in the storage and release of transmitters.