ribbon synapse

Ultrastructurally distinct type of synapse found in a variety of sensory receptor cells such as retinal photoreceptor cells, cochlear hair cells and vestibular organ receptors, as well as in a non-sensory neuron, the retinal bipolar cell. Unlike most neurons, these cells do not use regenerative action potentials but release transmitter in response to small graded potential changes. Ribbon synapses have different exocytotic machinery from conventional synapses in containing dense bars or ribbons anchored to the presynaptic membrane covered with a layer of synaptic vesices. The ribbons have been proposed to shuttle synaptic vesicles to exocytotic sites.