Ribbon Synapse - Function

Function

Features of the ribbon synapse enable it to process information extremely quickly. Bipolar neurons present a good model for how ribbon synapses function.

In photoreceptor and bipolar cells information is transferred via the release of the neurotransmitter glutamate at the ribbon synapse. Conventional neurons encode information by changes in the rate of action potentials, but for complex senses like vision, this is not sufficient. Ribbon synapses enable neurons to transmit light signals over a dynamic range of several orders of magnitude in intensity. This is achieved by encoding changes in tonic rate of transmitter release which requires the release of several hundred to several thousand synaptic vesicles per second.

To accomplish this level of performance, the sensory neurons of the eye maintain large pools of fast releasable vesicles that are equipped with ribbon synapses. This enables the cell to exocytose hundreds of vesicles per second, greatly exceeding the rate of normal neurons without the specialized ribbon synapse.

The current hypothesis of calcium-dependent exocytosis at retinal ribbon synapses suggests that the ribbon accommodates a reservoir of primed releasable vesicles. The vesicles that are in closest contact with the presynaptic plasma membrane at the ribbon base constitute the small, rapidly releasable pool of vesicles, whereas the remaining vesicles tethered to the ribbon constitute the large, readily (slower) releasable pool. These regularly aligned rows of synaptic vesicles tethered to either side of the ribbon along with the expression of the kinesin motor protein KIF3A at retinal ribbon synapses can move vesicles like a conveyor belt to the docking/release site at the ribbon base.