Figure 10.

Proposed vesicle cycle at the cone ribbon synapse. Vesicles can be resupplied to the synaptic ribbon through a fast mechanism (τ ∼800 ms) that is regulated by the actions of Ca²⁺/CaM on vesicle attachment sites on the ribbon, several hundred nanometers distant from Ca²⁺ entry through channels located at the ribbon base. Vesicles can also attach through a slower, Ca²⁺/CaM-independent process that has a time constant of ∼13 s. Vesicle priming appears to involve the synaptic ribbon (Snellman et al., 2011). Exocytosis occurs in two phases, with time constants of 6 ms and 170 ms that represent fast fusion of the IRP (blue) and movement of vesicles from the ribbon-associated reserve pool (yellow) to release sites near the ribbon base, respectively. Endocytosis is fast (τ = 250 ms; Van Hook and Thoreson, 2012) and returns vesicles to a cytoplasmic reservoir pool, where they are refilled with glutamate (τ ∼15 s; Hori and Takahashi, 2012).