Vesicles

Katz quantal model

Docked vesicles

Defines the number of readily releasable vesicles a synapse has available. A consequence of having a limited number is depletion at high stimulus frequency.
CNS synapses may have only a small number of docked vesicles on the order of 5-10 vesicles for a hippocampal CA1 synapse

Cycle

1. Docking
   Docked vesicles lie close to plasma membrane (within 30 nm)

2. Priming
   Primed vesicles can be induced to fuse with the plasma membrane by sustained depolarization, high K+, elevated Ca++, hypertonic sucrose treatment

Vesicles in the reserve pool undergo priming to enter the readily-
releasable pool

At a molecular level, priming corresponds to the assembly of the SNARE
complex

3. Fusion
   Vesicles fuse with the plasma membrane to release transmitter. Physiologically this occurs near calcium channels, but can be induced experimentally over a large area. The active zone is the site of physiological release, and can sometimes be recognized as an electron-dense structure.

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