Released fraction and total size of a pool of immediately available transmitter quanta at a calyx synapse

Neuron. 1999 Jun;23(2):399-409. doi: 10.1016/s0896-6273(00)80789-8.


The size of a pool of readily releasable vesicles at a giant brainstem synapse, the calyx of Held, was probed with three independent approaches. Using simultaneous pre- and postsynaptic whole-cell recordings, two forms of presynaptic Ca2+ stimuli were applied in rapid succession: uncaging of Ca2+ by flash photolysis and the opening of voltage-gated Ca2+ channels. The ensuing transmitter release showed a nearly complete cross-inhibition between the two stimuli, indicating the depletion of a limited pool of about 700 transmitter quanta. The pool size was confirmed in experiments using enhanced extracellular Ca2+ concentrations, as well as short, high-frequency stimulus trains. The results reveal a surprisingly large pool of functionally available vesicles, of which a fraction of about 0.2 is released by a single presynaptic action potential under physiological conditions.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Action Potentials
  • Algorithms
  • Animals
  • Brain Stem / physiology*
  • Calcium / metabolism
  • Calcium Channels / metabolism
  • Calcium Channels / physiology
  • Electric Stimulation
  • Excitatory Postsynaptic Potentials / physiology
  • In Vitro Techniques
  • Ion Channel Gating
  • Neurotransmitter Agents / physiology*
  • Patch-Clamp Techniques
  • Photolysis
  • Rats
  • Synapses / physiology*


  • Calcium Channels
  • Neurotransmitter Agents
  • Calcium