Are the presynaptic membrane particles the calcium channels?

Abstract

The number of large intramembrane particles associated with sites of synaptic vesicle release at the squid giant synapse was determined and compared to the average maximal presynaptic calcium current in order to derive an estimate of the conductance each particle would have if it were a calcium channel. This value, 0.21 pS, compares favorably with conductances of calcium channels in other preparations, substantiating the idea that the large intramembrane particles, which are concentrated at "active zones," represent calcium channels.