~200 μs. Simulations of the diffusion of glutamate – the most commonly found excitatory neurotransmitter in the central nervous
system – following release of a single vesicle from a presynaptic terminal has indicated that the peak concentration of glutamate at the postsynaptic receptors occurs ~10 μs after opening of the fusion pore
(Ventriglia & Di Maio, 2000). This interval includes the time it takes
the transmitter molecules to “escape” by random Brownian motion
from the vesicle through the rather narrow fusion pore and the diffusion of the transmitter molecules across the cleft.
Deficiencies in Understanding the
Concepts of Diffusion, Size & Scale as
Barriers to Understanding the Concept
of Synaptic Transmission
Chemical synaptic transmission is a complex biological process.
A deeper understanding of the underlying concept requires the
ability to break down the overall process into sub-processes and
Figure 1. Morphometric synapse model. (A) The proportions of the axon shaft, presynaptic terminal, and synaptic cleft have
been inferred from morphometric data. (B) Boxed area in A shown at higher magnification. The relative sizes of the synaptic
vesicle and the synaptic cleft were chosen so as to match the dimensions of these structures in electron micrographs. The sizes of
the calcium channels and the postsynaptic transmitter receptor/channel complex are not drawn to scale, but their spatial
proximity to the releasing vesicle has been inspired by morphometric findings. Courtesy: Günther K. H. Zupanc.