Synaptic vesicle dynamics: A simple model of phasic release

We present a simple model of phasic neurotransmitter release which reproduces the salient features of chemical neurotransmission. The synaptic vesicle cycle has been modelled as a set of biochemical reactions represented by a system of coupled differential equations. These equations have been solved analytically to obtain the time dependent behaviour of the system on perturbation from the steady state. The scheme of the synaptic vesicle network has been emphasized and its role in determining some of the major experimentally observed properties of synaptic transmission has been discussed, which includes the biphasic decay of the rate neurotransmitter release even under sustained stimulation. Another interesting outcome of this theoretical exercise is the saturation of total release with the calcium dependent rate constant. The theoretically calculated values of total release fit very well into a sigmoidal saturating function with a fourth order cooperativity exponent similar to the empirical Dodge-Rahamimoff equation. It appears that the synaptic vesicle network itself is responsible for some of the major properties associated with chemical neurotransmission.