The neurotransmitter release dynamics of the CA3-CA1 hippocampal synapse represent a commonly studied system. Nadkarni et al recently published a paper on the dynamics of en passant axon buttons that is pretty computationally intensive. Of interest are the button parameters they included in their model, which were:
- voltage-dependent calcium channels (VDCC), numbering between 1 and 208 units, with the max radius of the cluster = 66 nm
- plasma membrane calcium ATPase pumps (keep the calcium level at 100 nM), surface density is 180 μm^-2
- calbindin (the 28,000 dalton form), the mobile intracellular calcium buffer that modifies the calcium diffusion rate (~ 50 μm^2/s)
- an active zone with (7) docked vesicles, each with its own synaptotagmin calcium sensor,
- the distance between the active zone and the voltage-dependent calcium channels, marked as lc, and varying in length between 10 and 400 nm
- length of en passant synapse = 0.5 μm high /wide and 4 μm long
In their model, when an action potential arrives at the synapses, the voltage-dependent calcium channels open with some probability. Here, the amount of calcium that enters the button depends on the time course of the action potential, the number of VDCCs present on the membrane, the calcium conductance of open channels, and the total time each of the channels remains open. Calcium in the button can bind to calbiding, calcium pumps, or calcium sensors. If enough binds to the calcium sensors, the synaptotagmin sensor transitions to an active state and the vescicle containing the neurotransmitter is released. Moreover, each of these components has association and dissociation rates that can be tweaked.
Here is the visual display of their model with the above parameters included:
Now, variability in the parameters (number, length, distance between) of these components ought to affect the release probability and kinetics of vescicles. They found this to be the case. For example, in their model, decreasing the distance between the active zone and the voltage-dependent calcium channels, marked as lc, decreases the sensitivity of neurotransmitter release probability to calcium concentration:
So, in order to fully simulate the function of actual synapses, you’d probably need to get some of this structural data! However, some of the parameters could probably be assumed to the same between synaptic buttons. For example, the quantity and kinetics of calbidin could probably be assumed to be about enough so that the diffusion constant of calcium remains ~ 50 μm^2/s.
Nardkari S, et al. 2010 Title: Spatial and Temporal Correlates of Vesicular Release at Hippocampal Synapses arXiv:1004.2009v1 [q-bio.NC].