AlphaSynapse dynamics

[VENOM]

Hello NEURON community:

This post talks about the underlying equation of AlphaSynapse. https://www.sas.upenn.edu/LabManuals/BB ... phasyn.htm

I am wondering if the "v", postsynaptic voltage, is constant (and set as resting voltage) or should change during current injection. Biologically, it should be the latter as driving force changes as current is injected; but is the current computed during simulation (thus can know how postsynaptic voltage changes) or precomputed (thus have no idea how postsynaptic voltage changes)?

[ted]

Hello NEURON community:

This post talks about the underlying equation of AlphaSynapse. https://www.sas.upenn.edu/LabManuals/BB ... phasyn.htm

I am wondering if the "v", postsynaptic voltage, is constant (and set as resting voltage) or should change during current injection. Biologically, it should be the latter as driving force changes as current is injected; but is the current computed during simulation (thus can know how postsynaptic voltage changes) or precomputed (thus have no idea how postsynaptic voltage changes)?

You'll find the NMODL source code for the AlphaSynapse class at https://github.com/neuronsimulator/nrn/ ... oc/syn.mod. The actual calculation of synaptic current is done in the BREAKPOINT block by this statement: i = g*(v - e). Here's an important fact: v is a reserved name in NMODL, and it always means "local membrane potential". And local membrane potential is computed by NEURON's computational engine by numerical integration of the discretized charge balance equation
C*dv/dt = (sum of all currents flowing into this segment from adjacent segments) + (sum of all currents injected into this segment by electrodes (e.g. patch or sharp electrodes)) + (net transmembrane ionic current for this segment).
where C is the membrane capacitance of the segment in question.

So v may vary with time, and the statement "v (mV) is the resting potential" at https://www.sas.upenn.edu/LabManuals/BB ... phasyn.htm is correct only if the postsynaptic cell has been at rest for a long time, so that v will have settled to resting potential. But v will start to diverge from resting potential as soon as synaptic current starts to flow. It would be much better if the statement were revised to read something like this: "v (mV) is the membrane potential in the postsynaptic cell at the location where the synapse is attached"

revised 20250524 by NTC