www.neuron.yale.edu

This may be answered elsewhere, but i didn't find it.

I am wondering about the temporal dynamics of the alpha synapse... I think it shuts off at some point after activation, no? In more detail my question is as follows:

I am modeling a single spine, with an alpha synapse on the spine head. I measure voltage drop across the neck as a function of neck length. When I make the neck long, the head becomes electrically isolated and so the synapse has a big effect. As the synapse closes again, the voltage drops, but at some point i think NEURON stops computing the tail of the alpha function and simply shuts it. The current flow stops instantly, and the membrane potential drops very quickly (see also attached figure). I think usually that is not a big deaql because the small current at the end of the tail of the alphafunction does have a small effect. In the caes of my small and highly insulated spine heads, maybe it is a different story.

Is there a way around that? Or am I doing something wrong?

Thanks. Tim

Here is a response i got from Michael Hines. Thank you Michael.

>.. the alpha function and simply shuts it.

yes. nrn/src/nrnoc/syn.mod cuts it off at 10*tau


> Is there a way around that?

Two ways.
1) copy the mod file, change the name, and modify
FUNCTION alpha(x) {
if (x < 0 || x > 10) {
alpha = 0
}else{
alpha = x * exp(1 - x)
}
}

2) Use the built-in ExpSyn2 (which also allows superposition) and
stimulate with a NetStim connected to it with a NetCon. To be similar to
alpha, just make tau1 and tau2 very very close to each other

Tim