www.neuron.yale.edu

Hi,

Goal:
I am trying to ascertain the peak synaptic conductance value in my model (synaptic gbar).

I am using an Exp2Syn, so I believe I can just check my syn.g and this is synaptic gbar, but I am a little confused on a few things, so I'm not sure if that's correct.

Question 1: does syn.g represent synaptic gbar?
Question 2: what is the relationship between syn.g & netcon.weight? Is it g(t, V) = syn.g * netcon.weight ?
Question 3: when I measure and compute peak synaptic current / ionic driving force, I do not get the same value as syn.g. If syn.g = peak synaptic conductance, they should be equal. What's going wrong?

Setup:
- One soma1 with HH channels...
- ...connected via Exp2Syn and netcon to another soma2 with HH channels
- Soma1 is stimulated via vclamp to +55 mV which drives current through soma2 which is at -55 mV resting potential (driving force = 110 mV)

Results:
- syn.i = 0.677177 nA (to get this, I measured the max value of syn.i after running a simulation that elicits action potentials in soma2).
- ionic driving force = 110 mV (as mentioned above).
- As a result, gbar = 0.006156155. This does not equal my code value of syn.g which = .013.

Any help appreciated!

Thanks.

Good questions. You're not taking anything for granted.

alnurali wrote:I am trying to ascertain the peak synaptic conductance value in my model (synaptic gbar).

Exp2Syn does not have a gbar. It has a range variable g which is the instantaneous
(time varying) synaptic conductance. To see the source code for this mechanism, go to
http://www.neuron.yale.edu/neuron/install/getstd.html
and get the latest nrn*gz file, expand it, and descend the directory tree to
nrn-n.n/src/nrnoc/exp2syn.mod

Strictly speaking gbar is "teletype notation" for g with a horizontal bar over it, which has
been used in membrane biophysics to represent conductance density (conductance
per unit surface area) for ion channels that are distributed widely over the surface of
a cell. Channels that are involved in conductance change synaptic mechanisms,
however, are typically grouped in anatomically and electrotonically compact patches
(postsynaptic density). Synaptic conductances are therefore most efficiently represented
by point sources of current. As such, they are described in terms of net conductance at
a particular location on a cell, with units of conductance, unlike distributed membrane
conductances, which are specified in units of conductance/area.

Question 1: does syn.g represent synaptic gbar?

There is no synaptic gbar. There is only syn.g

Question 2: what is the relationship between syn.g & netcon.weight? Is it g(t, V) = syn.g * netcon.weight ?

No. g is the response of a 2nd order ODE, whose solution is described by two decaying
exponentials, to 0 or more Dirac delta function inputs. A mechanical analogy would be a
mass, spring, and dashpot (shock absorber), coupled to another mass, spring, and
dashpot, where dashpot friction is sufficiently high that the system is overdamped
(imagine a car with a very stiff ride). The first mass is driven by occasional taps of a
hammer, and the output of the system is the position of the second mass. netcon.weight
is analogous to the intensity of the hammer taps. For more information, please read
chapter 10 in The NEURON Book.

Question 3: when I measure and compute peak synaptic current / ionic driving force, I do not get the same value as syn.g. If syn.g = peak synaptic conductance, they
should be equal. What's going wrong?

To replicate your model, it is necessary to know the physical dimensions of the two
sections, the parameters of the Exp2Syn, and the netcon's weight.

Soma1 is stimulated via vclamp to +55 mV which drives current through soma2

You lost me with that "drives current through soma 2"? Is there electrical coupling
between the two sections?

which is at -55 mV resting potential (driving force = 110 mV)

Driving force for what?

This does not equal my code value of syn.g which = .013.

Your code specifies a value of syn.g? Perhaps, to eliminate ambiguities, you could
post your code?