EPSP/IPSP amplitude values?

I'm working with a Hodgkin-Huxley model that receives synaptic inputs from presynaptic neurons. If it receives a spike from an excitatory presynaptic neuron, the voltage of my HH neuron inceases by an amount $g$. If it receives a spike from an inhibitory neuron, the voltage jumps down by an amount $g \times K$ (so K is the factor controlling how much the voltage drops relative to how much it would increase from an excitatory neuron). I read in textbooks that the ratio of excitatory:inhibitory neurons is generally $4:1$. That implies that for the system to be "balanced", I should have $K=4$. If $K<4$, then there is a net drift upwards in voltage towards the threshold values, and for $K>4$, there is a net drift downwards away from the threshold.

I have two sub-questions:

1) Is there a typical value of $g$ (the EPSP amplitude) that is biologically plausible? I've seen anywhere from $0.1-0.5$ and above which is quite a big range. I know that there is often a big range based on stimulus and whatnot, but I'm looking for a biologically reasonable value.

2) What is a typical value of $K$? In other words, what is the size of a typical IPSP amplitude relative to the EPSP amplitude? I've seen anywhere from the amplitude being the same to the amplitude being 4x the EPSP amplitude (which of course is a mighty big range).

• this question is based on a misunderstanding that should be clarified here: cogsci.stackexchange.com/questions/8206/… – honi Jun 3 '16 at 17:12
• also, g is conductance, not a magnitude of voltage change. the amount of voltage change due to a particular conductance value depends on the driving force of that conductance. – honi Jun 3 '16 at 17:14
• what resource are you using? – honi Jun 3 '16 at 17:15
• @honi Sorry, I think $g$ was just bad notation (I should probably use $Q$ instead or something other than $g$). I was looking at these papers: arxiv.org/pdf/cond-mat/0608552v1.pdf (Eq 2) and link.springer.com/article/10.1140%2Fepjb%2Fe2012-30282-0 (bottom right of page 2) – Brenton Jun 3 '16 at 20:22
• I'm not sure why you would use Hodgkin-Huxley neurons if you are not using conductance based inputs... The whole point of the HH model is that it is in terms of conductances. – honi Jun 5 '16 at 1:41

1 Answer

By definition, current-based synapses are biologically implausible. After all, biological synapses are conductance-based [1]. It is possible that current based synapses are sufficient to answer the question you are asking. It is also possible that they are not. Generally, biological plausibility is not a matter of the parameter values you are using but rather a matter of whether the question you are asking has biological relevance. After all, unless you have a morphologically accurate simulation including every single receptor type, etc. your model is not biologically realistic.

The question you are asking doesn't have a good answer because you are asking a biologically non-sensical question: what is the value of current based synapse magnitudes?
The only way one could possibly answer that question is if one knew why that question needs an answer. Maybe you don't even need a reasonable value for the question you are trying to answer because the phenomenon is robust. Maybe you need the answer to the fifth decimal place because your model is very sensitive to that value.

Point is, biological realism is incredibly context-specific.

[1] Although I have heard a strange argument that the high resistance at the spine neck means that synapses are practically current sources. I don't know anyone that relies on that.

• EPSPs are potentials, not currents. EPSCs are the corresponding currents. Question seems to be on the former, which seems to be a valid question. But I don't see reason why the latter would be incorrect to ask either. – AliceD Jun 5 '16 at 6:00
• good point. Although my point still stands. A given synapse does not provide a constant amount of current or voltage to the soma. It provides conductance. The size of the EPSC or EPSP is incredibly dependent on what else is going on at the moment. What the current voltage is, how much shunting there is, how many voltage-activated channels there are, where in their activation range the current voltage is, etc. There is no "biologically-plausible" value of EPSP magnitudes for a single synapse. – honi Jun 5 '16 at 14:56