action potential initiation site

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luciana
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Re: action potential initiation site

Post by luciana »

ted wrote:What happens if you make soma.diam even larger? Does the range of stimulus amplitudes that make the axon fire first become wider or narrower
Ted, if soma.diam become even larger, I can't find any stimulus amplitude that make the axon fire first. I pass from soma firing first to none fire state.
However, if I decrease stimulus amplitude from 0.1 nA to 0.09 nA, the range of soma.diam that make the axon fire first become wider:
soma.diam <= 3, none fire
4 <= soma.diam <= 9, axon fire first
soma.diam >= 10, none fire

So, it seems that we obtain this behavior only with lower stimulus amplitudes and therefore, lower somatic diameters.
I'm wondering about the math behind this, soma has higher Cm (Cm=cm*surface) than axon, but also more current is flowing across its membrane.
I might think Cm delay the action potential occurrence, but more channels accelerate the action potential occurrence. So what is the deal between them?
ted
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Re: action potential initiation site

Post by ted »

luciana wrote:if soma.diam become even larger, I can't find any stimulus amplitude that make the axon fire first. I pass from soma firing first to none fire state.
In the context of this discussion, the plain meaning of the words
"none fire state"
is that after increasing the diameter of the soma, you found it impossible to make the model generate an action potential. But the soma membrane has the hh mechanism so making the soma larger only adds more excitable membrane to the soma, and a large enough stimulus current would easily make that membrane spike. "I found it impossible to make the model generate an action potential" may not be what you intended to say, but the use of private jargon invites misunderstanding. It is best to avoid jargon entirely. In plain terms, what was your observation?

Before discussing the effects of changing soma diameter, let me ask one question: what were the values of nseg in your model?
luciana
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Joined: Sun Mar 09, 2014 2:29 pm

Re: action potential initiation site

Post by luciana »

I'm sorry my english is not very good. Let's start again.
ted wrote: What happens if you make soma.diam even larger? Does the range of stimulus amplitudes that make the axon fire first become wider or narrower?
When I increase the diameter of the soma (soma.diam), the range of stimulus amplitudes that make the axon fire first keep constant (~0.01 nA). But, absolute values become bigger:
soma.diam=20 -> 0.24 - 0.25 nA
soma.diam=40 -> 0.27 - 0.28 nA

However, when I increase the diameter of the soma, the temporal difference between action potential peak in the axon and in the soma become bigger.
Is this clearer than my last post?
luciana
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Joined: Sun Mar 09, 2014 2:29 pm

Re: action potential initiation site

Post by luciana »

Oh, I forgot answer your other question.
ted wrote: Before discussing the effects of changing soma diameter, let me ask one question: what were the values of nseg in your model?
nseg = 1 for both compartments (soma and axon).
ted
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Re: action potential initiation site

Post by ted »

Thanks for the extra information.

If dend is 500 um long, 2 um diam, and axon is 500 um long, 1 um diam, their nseg values should be at least 9 and 11*, respectively, to get good spatial accuracy. Those are the values that would be appropriate for Ra = 35.4 ohm cm and specific membrane capacitance cm = 1 uf/cm2. Changing spatial discretization will affect stimulus threshold. I wonder what it will do to how soma diameter affects stimulus threshold and the timing of spike onset. At the very least I expect that you will see axonal spike initiation when stimulus amplitude is low.

*--those values are what the d_lambda rule would recommend. If you're not familiar with that, see
Hines, M.L. and Carnevale, N.T.
NEURON: a tool for neuroscientists.
The Neuroscientist 7:123-135, 2001.
PDF is available from a link at http://www.neuron.yale.edu/neuron/nrnpubs
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