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Posted: **Sun Oct 31, 2010 8:30 am**

Hi Ted,
i am simulating a extracellular stimulation, the model is a myelined axon with 21 myelins and 21 nodes. The extracellular electric potential is calculated from Comsol. first i get the center coordinate of each segment for all the myelined and nodes in NEURON, this is accroding your "interpxyz.hoc", then get the voltage potential as a function of time for those points in Comsol, and then add those voltage potential to these points.

but i have a question, what is the boundary condition for my model？How is Vm defined at the first node and the last node?

because here i insert pas to each myelin and insert fh to each node, need i add the extracellular potential to the myelin? When the comsol voltage is applied as an external extracellular stimulus to the segments defined as myelin, is Vm(t) =0 ?

Posted: **Sun Oct 31, 2010 12:29 pm**

Good questions.

what is the boundary condition for my model？

The default values of extracellular's xraxial, gc, and xc are such that vext, the potential immediately adjacent to the outer surface of a section, will be equal to e_extracellular.

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How is Vm defined at the first node and the last node?

What do you mean by "first node" and "last node"--the nodes at the 0 and 1 locations of a section?

here i insert pas to each myelin and insert fh to each node

I guess that this "node" means "node of Ranvier", but I have no idea what the word "here" is supposed to tell me. Perhaps you meant to say "In my model . . . " ?

need i add the extracellular potential to the myelin?

Yes.

When the comsol voltage is applied as an external extracellular stimulus to the segments defined as myelin, is Vm(t) =0 ?

Why would it ever be 0? Membrane potential is the difference in potential between the inside of a cell and the extracellular space immediately adjacent to the outside surface of the cell. In the diagram at http://www.neuron.yale.edu/neuron/stati ... racellular note that the potentials at these two locations are labeled vext+v and vext, respectively. The difference is v, which is membrane potential. Another way of saying this is "membrane potential rides on top of vext".

Posted: **Mon Nov 01, 2010 6:51 am**

ted wrote:Membrane potential is the difference in potential between the inside of a cell and the extracellular space immediately adjacent to the outside surface of the cell.

sorry, my english is not so good, do you mean the membran potential will immediately adjacent to the outside surface of the cell or the vext(the potential for extracellular space)?

ted wrote:what is the boundary condition for my model？
The default values of extracellular's xraxial, gc, and xc are such that vext, the potential immediately adjacent to the outer surface of a section, will be equal to e_extracellular.

I found in the NEURON book that said "Nerve boundary conditions are that no axial current flows at the end of the cable, i.e. the end is sealed." do you mean at the very beginning the potential is sealed and then immediately adjacent to the outer surface of a section and equal to e_extracellular?

ted wrote:How is Vm defined at the first node and the last node?
What do you mean by "first node" and "last node"--the nodes at the 0 and 1 locations of a section?

no, i mean the end of the cable, the node[0], node[20](i create 21 nodes), yes, node is node of ranvier.

ted wrote:I guess that this "node" means "node of Ranvier", but I have no idea what the word "here" is supposed to tell me. Perhaps you meant to say "In my model . . . " ?

yes, it is in my model.

Posted: **Mon Nov 01, 2010 7:17 am**

i would like to write my question clearly,
Boundary conditions in this case refer to the conditions at the ends of a simulated neuron because i have a fibre model with a finite length. All nodes of Ranvier will use the FH equations to describe the changes in Vm(membran potential), but the end nodes will have an additional specification in a computer model because the action potential will not be able to propagate into the neighboring nodes (since there is no neighboring node beyond the end of the fibre). so how is Vm defined at the node[0] and node[20] ?there are 21 nodes of Ranvier in my model.

Posted: **Mon Nov 01, 2010 11:49 am**

melissamou wrote:
ted wrote:Membrane potential is the difference in potential between the inside of a cell and the extracellular space immediately adjacent to the outside surface of the cell.
sorry, my english is not so good, do you mean the membran potential will immediately adjacent to the outside surface of the cell or the vext(the potential for extracellular space)?

Look at the diagram for the extracellular mechanism
http://www.neuron.yale.edu/neuron/stati ... racellular
The potential inside the cell is vext+v.
The potential in the extracellular space immediately adjacent to the outside surface of the cell is vext.
The difference between these is the membrane potential v.

I found in the NEURON book that said "Nerve boundary conditions are that no axial current flows at the end of the cable, i.e. the end is sealed." do you mean at the very beginning the potential is sealed and then immediately adjacent to the outer surface of a section and equal to e_extracellular?

I can't parse this:
"at the very beginning the potential is sealed and then immediately adjacent to the outer surface of a section and equal to e_extracellular"
The first part
"at the very beginning the potential is sealed"
is grammatically correct but I don't know which "potential" is being referred to, nor do I understand the meaning of "the potential is sealed".
The second part
"immediately adjacent to the outer surface of a section and equal to e_extracellular"
is a sentence fragment that I don't understand.

ted wrote:How is Vm defined at the first node and the last node?
What do you mean by "first node" and "last node"--the nodes at the 0 and 1 locations of a section?
no, i mean the end of the cable, the node[0], node[20](i create 21 nodes), yes, node is node of ranvier.

Vm has the same definition everywhere: it is the difference between the potential inside the cell and the potential in the extracellular space immediately adjacent to the outside surface of the cell. In a section that has no extracellular mechanism, the potential of the outside surface is 0; if extracellular has been inserted into a section, the potential of the outside surface is vext_extracellular at that location.

ted wrote:I guess that this "node" means "node of Ranvier", but I have no idea what the word "here" is supposed to tell me. Perhaps you meant to say "In my model . . . " ?
yes, it is in my model.

i insert pas to each myelin and insert fh to each node, need i add the extracellular potential to the myelin?

If the question is "should I insert the extracellular mechanism into the myelinated sections?" the answer is yes, especially if you are simulating the effects of extracellular fields on a cell. It is possible to simulate the effect of myelination by merely eliminating most ion channels and reducing cm, but every NEURON model of myelinated axon that I have seen has used extracellular to represent the properties of the myelin. It is not possible to simulate the effects of an extracellular field without doing one of two things: use a direct representation of what the field does to extracellular potential (that's why the extracellular mechanism is needed), or represent the effect of the field on membrane current (Rattay's "activating function" approach).

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