how to define the diameter of myelin

[melissamou]

Hi Ted,
I just confused about the diameter of the myelin, if we name d as diameter of the node, and D is the diameter of the fiber(include the sheath), so is the diameter of the myelin equal D or d?

and somebody talks about the thickness of the myelin, what the thickness of myelin means? how does NEURON deal with the thickness of myelin?

the cm for myelin, in the model"Myelinated axon conduction velocity (Brill et al 1977) ", they define the cm as cm = 1.87e-11*l2a()*1e6 // end up as uF/cm2
, why isn't a constant here?

[ted]

In NEURON the extracellular mechanism can be used to represent the electrical effects of myelination--read about it here
http://www.neuron.yale.edu/neuron/stati ... racellular
Myelin thickness is not a parameter of the extracellular mechanism; instead, the electrical properties of the myelin sheath are represented directly by xc, xg, and xraxial, as indicated by the diagram.

An alternative way to represent the effects of myelination that does not involve the use of extracellular is to simply reduce membrane specific conductance and capacitance. That's what Brill et al. did.

they define the cm as cm = 1.87e-11*l2a()*1e6 // end up as uF/cm2
, why isn't a constant here?

NEURON requires membrane conductance and capacitance parameters to be in units per area. The comments in
http://senselab.med.yale.edu/modeldb/Sh ... \cable.hoc
suggest that their membrane conductance and capacitance parameters were in units per length. The expression that puzzles you simply multiplies their parameter values by the scale factor necessary to convert from units per length to units per area.

[hallockk]

how does NEURON deal with the thickness of myelin?

If you wanted to add it in, you could do a little programming and include it as part of the calculation for the a diameter.