melissamou wrote:i am sure the diameter and other geoetry is right but still the result is not exactly linear, the slope is smaller when the diameter is bigger.

One factor I omitted from my previous post: spatial discretization can affect simulation results. Diameter and effective membrane capacitance affect the appropriate value for nseg. How are you deciding what value to use for nseg in your models? If you change diameter, you may also have to change nseg of internodes. If you are using the d_lambda rule, you'll have to calculate lambda_100 yourself, using the "effective specific membrane capacitance" of myelinated internodes; the code built into NEURON for calculating lambda_100 uses the specific membrane capacitance cm, but the "effective specific membrane capacitance" is much smaller--equals cm*cmyel/(cm+cmyel), where cmyel is the specific capacitance of the myelin itself.

There are many experimental and computational modeling articles on the anatomical and biophysical properties of myelinated axons, and the relationship between fiber diameter and conduction velocity. Have you read any of Stephen Waxman's papers, especially

Brill MH, Waxman SG, Moore JW, Joyner RW (1977)

Conduction velocity and spike configuration in myelinated fibres: computed dependence on internode distance.

J Neurol Neurosurg Psychiatry 40:769-74

or

Moore JW, Joyner RW, Brill MH, Waxman SD, Najar-Joa M (1978)

Simulations of conduction in uniform myelinated fibers. Relative sensitivity to changes in nodal and internodal parameters.

Biophys J 21:147-60

which have models in ModelDB?

Or have you read anything in his books

Physiology & Pathobiology of Axons

The Axon: Structure, Function and Pathophysiology

i think the total sodium conductance total potassium conductance, et al, are related to diameter. how does neuron deal with this?

You may have noticed that

(1) gnabar_hh, gkbar_hh, and gl_hh are in units of conductance/area

(2) sections have length and diameter

(3) segments are cyilndrical

Guess what NEURON does with this information . . .