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I understand that the nseg should be increased (by ODD multiples) until sufficient spatial resolution is reached that no improvements are seen with further increase in resolution. Once a desireable nseg has been determined, is it possible to have non-uniform channel densities such that the density at different points in a section falls off according to some function? In other words can different segments of the same section have different channel densities? If so, how does one implement this?

That's the whole point of range variables, isn't it? If section foo has a density mechanism
called mech that has a range variable fap, you can write a simple assignment statement
foo fap_mech(x) = whatever
where 0<=x<=1 and whatever is . . . whatever. The value of fap in the segment that
contains x will then be equal to whatever.

Actually you will probably be doing something more like
forsec subset for (x,0) { rangevar_suffix(x) = f(p(x)) }
where subset is a SectionList (essentially a set of sections),
p is some measure of the position of the current segment in space (e.g. distance from
the segment's midpoint to some point or plane),
and f is a function that tells how rangevar_suffix should vary with position in space.
Of course, if you then change nseg, you should again assert
forsec subset for (x,0) { rangevar_suffix(x) = f(p(x)) }
to ensure that the inhomogeneous range variable samples f at the proper locations in
space.

In recent versions of NEURON, the CellBuilder can take care of setting up nonuniform
biophysical properties very efficiently and flexibly--and it even automatically reasserts
forsec subset for (x,0) { rangevar_suffix(x) = f(p(x)) }
if you do anything in the CellBuilder that changes nseg. See the latest CellBuilder tutorial
http://www.neuron.yale.edu/neuron/stati ... /main.html