/* Sets nseg in each section to an odd value
so that its segments are no longer than
d_lambda x the AC length constant
at frequency freq in that section.
Be sure to specify your own Ra and cm before calling geom_nseg()
To understand why this works,
and the advantages of using an odd value for nseg,
see Hines, M.L. and Carnevale, N.T.
NEURON: a tool for neuroscientists.
The Neuroscientist 7:123-135, 2001.
*/
// these are reasonable values for most models
// freq = 100 // Hz, frequency at which AC length constant will be computed
// d_lambda = 0.1
func lambda_f() { local i, x1, x2, d1, d2, lam
if (n3d() < 2) {
return 1e5 * sqrt(diam / (4 * PI * $1 * Ra * cm))
}
// above was too inaccurate with large variation in 3d diameter
// so now we use all 3-d points to get a better approximate lambda
x1 = arc3d(0)
d1 = diam3d(0)
lam = 0
for i = 1, n3d() - 1 {
x2 = arc3d(i)
d2 = diam3d(i)
lam += (x2 - x1)/sqrt(d1 + d2)
x1 = x2
d1 = d2
}
// length of the section in units of lambda
lam *= sqrt(2) * 1e-5 * sqrt(4 * PI * $1 * Ra * cm)
return L / lam
}
proc geom_nseg() { local freq, d_lambda
freq = $1
d_lambda = $2
forall {
nseg = int((L / (d_lambda * lambda_f(freq)) + 0.9) / 2) * 2 + 1
}
}