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axon5.hoc
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axon5.hoc
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/*
Frankenhaeuser-Huxley model of myelinated axon
with parameters from Reilly.
External diameter is 5 um.
Origin of cell is the 0 node of the first node of Ranvier.
Note: Resting potential is close to, but not exactly, -70 mV.
Actual value is approximately -69.77 mV.
Spike threshold for intracellular stimulus at middle of first node
is 0.943 nA x 0.1 ms for adaptive integration
and
1.015 nA x 0.1 ms for integration with fixed dt = 0.025 ms.
Distance between centers of first and last nodes is 100250 um = 10.025 cm.
Conduction velocity
spike detection threshold = -20 mV
path length = 81201.25 - 21051.25 = 60150 um
integrator internode[20](1) [80](1) latency CV
fixed dt 2.7 ms 6.05 ms 3.35 ms 17960 um/ms = 17.96 mm/ms = 17.96 m/s
adaptive 2.315 ms 4.967 ms 2.652 ms 22680 um/ms = 22.67 mm/ms = 22.67 m/s
*/
NNODES = 101 // # nodes of Ranvier--must be an odd number!
// NNODES = 5 // # nodes of Ranvier for debugging
GAP = 2.5e-4 // width of node of Ranvier in cm
// DIAM = 10 // fiber diameter in um including myelin
DIAM = 5 // fiber diameter in um including myelin
SDD = 0.7 // axon diameter is SDD*DIAM
ELD = 100 // length of internode/fiber diameter
RHOI = 100 // cytoplasmic resistivity in ohm cm
// RHOI = 110 // cytoplasmic resistivity in ohm cm (McNeal 1976)
// note: Hines and Shrager 1991 agree with McNeal 1976
// RHOE = 300 // extracellular resistivity in ohm cm, now defined in initxstim.hoc
CM = 2 // specific membrane capacitance in uf/cm2
///// topology
create node[NNODES], internode[NNODES-1] // node at each end
access node[0]
for ii=0,NNODES-2 {
connect internode[ii](0), node[ii](1)
connect node[ii+1](0), internode[ii](1)
}
// topology() // to verify node-internode-node architecture
///// subsets
objref all, nodes, internodes
all = new SectionList()
nodes = new SectionList()
internodes = new SectionList()
forall all.append() // all sections
forsec "internode" internodes.append() // all internodes
forsec "node" nodes.append() // all nodes and internodes
forsec internodes nodes.remove() // now nodes contains only nodes
/*
print "NODES:"
forsec nodes print secname()
print "INTERNODES:"
forsec internodes print secname()
*/
///// geometry
forsec nodes {
pt3dclear()
pt3dadd(0,0,0, SDD*DIAM)
pt3dadd(GAP*1e4,0,0, SDD*DIAM)
}
forsec internodes {
pt3dclear()
pt3dadd(0,0,0, SDD*DIAM)
pt3dadd(ELD*DIAM,0,0, SDD*DIAM)
}
define_shape() // adjust all pt3d data relative to root section
/* don't bother with this--control location of stimulus electrode instead
// move axon to left so the middle of its middle node is at (0,0,0)
proc centerit() { local i, axlen, offset
node[0] distance() // distance is relative to node[0](0)
node[NNODES-1] axlen = distance(1) // total length of model
// move node[0](0) to (-axlen/2, 0, 0)
node[0] for i = 0,n3d()-1 pt3dchange(i, -axlen/2 + x3d(i), y3d(i), z3d(i), diam3d(i))
define_shape() // adjust all pt3d data relative to root section
node[(NNODES-1)/2] print "middle of node ", (NNODES-1)/2, \
"is now at ", (x3d(n3d()-1)+x3d(0))/2
}
centerit()
*/
///// biophysics
/*
fh and related parameters from MLH implementation of
Frankenhaeuser B, Huxley AF (1964)
The action potential in the myelinated nerve fiber
of Xenopus Laevis as computed on the basis of
voltage clamp data. J Physiol 171:302-15
ModelDB entry 3507
according to which v_init = -70
Values for pnabar_fh, pkbar_fh, ppbar_fh, and gl_fh
are from
Hines M, Shrager P (1991)
A computational test of the requirements for conduction
in demyelinated axons
Restorative Neurology And Neuroscience 3:81-93
ModelDB entry 3798
Hines and Shrager included FH space
and detailed electrical properties of myelin,
which the following omits.
They also assumed extracellular resistivity
adjacent to the fiber was 80 ohm cm.
*/
// forall Ra = 110 // from Hines and Shrager 1991
forall Ra = RHOI // Reilly
PNabar = 8e-3 // cm/s from Hines and Shrager 1991
PKbar = 1.2e-3
PPbar = 0.54e-3
nGm = 0.0303
// nGm = 0.0304 (table 1 in McNeal 1977
// but note 0.0303 on p. 336 of McNeal 1977)
// iGm = .0005 // internode membrane conductance--unused here
forsec nodes {
cm = CM
insert fh
pnabar_fh = PNabar
pkbar_fh = PKbar
ppbar_fh = PPbar
gl_fh = nGm
nai = 13.74
nao = 114.5
ki = 120
ko = 2.5
}
forsec internodes {
cm = 1e-6 // unless using extracellular, in which case set it to CM
}
celsius = 20
// celsius = 22.03 // 295.18 - 273.15 (McNeal 1977)