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TITLE FH channel
: Frankenhaeuser - Huxley channels for Xenopus
NEURON {
SUFFIX fh
USEION na READ nai, nao WRITE ina
USEION k READ ki, ko WRITE ik
NONSPECIFIC_CURRENT il, ip
RANGE pnabar, pkbar, ppbar, gl, el, il, ip
GLOBAL inf,tau
}
INCLUDE "standard.inc"
PARAMETER {
v (mV)
celsius (degC) : 20
pnabar=8e-3 (cm/s)
ppbar=.54e-3 (cm/s)
pkbar=1.2e-3 (cm/s)
nai (mM) : 13.74
nao (mM) : 114.5
ki (mM) : 120
ko (mM) : 2.5
gl=30.3e-3 (mho/cm2)
el = -69.74 (mV)
}
STATE {
m h n p
}
ASSIGNED {
ina (mA/cm2)
ik (mA/cm2)
ip (mA/cm2)
il (mA/cm2)
inf[4]
tau[4] (ms)
}
INITIAL {
mhnp(v*1(/mV))
m = inf[0]
h = inf[1]
n = inf[2]
p = inf[3]
}
BREAKPOINT {
LOCAL ghkna
SOLVE states METHOD cnexp
ghkna = ghk(v, nai, nao)
ina = pnabar*m*m*h*ghkna
ip = ppbar*p*p*ghkna
ik = pkbar*n*n*ghk(v, ki, ko)
il = gl*(v - el)
}
FUNCTION ghk(v(mV), ci(mM), co(mM)) (.001 coul/cm3) {
:assume a single charge
LOCAL z, eci, eco
z = (1e-3)*FARADAY*v/(R*(celsius+273.15))
eco = co*efun(z)
eci = ci*efun(-z)
ghk = (.001)*FARADAY*(eci - eco)
}
FUNCTION efun(z) {
if (fabs(z) < 1e-4) {
efun = 1 - z/2
}else{
efun = z/(exp(z) - 1)
}
}
DERIVATIVE states { : exact when v held constant
mhnp(v*1(/mV))
m' = (inf[0] - m)/tau[0]
h' = (inf[1] - h)/tau[1]
n' = (inf[2] - n)/tau[2]
p' = (inf[3] - p)/tau[3]
}
UNITSOFF
FUNCTION alp(v(mV),i) { LOCAL a,b,c,q10 :rest = -70 order m,h,n,p
v = v+70
q10 = 3^((celsius - 20)/10)
if (i==0) {
a=.36 b=22. c=3.
alp = q10*a*expM1(b - v, c)
}else if (i==1){
a=.1 b=-10. c=6.
alp = q10*a*expM1(v - b, c)
}else if (i==2){
a=.02 b= 35. c=10.
alp = q10*a*expM1(b - v, c)
}else{
a=.006 b= 40. c=10.
alp = q10*a*expM1(b - v , c)
}
}
FUNCTION bet(v,i) { LOCAL a,b,c,q10 :rest = -70 order m,h,n,p
v = v+70
q10 = 3^((celsius - 20)/10)
if (i==0) {
a=.4 b= 13. c=20.
bet = q10*a*expM1(v - b, c)
}else if (i==1){
a=4.5 b= 45. c=10.
bet = q10*a/(exp((b - v)/c) + 1)
}else if (i==2){
a=.05 b= 10. c=10.
bet = q10*a*expM1(v - b, c)
}else{
a=.09 b= -25. c=20.
bet = q10*a*expM1(v - b, c)
}
}
FUNCTION expM1(x,y) {
if (fabs(x/y) < 1e-6) {
expM1 = y*(1 - x/y/2)
}else{
expM1 = x/(exp(x/y) - 1)
}
}
PROCEDURE mhnp(v) {LOCAL a, b :rest = -70
TABLE inf, tau DEPEND celsius FROM -100 TO 100 WITH 200
FROM i=0 TO 3 {
a = alp(v,i) b=bet(v,i)
tau[i] = 1/(a + b)
inf[i] = a/(a + b)
}
}
UNITSON
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