cat_modified.mod

TITLE t-type calcium channel with high threshold for activation
: used in somatic and dendritic regions 
: it calculates ica using channel permeability instead of conductance

UNITS {
  (mA) = (milliamp)
  (mV) = (millivolt)
  (M)  = (1/liter)
  (mM) = (milliM)

  FARADAY = 96520 (coul)
  KTOMV   = .0853 (mV/degC)
}

INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}

PARAMETER {
  : parameters that can be entered when function is called in cell-setup 
  dt              (ms)
  v               (mV)
  tBase   = 23.5  (degC)
  celsius = 22    (degC)
  gcatbar = 0     (mho/cm2)    : initialized conductance
  ki      = 0.001 (mM)
  cai     = 5.e-5 (mM)
  cao     = 2     (mM)
  tfa     = 1                  : activation time constant scaling factor
  tfi     = 0.68               : inactivation time constant scaling factor
  eca     = 140                : Ca++ reversal potential
}

NEURON {
  SUFFIX cat2
  USEION ca READ cai,cao WRITE ica
  RANGE gcatbar, hinf, minf, taum, tauh
}

STATE {	m h }  : unknown activation and inactivation parameters to be solved in the DEs 

ASSIGNED {
  : parameters needed to solve DE
  ica  (mA/cm2)
  gcat (mho/cm2) 
  minf
  hinf
  taum
  tauh
}

INITIAL {
  : tadj = 3^((celsius-tBase)/10)   : assume Q10 of 3
  rates(v)
  m    = minf
  h    = hinf
  gcat = gcatbar*m*m*h*h2(cai)
}

BREAKPOINT {
  SOLVE states
  gcat = gcatbar*m*m*h*h2(cai)  : maximum channel permeability
  ica  = gcat*ghk(v,cai,cao)    : dummy calcium current induced by this channel
}

UNITSOFF
FUNCTION h2(cai(mM)) {
  h2 = ki/(ki+cai)
}

FUNCTION ghk(v(mV), ci(mM), co(mM)) (mV) {
  LOCAL nu,f
  f   = KTF(celsius)/2
  nu  = v/f
  ghk = -f*(1. - (ci/co)*exp(nu))*efun(nu)
}

FUNCTION KTF(celsius (degC)) (mV) {
  : temperature-dependent adjustment factor
  KTF = ((25./293.15)*(celsius + 273.15))
}

FUNCTION efun(z) {
  if (fabs(z) < 1e-4) {
    efun = 1 - z/2
  } else {
    efun = z/(exp(z) - 1)
  }
}

FUNCTION alph(v(mV)) {
  TABLE FROM -150 TO 150 WITH 200
  alph = 1.6e-4*exp(-(v+57)/19)
}

FUNCTION beth(v(mV)) {
  TABLE FROM -150 TO 150 WITH 200
  beth = 1/(exp((-v+15)/10)+1.0)
}

FUNCTION alpm(v(mV)) {
  TABLE FROM -150 TO 150 WITH 200
  alpm = 0.1967*(-1.0*v+19.88)/(exp((-1.0*v+19.88)/10.0)-1.0)
}

FUNCTION betm(v(mV)) {
  TABLE FROM -150 TO 150 WITH 200
  betm = 0.046*exp(-v/22.73)
}

UNITSON
LOCAL facm,fach

: if state_cagk is called from hoc, garbage or segmentation violation will
: result because range variables won't have correct pointer. This is because
: only BREAKPOINT sets up the correct pointers to range variables.
PROCEDURE states() {
  : exact when v held constant; integrates over dt step
  rates(v)
  m = m + facm*(minf - m)
  h = h + fach*(hinf - h)
  VERBATIM
  return 0;
  ENDVERBATIM
}

PROCEDURE rates(v (mV)) {
  : callable from hoc
  LOCAL a
  a    = alpm(v)
  taum = 1/(tfa*(a + betm(v))) : estimation of activation tau
  minf =  a/(a+betm(v))        : estimation of activation steady state
  facm = (1 - exp(-dt/(taum*1(ms))))
  a    = alph(v)
  tauh = 1/(tfi*(a + beth(v))) : estimation of inactivation tau
  hinf = a/(a+beth(v))         : estimation of inactivation steady state
  fach = (1 - exp(-dt/(tauh*1(ms))))
}