cal.mod

TITLE L-type calcium channel with low threshold for activation
: used in somatic and proximal dendritic regions 
: it calculates I_Ca using channel permeability instead of conductance

UNITS {
  (mA)    = (milliamp)
  (mV)    = (millivolt)
  FARADAY = 96520 (coul)
  KTOMV   = .0853 (mV/degC)
  (M)     = (1/liter)
  (mM)    = (milliM)
}

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)
  celsius = 34    (degC)
  gcalbar = 0     (mho/cm2) : initialized conductance
  ki      = 0.001 (mM)
  cai     = 5.e-5 (mM)
  cao     = 2     (mM)
  tfa     = 5               : time constant scaling factor
  eca     = 140             : Ca++ reversal potential
}

NEURON {
  SUFFIX cal
  USEION ca READ cai,cao WRITE ica
  RANGE gcalbar, minf,taum
}

STATE {	m }                      : unknown parameter to be solved in the DEs 

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

INITIAL {
  : initialize the following parameter using rates()
  rates(v)
  m    = minf
  gcal = gcalbar*m*h2(cai)
}

BREAKPOINT {
  SOLVE states
  gcal = gcalbar*m*h2(cai)   : maximum channel permeability
  ica  = gcal*ghk(v,cai,cao) : 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 alpm(v(mV)) {
  TABLE FROM -150 TO 150 WITH 200
  alpm = 0.055*(-27.01 - v)/(exp((-27.01-v)/3.8) - 1)
}


FUNCTION betm(v(mV)) {
  TABLE FROM -150 TO 150 WITH 200
  betm = 0.94*exp((-63.01-v)/17)
}

UNITSON
LOCAL facm
: 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)
  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 value
  facm = (1 - exp(-dt/(taum*1(ms))))
}