TITLE CaPCalyx.mod  The presynaptic calcium current at the MNTB calyx of Held

COMMENT

NEURON implementation of Calcium current from Borst and Sakmann, 1998.
Equations are basic HH; parameters are taken from Figure 8 legend in that paper.

Original implementation by Paul B. Manis, October 2007

Contact: pmanis@med.unc.edu

Modified 2/2014: Use Derivative block format.

ENDCOMMENT

UNITS {
        (mA) = (milliamp)
        (mV) = (millivolt)
        (nA) = (nanoamp)
}

NEURON {
THREADSAFE
    SUFFIX CaPCalyx
    USEION ca READ eca WRITE ica
    RANGE gbar, gcap, ica
    GLOBAL minf, taum, alpha, beta
}

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

PARAMETER {
        v (mV)
:        celsius = 22 (degC)  : model is defined on measurements made at 23-24 deg in Germany
        celsius (degC)
        dt (ms)
:        eca = 43.9 (mV)
        eca (mV)
        gbar = 0.01 (mho/cm2) <0,1e9> : target is 48.9 nS total
        alpha (1/ms)
        beta (1/ms)
}

STATE {
        m
}

ASSIGNED {
    ica (mA/cm2)
    gcap (mho/cm2)
    minf (1)
    taum (ms)

}

LOCAL mexp

BREAKPOINT {
    SOLVE states METHOD cnexp

    gcap = gbar*m*m
    ica = gcap*(v - eca)
}

UNITSOFF

INITIAL {
    trates(v)
    m = minf
}

DERIVATIVE states {  : Computes state variables m
    rates(v)         : at the current v and dt.
    m' = (minf - m)/taum
    :m = m + mexp*(minf-m)
:VERBATIM
:	return 0;
:ENDVERBATIM
}

LOCAL q10

PROCEDURE rates(v) {  :Computes rate and other constants at current v.
                      :Call once from HOC to initialize inf at resting v.

    q10 = 3^((celsius - 24)/10) : if you don't like room temp, it can be changed!

    minf = (1 / (1 + exp(-(v + 23.2) / 9.1)))

    alpha = 1.78*exp(v/23.3)
    beta = 0.140*exp(-v/15.0)
    taum = 1/(alpha + beta)
}

PROCEDURE trates(v) {  :Computes rate and other constants at current v.
                      :Call once from HOC to initialize inf at resting v.
    LOCAL minc
    TABLE minf, mexp
    DEPEND dt, celsius FROM -150 TO 150 WITH 300

    rates(v)    : not consistently executed from here if usetable_hh == 1
        : so don't expect the tau values to be tracking along with
        : the inf values in hoc

    minc = -dt * q10
    mexp = 1 - exp(minc/taum)
}

UNITSON