TITLE Nap.mod   persistent sodium current
 
COMMENT
This is the original Hodgkin-Huxley treatment for the set of sodium, potassium,
and leakage channels found in the squid giant axon membrane.
Some parameters have been changed to correspond to McIntyre and Grill (2002) "Extracellular
stimulation of central neurons"
Author: Balbi
ENDCOMMENT
 
UNITS {
        (mA) = (milliamp)
        (mV) = (millivolt)
		(S) = (siemens)
}
 
NEURON {
        SUFFIX Nap
        USEION na READ ena WRITE ina
        RANGE gnamax, gna
        RANGE pinf, ptau
}
 
PARAMETER {
	gnamax = 0.3 (S/cm2)   <0,1e9>
}
 
STATE {
        p
}
 
ASSIGNED {
        v (mV)
        ena (mV)

		gna (S/cm2)
        ina (mA/cm2)
        pinf
		ptau (ms)
}
 
BREAKPOINT {
        SOLVE states METHOD cnexp
        gna = gnamax*p*p*p
		ina = gna*(v - ena)
}
 
INITIAL {
	rates(v)
	p = pinf
}

DERIVATIVE states {
        rates(v)
        p' = (pinf-p)/ptau
}
 
PROCEDURE rates(v(mV)) {  
		:Call once from HOC to initialize inf at resting v.
        LOCAL  alpha, beta, sum

UNITSOFF
                :"p" persistent sodium activation system
        alpha = 0.0353 * vtrap(-28.4 - v, 5)
        beta =  0.00353 * vtrap(v + 32.7, 5)
        sum = alpha + beta
		ptau = 1/sum
        pinf = alpha/sum
}
 
FUNCTION vtrap(x,y) {  :Traps for 0 in denominator of rate eqns.
        if (fabs(x/y) < 1e-6) {
                vtrap = y*(1 - x/y/2)
        }else{
                vtrap = x/(exp(x/y) - 1)
        }
}
 
UNITSON