COMMENT
Two state kinetic scheme synapse described by rise time tau1,
and decay time constant tau2. The normalized peak condunductance is 1.
Decay time MUST be greater than rise time.
The solution of A>G>bath with rate constants 1/tau1 and 1/tau2 is
A = a*exp(t/tau1) and
G = a*tau2/(tau2tau1)*(exp(t/tau1) + exp(t/tau2))
where tau1 < tau2
If tau2tau1 > 0 then we have a alphasynapse.
and if tau1 > 0 then we have just single exponential decay.
The factor is evaluated in the
initial block such that an event of weight 1 generates a
peak conductance of 1.
Because the solution is a sum of exponentials, the
coupled equations can be solved as a pair of independent equations
by the more efficient cnexp method.
20120413 TMM modified to include conductance saturation: the
conductance, g, will not exceed "saturation"; however when simulated
past saturation, g will take longer to drop back below saturation.
ENDCOMMENT
NEURON {
POINT_PROCESS Exp2SynSat
RANGE tau1, tau2, e, i, saturation
NONSPECIFIC_CURRENT i
RANGE g
}
UNITS {
(nA) = (nanoamp)
(mV) = (millivolt)
(uS) = (microsiemens)
}
PARAMETER {
tau1=.1 (ms) <1e9,1e9>
tau2 = 10 (ms) <1e9,1e9>
e=0 (mV)
saturation (uS) : assign in hoc (typical real synapse
: value 0.0004 = 0.4 nS)
}
ASSIGNED {
v (mV)
i (nA)
g (uS)
factor
}
STATE {
A (uS)
B (uS)
}
INITIAL {
LOCAL tp
if (tau1/tau2 > .9999) {
tau1 = .9999*tau2
}
A = 0
B = 0
tp = (tau1*tau2)/(tau2  tau1) * log(tau2/tau1)
factor = exp(tp/tau1) + exp(tp/tau2)
factor = 1/factor
}
BREAKPOINT {
SOLVE state METHOD cnexp
g = B  A
if (g>saturation) {
g = saturation
}
i = g*(v  e)
}
DERIVATIVE state {
A' = A/tau1
B' = B/tau2
}
NET_RECEIVE(weight (uS)) {
A = A + weight*factor
B = B + weight*factor
}
