(PSPICE) Memristor-Based IF Neuron Circuit: What is this NMOS?
It seems that \$A_2\$ works as comparator (with hysteresis, because of \$R_2\$) so its output is digital. Also the following inverter operates digitally. 

So you can assume that the MOSFET is used simply as analog switch and the only thing you have to care about is that it is turned on properly when \$V_K\$ is high, i.e. that \$V_{th}\$ of the MOSFET (not the \$V_{th}\$ shown in the circuit diagram) is small enough; or just use the more abstract voltage controlled Switch element of PSPICE instead of a MOSFET: