function I = Na(V)
global G1 G2 V1 V2
% 
% Two resistors in parallel to generate the `N' IV-characteristics (of the
% Na-channel
% Need |G2|>G1???
%

n1 = G1*V; % parallel resistor 1
n2 = (V>V1).*(V<=V2).*((-G1+G2)*(V-V2))+(V<V1).*((-G1+G2)*(V1-V2)); % parallel resistor 2
I = n1 + n2; % equivalent resistor by Kirchhoff's Laws.

