function out = KNa(V,I)
global E_Na E_K
%
% 'N' resistor (i_1=K(V+E_K)) and `S' resistor (V=Na(i_2)+E_Na) parallel to generate
% Spike Burster characteristics by Kirchhoff Laws so that I = i_1 + i_2, or
% i_2 = I - i_1 = I - K(V+E_K)
% E_Na -- Nernst potential for Na-channel
% E_K -- Nernst potential for K-channel
% 
out = V-Na(I-K(V+E_K))-E_Na;