Having trouble modeling summation within a differential equation

So I am having trouble modeling a differential equation within matlab. I can either solve at steady state where dx/dt = 0 and then solve for xi which is ideal. Or I can simply model over time. I am having trouble creating the summations using the symsum function. If anyone can help that would be appreciated.

Currently, this is what I have so far:

if true
  % clear all
%valuesexcitatoryG = [];
%valuesinhibitoryG = [];
%mu =0;
%distancexx = [0 1 2 3 4];
%hold on
%for x = -4:1:4
 %excitatoryG =  (2.7183 ^ (((-x-mu) ^ 2)/4));
 %valuesexcitatoryG =  [valuesexcitatoryG excitatoryG];
%end
%VEG = valuesexcitatoryG(5:9);
%plot(distancexx, VEG); 
%title('Receptive Field Coefficients')
%for x = -4:1:4
%   inhibitoryG = (0.5 .* 2.7183 ^ -(((x - mu) ^ 2)/(16)));
%   valuesinhibitoryG = [valuesinhibitoryG inhibitoryG]; 
%end 
%VIG = valuesinhibitoryG(5:9);
%plot(distancexx, VIG);
%xlabel ('|k - i|') 
%legend ('C_ki','E_ki')
%hold off
%
%%%%%%%%%%%%%%%%%%%%%*Section which I need help*
%solve at steady state EQ
%
% Ioverall = [ .1 .1 .1 .1 .1 .8 .8 .8 .8 .8];
%time = 0:.01:23; %time
%dt = .1; %step
%a= .1;
%b = 1;
% for Ioverall = [ .1 .1 .1 .1 .1 .8 .8 .8 .8 .8]
  %xi(Ioverall) = ((b .* (VIG.*VEG))/ (a + (VEG.*Ioverall(i)) + %(VIG.*Ioverall(i)))); 
%    
%end
%
%plot(xi)

end