transfer function for these system of differential equations

Can anyone help me write the " transfer functions" for this system of equations please.

I have managed to solve the ODE's using the code below

function diffeqs= ode_sys(t,D)
x0= -1.6;
y0=1;
tauknot= 2857;
tautwo=10;
Irest1=3.1;
Irest2=0.45;
gama=0.01;
g = @(x1,t0,t,gama) integral(@(tau) exp(-gama*(t-tau)).*x1, t0, t); 
f1= @(x1,x2,z) (x1.^3 - 3*x1.^2) .* (x1<0)+ (x2-0.6.*(z-4).^2).*(x1).*(x1>=0);
f2= @(x1,x2,z)((0).*(x2<-0.25))+(6.*(x2+0.25)).*(x2>=-0.25);

x1= D(1);
y1= D(2);
z = D(3);
x2= D(4);
y2= D(5);

Ddv_Div = [y1-f1(x1,x2,z)-z+Irest1;
          (y0-(5.*x1^2)-(y1));
        ((1/tauknot).*((4*x1-4*x0)-z));
        (-y2+x2-(x2.^3)+Irest2+0.002*(g(x1,t0,t,gama))- (0.3)*(z-3.5));
        ((1/tautwo).*(-y2+f2(x1,x2,z)));]

end
% the code is run through the lines below
range =[1:1:1];
%Initial condition
IC =[0,-5,3,0,0];

[tsol, Dsol ] = ode45(@ode_sys,range,IC);