function ydot=myfun(t,y,a)

           S1 = 2 *a* 2 * y(2)  -  2 * 3 * y(1)*y(1);
           S2 = 3 * y(1)*y(1) + a -  5 * y(2);
           ydot = [S1; S2];
           y_0=[0,0];
           %[t,y]=ode45(@myfun,[0:0.1:Tfinal],y_0);


and run following with nested myfun function:


%function events=events_in_Matlab(a)
       dt=0.01;
       T=0:dt:30;
       y_0=[0 0];
       Y=zeros(length(T),length(y_0));


       %t=0 to t=10, pass parameter a=0 to add to ODEs
       a=0;
       [~,Y(1:10/dt+1,:)]=ode45(@(t,y)myfun(t,y,a),T(1:10/dt+1),y_0);
       %t=10 to t=20, pass parameter a=10 to add to ODEs
       a=10;
       [~,Y(10/dt+1:20/dt+1,:)]=ode45(@(t,y)myfun(t,y,a),T(10/dt+1:20/dt+1),
       Y(10/dt+1,:));
       %t=20 to t=30, pass parameter a=20 to add to ODEs
       a=20;

      [~,Y(20/dt+1:end,:)]=ode45(@(t,y)myfun(t,y,a),T(20/dt+1:end),(20/dt+1,:));