You indicate that F(x) = x>0 (in terms of MATLAB logical expression processing.)
Is that the same F as where you have
fv(2)= p2*y1(2)*(1-(y1(2)+y1(3))/c2)-s2*y1(1)*y1(2)-u*F*(y1(5))*y1(1)-(i2*y1(1)*y1(5))/a2+y1(2);
so for example did you intend F*(y1(5)) to handle what is shown in your diagram as 
? If so then remember that * is multiplication not function application, so you would have needed F(y1(5)) not F*(y1(5))
? If so then remember that * is multiplication not function application, so you would have needed F(y1(5)) not F*(y1(5)) However...
F(x) = 0 if x <= 0, F(x) = 1 if x > 0, is a discontinuous function. Which is a problem for the mathematics that is used for all of the ode*() functions: the way that they determine which points are suitable ones to estimate the curve, depends upon the first and second derivatives of all of the functions being continuous.
To deal with this, in ode45() you need to use event functions to detect the places where F(q(t)) or F(-q(t)) would change between 0 and 1, and you have to terminate the calculation there. Then you restart ode45 from the same place as you left off. When you use the ode*() routines, it is not exactly the case that you cannot use if statements or logical comparisons such as F = @(x) x>0 but it is the case that for any one call to ode45() the same branch needs to be used each time.
