Solving System of ODE with global condition

I'm currently working on a system of 10 differential equations describing the cell devision cycle.

I'm struggling with the implementation of the mass devision, which happens whenever the first varible crosses zero with a negative slope. This is a globsl condition which happens multiple times during the chose timeframe.

So far I've done it this way:

But the problem is that the devision of the mass dy(10) by 0.5 happens EVERYTIME it's below 0.

I want this division only to be executed when dy(1) crosses zero with a negative slope only.

Kind regard for your help!

function dy=cellcycle(t,y)
dy(1)=..;
dy(2)=...;
    ..
dy(10)=;

if(dy(1))
dy(10)=0.5*dy(10) %need to take care of the slope as well....
end
    
end

[Stephan]

Maybe this helps:

https://de.mathworks.com/help/matlab/math/ode-event-location.html

[Cone]

Thanks a lot for your immediate answer, Stephan!

This should be the solution.