Simulink Model for summation of stimulation of muscle

I'm working on a problem to model muscle twitches that leads up to tetanus . I give an attempt to use signal generator, falling integrator and a transport delay. I wasn't able to model the "falling" phases of this graph; my output was the same as an input, a uniform sine wave. I want the wave to be decreasing unless I have an additional signal coming in and "bringing up" the wave to some threshold level. I can use a switch to trigger the constant function but I can't get these things to sum properly. How can I model this behavior?

[Irem Nur Altintas]

hey did u solved it ?