How to create time varying filters in MATLAB?

Hi,

I have a Simulink model for filtering some sort of signals and my model includes time varying IIR filter blocks. Now, I am trying to create the same model on MATLAB. But, I have problems with time varying filter parameters. First I used "filter" command wihch applies to whole signal vector and changing filter parameters with time is not possible as far as I know. Then, I tried "step" command but it does not work, I mean I cannot get the same (or approximate) results from the two approaches.

How should I use "step/filter" command to model filters with time varying parameters? Or which command should I use?

Thanks