I have a nonlinear high dimension system how could I write codes for this system as a function and using ode?my state is a vector with22 arrays.

I have done modelling this system with matlab simulink and I run it well. but the problem is when i want to use extended Kalman filter in matlab state transition should be defined as a function. so i need write codes for it and using commands like ode but the problem is high coupling of these equations.