Frenet frame from a spline structure

Hello people,
I would like to apply the frenet frame in a spline curve, as follow in the example:

t = [-5:1:5].';
x = t ;
y = t.^2;
splx = spline(t,x);
sply = spine(t,y);
spldx = fnder(splx);
spldy = fnder(sply);
fdx = @(t) ppval(spldx,t);
fdy = @(t) ppval(spldy,t);
% frenet frame's vector T(t)
T = @(t) [fdx;fdy]/norm(fdx;fdy);
N = ...

Well, as you can see, I don't know how to determine the frenet's normal vector N(t), that is the derived of T(t) divided by its norm. There some way to derive the function handle T or someonw knows other solution?

Thanks