Taking a partial derivative with respect to a model

"[...] These variables give a measure of correspondence between signal $\displaystyle {\bf X}(n)$ and model $\displaystyle {\bf M}_h$ relative to all other models, $\displaystyle h'$. A mechanism of concept formation and learning, an internal dynamics of the modeling fields (MF) is defined as follows,

$\displaystyle {\bf S}_h = {\bf S}_h + \alpha \displaystyle\sum\limits_{n}f(h|n)\left(\frac{\par tial \ln l(n|h)}{\partial {\bf M}_h}\right)\frac{\partial {\bf M}_h}{\partial {\bf S}_h} $

[...]

Parameter $\displaystyle \alpha$ determines the iteration step and speed of convergence of the MF system; $\displaystyle N_h$ can be interpreted as a number of signals $\displaystyle {\bf X}(n)$ associated with or coming from a concept object $\displaystyle n$. As already mentioned, in the MF internal dynamics, similarity measures are adapted so that their fuzziness is matched to the model uncertainty. [...]"

http://www.leonid-perlovsky.com/4%20-%20Mehler.pdf (page 368)

What does it mean to take a partial derivative with respect to a model $\displaystyle {\bf M}_h$ i.e. $\displaystyle \left(\frac{\partial \ln l(n|h)}{\partial {\bf M}_h}\right)$?

Thanks in advance,

jrobot