Question about xticks

Hi, this is my first message on the list.

I’m new at using python and matplotlib so maybe this is an easy thing, but I would like to know how can I write a text for each ‘xtick’ on the upper side of a plot figure (a different label than on the lower side, I mean).

I would also like to ask wether I could draw different number of xticks on the upper side than in the lower side.

Thank you all!