how to put an action to a tree node

hello
I don't know how to put an action to a tree node
for example I have de tree
a
|_ b
|_x
|_y
|_c
|_1
|_2

and when I click b or x or any other node I want to put a function, for example to load something from a database [without changing the tree expansion]...
can you help me pls?
thank you

the function should make something depending on the node you click (for example if you click 'b' show some data, if you click x show some other data.. which depends on "b", "x",.. )