To understand Second messenger signalling in I/c and E/c
I think the schematic you have there and the one that @Masi posted are pretty good.  A better way of getting a grasp of intercellular signaling would be to study a specific hormone and understand how each of the elements in the block diagrams here could vary.

For instance take a look at this diagram from a journal article where they are tinkering with yeast mating factor, one of the first discovered extracellular signaling pathways. 



alpha factor and 'ligand' (I assume its not known at this point) bind to their receptors Ste2 and hGPCR respectively.  They then activate G protein complexes and both activate the same kinase cascade Ste4/18 -> Cdc42 -> Ste20 -> MEKK/MEK/MAPK/Ste5 which then may activate Far1 (or not) and Ste12.  Far1 and Ste12 are migrated into the nucleus to activate genes for cell reproduction.

By actually filling out the boxes in your diagram you can see dozens of different paradigms for how cell signaling may work, and why the diagram you have needs to be so simple.  Hope this helps.