Do other ingredients in brines penetrate the meat?
Good Eats describes what happens in a brine very well

Essentially, the thing that penetrates the meat is water. Because water is a solvent it can carry things with it into the meat. Salt and sugar are the two most common ingredients in a brine. Salt is required and sugar dissolves really easily in water and adds complexity to the flavor profile. But you can add anything that will be carried by the water into the meat. If your brine isn't penetrating your meat past the surface you aren't doing it right.