Medial nerves are connected to each facet joint located between every vertebrae. They carry facet pain signals to the brain. A Medial Branch Block is done to ensure that it is indeed the facet that is of issue and to treat the pain.
A Medial Branch Block is performed by injecting an anesthetic into the medial nerves connected to a problematic facet joint. Multiple levels are often injected during the procedure. It is first done as a diagnostic. If the patient feels immediate relief then the facet is indeed the issue. From there if needed further blocks can be done.
In essence the pain nerves are blocked from sending messages and therefore the pain is no longer felt. It is like when a cell phone tower goes out. Someone may be calling you but your phone will not ring. a nerve block works the same way and since the medial nerves only carry pain messages there is no concern for other interference.