Where should a dentist inject the lidocaine to block pain perception?

Depends: It depends on what or which teeth they wish to anesthetize. Different types of injections are used in different areas. They are dependent on bone structure, nerve location and number of teeth to be numb. A qualified dentist should be able to know the correct procedures.

By the nerve: Adjacent to the nerve which supplies the affected area, so that the lidocane spreads to the nerve.