This image shows a dentaform manufactured by Kilgore International inc. It represents a number pathological conditions in the teeth including caries, inflamed periodontal ligaments, periapical granuloma and inflamed nerves. Its purpose here is to illustrate the anatomical appearance of the inferior alveolar branch of the mandibular branch of the trigeminal nerve as it would be seen in a dissection of the mandible. It enters the inside aspect of the mandibular bone through a small hole called the mandibular foramen which is visible on the diagram to the right. When a dentist gives a mandibular block to get your lower teeth numb, he aims for the mandibular foramen. He doesn't actually have to hit the nerve, he only has to be in the general vicinity to get the teeth numb.
In the image above, a small tag of the cortical bone has been left intact in order to show the position of the mental foramen (it looks like a teardrop). The mental foramen is also visible in the chin area on the diagram to the right. The mental nerve extends through the foramen and gives feeling to the lip and chin. The small nerves that branch off from the inferior alveolar nerve inside the mandible toward the teeth are known collectively as the inferior dental plexus. This plexus of nerves innervates the lower teeth and their associated gingiva.
In the illustration to the right, it can be seen that the inferior alveolar nerve is only one of three main branches into which the mandibular nerve divides (all three are pink in the diagram). The branch that seems to split to the back of the jaw really runs forward to the buccal (cheek) side of the teeth and is called the long buccal nerve. It innervates (gives feeling to) the buccal gingiva and the buccal mucosa. The branch that appears to split off to the front of the face is called the lingual nerve, and it innervates the tongue, lingual (tongue side) gingiva and the floor of the mouth.
The nerve trunk itself has a "layered" structure so that the nerves that innervate the back teeth lie deeper in the trunk than the nerves that innervate the front teeth and the chin. This means that anesthetic administered in the pterygomandibular space (an ordinary mandibular block) will affect the front teeth and lips before it affects the back teeth. This is why the lip, chin and the front teeth may be profoundly numb, but the molar that the dentist is drilling remains hot. The anesthetic enters the superficial layers of the nerve trunk first causing the incisors to become numb well before it reaches the deeper fibers from the posterior teeth.