Why a dentist can numb exactly half your jaw and nothing else
Most of the skin's nerve territories blur into one another, but the face is mapped with unusually crisp borders. Its three sensory zones, served by branches of the trigeminal nerve, barely overlap, so a single injection can silence the teeth on one side of the jaw while the rest of the mouth still feels everything.
The trigeminal is the fifth cranial nerve and the most complex of the twelve. Its name means triplet: on each side of the pons, part of the brainstem, one nerve splits into three branches. The ophthalmic branch covers the forehead, scalp, upper eyelid, cornea and most of the nose. The maxillary branch handles the cheek, lower eyelid, upper lip, upper teeth and palate. The mandibular branch serves the lower lip, lower teeth, chin and jaw, apart from the angle of the jaw, which belongs to neck nerves. Each branch leaves the skull through its own opening, and all three meet at the trigeminal ganglion, tucked in a pocket called Meckel's cave.
Only the mandibular branch carries motor fibres as well. It drives eight muscles, including the four chewing muscles: the masseter, the temporalis and two pterygoids. Because both sides of the brain control these muscles, a stroke on one side rarely weakens chewing. Damage to the nerve itself is different: the jaw swings toward the paralysed side on opening, pushed by the healthy pterygoids opposite. Curiously, the left half of the jaw carries a few more nerves than the right.
Sensation arrives in two streams handled by separate wiring all the way to the cortex. Touch and position travel on fast, insulated fibres; pain and temperature ride slower bare ones. That is why stepping on a pin registers instantly as contact, while the sting follows a moment later. The pain system is considered older and more primitive, and its processing is tangled up with memory and emotion, part of the reason chronic pain is so hard to treat.
In other vertebrates, the equivalent of this nerve does more exotic work, handling the perception of magnetic and electric fields, low-frequency vibration and infrared radiation in some species.
Source: Trigeminal nerve
