There are 12 nerves which emerge from the brain to supply the head and face, including the optic nerve (for vision), olfactory nerve (for smell). These are called the cranial nerves.
The 5th cranial nerve, the trigeminal nerve, looks after sensation in each half of the face. In some unlucky individuals, there can be severe pain affecting one of these nerves. This condition is known as trigeminal neuralgia and can be very disabling. Treatment is initially medical with medicines such as carbamazepine, gabapentin and phenytoin. If they do not work or there are too many side effects, radiosurgery can be offered. But the more successful treatment for this is an operation known as microvascular decompression. Many neurosurgeons feel the cause of this pain is a blood vessel that rubs on the trigeminal nerve, close to where it emerges from the brain. In this operation, any blood vessel that is in contact with the nerve is moved away from it, and held in this position with Teflon felt.
This same technique can be used for hemifacial spasm. In this condition, blood vessels irritating the facial nerve cause involuntary contractions of half of the face.