Trigeminal nerve

The trigeminal nerve is the fifth (V) cranial nerve, and carries sensory information from most of the face, as well as motor supply to the muscles of mastication (the muscles enabling chewing), tensor tympani (in the middle ear), and other muscles in the floor of the mouth, such as the mylohyoid and anterior digastric muscle.

Branches
It is named trigeminal because it divides into three branches:
 * the ophthalmic nerve (V1)
 * the maxillary nerve (V2)
 * the mandibular nerve (V3)

The ophthalmic, maxillary, and mandibular branches leave through various foramina (holes) in the skull: the superior orbital fissure, foramen rotundum and foramen ovale, respectively. The mnemonic device standing room only can help one remember that the V1 segment branch passes through the s uperior orbital fissure, V2 passes through the foramen r otundum, and V3 passes through the foramen o vale.

Sensory
The trigeminal nerve is the major cutaneous sensory nerve of the head, and is responsible for sensation over most of the skin on the head.

Motor
While all three trigeminal branches carry sensory information, only the mandibular branch carries motor input. The mandibular nerve supplies motor fibres to the temporalis, lateral pterygoid, medial pterygoid, masseter (the four main muscles involved in mastication), tensor veli palatini, mylohyoid, the anterior belly of the digastric muscle, and the tensor tympani.

Proprioceptive
The trigeminal nerve caries proprioceptive fibers from the tissues it innervates as well as from the muscles supplied by the facial nerve.

Testing the trigeminal nerve
As the trigeminal nerve contains both sensory and motor fibers, and also branches into different regions, these should be tested separately and on both sides of the face.

To test the sensation, the skin is touched lightly (for instance with cotton wool), and also tested for pain (such as the sharpness of a pin prick). This is done on the three regions: forehead, cheeks and chin, as well as around the nose and mouth.

The corneal reflex is dependent on the trigeminal nerve sensing touching the eye (as well as the facial nerve for the motor response). This can be done with a wisp of cotton wool, and approaching from the side (so the person being investigated doesn't blink at the sight approaching). The corneal reflex elicits a blink with both eyes, and feels very uncomfortable for the patient.

To test whether the motor pathways are functional, the temporalis and masseter muscles may be felt. The temporalis can be felt contracting at the temples (just lateral to the eyes) when a person is chewing (or clenching and unclenching his/her teeth). The masseter can be felt just above the angle of the jaw in the same movement.

The strength of the jaw muscles can be tested by holding a person's jaw closed and telling the person to open it (which tests the pterygoids) and opening the jaw and getting the person to close it (which tests the masseters). The jaw muscles are normally quite strong, and should have little difficulty overcoming pressure from a hand.

If there is unilateral damage to the motor root of the trigeminal nerve, the jaw will tend to deviate towards the paralysed side.

The jaw jerk reflex will be very brisk if there is any upper motor neuron damage above the pons.

Conditions involving the trigeminal nerve

 * Trigeminal neuralgia is an example of a disorder of the trigeminal nerve where the sufferer experiences pain in the territory of the trigeminal nerve innervation.
 * Cluster headaches are an intense type of headache involving the trigeminal nerve.
 * Herpes zoster of the face can arise from an infection of the ophthalmic nerve by the varicella-zoster virus.