Nuclei and fibre types
CN V carries two fibre types, general sensation (GSA) and motor to first pharyngeal arch muscles (SVE), from four nuclei spanning the midbrain to the upper cervical cord.
| Nucleus | Site | Fibre type and role |
|---|---|---|
| Mesencephalic nucleus | Midbrain | GSA: proprioception from muscles of mastication and teeth; its cell bodies are primary sensory neurons sitting inside the CNS |
| Principal (chief) sensory nucleus | Pons | GSA: fine touch and pressure |
| Spinal nucleus | Pons to upper cervical cord (about C2) | GSA: pain and temperature |
| Motor nucleus | Pons | SVE: muscles of mastication and other first arch muscles |
CN V has no parasympathetic nucleus of its own. Parasympathetic fibres from CN III, VII and IX hitch a ride on its branches to reach the ciliary, pterygopalatine, submandibular and otic ganglia and their targets.
Emergence and skull foramen
CN V emerges from the lateral side of the pons, near its junction with the middle cerebellar peduncle, as a large sensory root and a small motor root. The roots cross the apex of the petrous temporal bone into the trigeminal cave (Meckel's cave), a dural pocket in the middle cranial fossa that holds the trigeminal ganglion, the sensory ganglion of the nerve.
Each division leaves the skull through its own opening: V1 through the superior orbital fissure, V2 through the foramen rotundum, V3 through the foramen ovale. The motor root passes under the ganglion and joins V3 only.
Course and branches
Each division supplies a band of skin and the deeper structures beneath it.
Ophthalmic division (V1): sensory
V1 runs forward in the lateral wall of the cavernous sinus and divides into three branches that enter the orbit through the superior orbital fissure:
- Frontal nerve: runs on levator and splits into supraorbital and supratrochlear nerves to the forehead and scalp as far back as the vertex.
- Lacrimal nerve: to the lacrimal gland region, lateral upper lid and conjunctiva.
- Nasociliary nerve: gives the long ciliary nerves (cornea), the ethmoidal nerves, the infratrochlear nerve and the external nasal nerve to the tip of the nose.
Maxillary division (V2): sensory
V2 runs in the lower lateral wall of the cavernous sinus, passes through the foramen rotundum into the pterygopalatine fossa and gives:
- Branches through the pterygopalatine ganglion to the nasal cavity, palate and nasopharynx, including the greater and lesser palatine nerves.
- The zygomatic nerve to the cheek and temple, which also carries parasympathetic fibres to the lacrimal gland.
- The posterior superior alveolar nerve to the upper molars.
- The infraorbital nerve, which enters the orbit through the inferior orbital fissure, runs in the infraorbital groove and canal, gives the middle and anterior superior alveolar nerves, and emerges at the infraorbital foramen to the lower lid, cheek, side of the nose and upper lip.
Mandibular division (V3): sensory and motor
V3 passes through the foramen ovale into the infratemporal fossa. The main trunk gives a meningeal branch and the nerve to medial pterygoid, which also supplies tensor veli palatini and tensor tympani. The anterior division is mostly motor, to masseter, temporalis and lateral pterygoid, plus the sensory buccal nerve. The posterior division is mostly sensory:
- Auriculotemporal nerve: splits around the middle meningeal artery, supplies the temple and part of the ear, and carries parasympathetic fibres from the otic ganglion to the parotid gland.
- Lingual nerve: general sensation to the anterior two-thirds of the tongue; it is joined by the chorda tympani (CN VII), carrying taste and secretomotor fibres.
- Inferior alveolar nerve: gives the nerve to mylohyoid (mylohyoid and anterior belly of digastric), enters the mandibular foramen to supply the lower teeth, and ends as the mental nerve to the chin and lower lip.
Clinical testing
CN V is tested by sensation in each division, the corneal reflex, the muscles of mastication and the jaw jerk.
- Sensation: light touch and pinprick on the forehead (V1), cheek (V2) and chin (V3), comparing sides. The skin over the angle of the mandible is supplied by the great auricular nerve (C2, C3), not CN V, which helps detect non-organic loss.
- Corneal reflex: touching the cornea makes both eyes blink. The afferent limb is V1 (nasociliary nerve) and the efferent limb is CN VII.
- Motor: feel masseter and temporalis as the patient clenches, then ask them to open the mouth against resistance. With a unilateral V3 lesion the jaw deviates toward the weak side, pushed across by the intact lateral pterygoid.
- Jaw jerk: a tap on the chin with the mouth slightly open. Both limbs run in CN V; a brisk jerk points to an upper motor neurone lesion above the pons.
Lesions and palsies
Trigeminal lesions cause facial sensory loss, pain syndromes or weakness of chewing, depending on the level.
- Trigeminal neuralgia: brief, electric-shock pain, usually in V2 or V3, triggered by touch, chewing or shaving. It is often caused by a loop of artery, commonly the superior cerebellar artery, compressing the root near the pons. Carbamazepine is first-line; microvascular decompression is an option.
- Herpes zoster ophthalmicus: shingles in V1. Vesicles on the tip of the nose (Hutchinson's sign) mean the nasociliary nerve is involved and the eye is at risk.
- Cerebellopontine angle tumour: a vestibular schwannoma can compress the root; loss of the corneal reflex is an early sign.
- Cavernous sinus lesion: numbness in V1 and V2 with ophthalmoplegia; V3 is spared because it does not enter the sinus.
- Lateral medullary syndrome: damage to the spinal nucleus causes loss of pain and temperature on the same side of the face, in an onion-skin pattern.
- Peripheral injury: the lingual nerve lies against the inner surface of the mandible beside the lower third molar and can be damaged during extraction; the inferior alveolar nerve is injured in mandibular fractures, numbing the lower lip.