2.1 The Trigeminal Nerve and Dental Innervation

The Trigeminal Nerve (Cranial Nerve V)

The trigeminal nerve is the largest cranial nerve and serves as the principal sensory supply to the head, face, oral cavity, teeth, and intracranial dura mater, while providing motor innervation to the muscles of mastication, mylohyoid, anterior belly of the digastric, tensor veli palatini, and tensor tympani.

From the sensory trigeminal (semilunar or Gasserian) ganglion situated in Meckel's cave on the petrous apex of the temporal bone, three distinct divisions emanate through dedicated cranial base foramina:

Trigeminal Ganglion (CN V)
  ├── V1: Ophthalmic Division ──────> Superior Orbital Fissure (Sensory)
  ├── V2: Maxillary Division  ──────> Foramen Rotundum (Sensory)
  └── V3: Mandibular Division ──────> Foramen Ovale (Sensory + Motor Root)

Ophthalmic Division (CN V1)

CN V1 is purely sensory and the smallest division. It passes through the lateral wall of the cavernous sinus and enters the orbit through the superior orbital fissure. It bifurcates into three primary branches within the orbit:

1. Frontal Nerve: Traverses the superior orbit above the levator palpebrae superioris, dividing into:
   • Supratrochlear nerve: Innervates the medial upper eyelid, bridge of the nose, and medial forehead.
   • Supraorbital nerve: Exits via the supraorbital notch/foramen to supply the central forehead, upper eyelid, and scalp as far posterior as the lambdoid suture.
2. Lacrimal Nerve: Passes through the lateral orbit to supply sensory innervation to the lacrimal gland, lateral upper eyelid, and conjunctiva (carrying postganglionic parasympathetic secretomotor fibres from the pterygopalatine ganglion hitchhiking via the zygomatic nerve).
3. Nasociliary Nerve: Traverses the common tendinous ring to give off:
   • Long ciliary nerves: Carry sensory fibres from the cornea and sympathetics to the dilator pupillae (mediating the afferent limb of the corneal blink reflex via CN V1, with CN VII orbicularis oculi acting as the efferent limb).
   • Posterior and anterior ethmoidal nerves: The anterior ethmoidal terminates as the external nasal nerve, supplying the tip and dorsum of the nose. Involvement of this branch in herpes zoster ophthalmicus produces vesicles on the nasal tip (Hutchinson's sign), signifying imminent corneal ulceration.
   • Infratrochlear nerve: Supplies the medial eyelids, caruncle, and skin of the nasal bridge.

Maxillary Division (CN V2)

CN V2 is purely sensory. It leaves the middle cranial fossa via the foramen rotundum, crosses the superior aspect of the pterygopalatine fossa, and enters the orbit via the inferior orbital fissure as the infraorbital nerve before exiting onto the midface via the infraorbital foramen.

• Intracranial / Pterygopalatine Branches:
  • Meningeal branch: Supplies the dura of the middle cranial fossa.
  • Zygomatic nerve: Divides into zygomaticofacial and zygomaticotemporal nerves (malar skin and temple sensation).
  • Ganglionic branches: Suspend the parasympathetic pterygopalatine (sphenopalatine) ganglion.
  • Greater and Lesser Palatine Nerves: Descend through the greater palatine canal. The greater palatine nerve emerges through the greater palatine foramen (located palatal to the maxillary second/third molar, ~3–4 mm anterior to the junction of the hard and soft palate) to supply the palatal mucoperiosteum from the third molar anteriorly to the canine. The lesser palatine nerves emerge via lesser palatine foramina to supply the soft palate and tonsillar bed.
  • Nasopalatine Nerve: Passes through the sphenopalatine foramen, traverses the nasal septum, and passes through the incisive canal/fossa to innervate the palatal mucosa of the six anterior maxillary teeth (canine to canine), overlapping with the greater palatine nerve palatal to the canine.
• Dental Plexus Branches:
  • Posterior Superior Alveolar (PSA) Nerve: Arises in the pterygopalatine fossa, descends along the maxillary tuberosity, and enters posterior alveolar foramina to innervate the maxillary sinus mucosa, buccal gingiva, and the maxillary second and third molars, plus the distobuccal and palatal roots of the maxillary first molar.
  • Middle Superior Alveolar (MSA) Nerve: Present in ~70% of individuals, arises from the infraorbital nerve within the infraorbital groove; innervates the maxillary premolars and the mesiobuccal root of the maxillary first molar, alongside the adjacent buccal gingiva.
  • Anterior Superior Alveolar (ASA) Nerve: Arises from the infraorbital nerve within the infraorbital canal ~8–10 mm posterior to the infraorbital foramen; descends through the anterior maxillary sinus wall to innervate the maxillary central and lateral incisors and canines, plus labial gingiva and anterior nasal floor.

Mandibular Division (CN V3)

CN V3 is the largest division and the only mixed division, carrying both sensory fibres and the entire motor root of CN V. It exits the middle cranial fossa vertically through the foramen ovale into the infratemporal fossa. Immediately below the foramen, the main trunk gives off two branches before splitting into anterior and posterior divisions:

• Main Trunk Branches:
  • Nervus spinosus (recurrent meningeal nerve): Re-enters the cranium via the foramen spinosum alongside the middle meningeal artery to supply the dura mater.
  • Nerve to medial pterygoid: Motor to medial pterygoid; passes through/near the otic ganglion without synapsing to supply the tensor veli palatini and tensor tympani.
• Anterior Division (Predominantly Motor):
  • Masseteric nerve: Passes through the mandibular (sigmoid) notch to innervate the masseter muscle and send sensory twigs to the TMJ.
  • Deep temporal nerves: (Anterior and posterior) pierce the temporalis muscle.
  • Nerve to lateral pterygoid: Enters the deep surfaces of both superior and inferior bellies.
  • Buccal nerve (Long buccal nerve): The sole sensory branch of the anterior division. Passes between the two heads of the lateral pterygoid, crosses the anterior border of the ramus over the retromolar pad, and supplies sensory innervation to the skin of the cheek, buccal mucosa, and the buccal gingiva adjacent to the mandibular molars (anesthetised via a long buccal nerve block).
• Posterior Division (Predominantly Sensory):
  • Auriculotemporal Nerve: Encircles the middle meningeal artery with two roots, passes posterior to the TMJ condyle, traverses the upper parotid gland, and ascends over the zygomatic arch with the superficial temporal vessels to supply sensory innervation to the auricle, external acoustic meatus, tympanic membrane, TMJ capsule, and temporal scalp. It carries postganglionic parasympathetic secretomotor fibres from the otic ganglion (derived from CN IX via the lesser petrosal nerve) to the parotid gland.
  • Lingual Nerve: Descends medial to the lateral pterygoid, enters the pterygomandibular space anterior and medial to the inferior alveolar nerve. In the infratemporal fossa, it is joined by the chorda tympani (branch of CN VII carrying preganglionic parasympathetic fibres to the submandibular and sublingual glands, and special sensory taste fibres from the anterior two-thirds of the tongue). The lingual nerve runs subperiosteally along the lingual plate of the mandible adjacent to the third molar roots, loops under the submandibular (Wharton's) duct from lateral to medial on the hyoglossus muscle, and provides general somatic sensation to the anterior two-thirds of the tongue, floor of mouth, and lingual gingiva.
  • Inferior Alveolar Nerve (IAN): Descends between the sphenomandibular ligament and the medial surface of the mandibular ramus. Just prior to entering the mandibular foramen, it gives off the nerve to mylohyoid (motor to the mylohyoid and anterior belly of digastric). The IAN traverses the mandibular (inferior dental) canal, innervating all mandibular molars and premolars via the mandibular dental plexus. At the mental foramen (typically apical to the mandibular second premolar or between first and second premolars), it bifurcates into:
    • Mental nerve: Exits through the mental foramen to supply sensation to the skin and mucous membrane of the lower lip, labial gingiva from midline to premolars, and the chin.
    • Incisive nerve: Remains within the intraosseous incisive canal to innervate the mandibular canine and incisor teeth.