The Trigeminal Nerve (CN V)

Core Review

Overview of the Trigeminal Nerve

The trigeminal nerve is the largest cranial nerve. It emerges from the lateral aspect of the pons and forms the trigeminal (semilunar, Gasserian) ganglion, located in Meckel's cave (a dural recess in the middle cranial fossa). From the ganglion, three major divisions emerge:

• V1 — Ophthalmic division: Entirely sensory
• V2 — Maxillary division: Entirely sensory
• V3 — Mandibular division: Sensory AND motor

The trigeminal nerve provides general somatic afferent (sensory) innervation to the face, scalp (anterior to the vertex), cornea, nasal cavity, paranasal sinuses, oral cavity, teeth, periodontium, and anterior two-thirds of the tongue (general sensation, NOT taste). It also provides branchial motor innervation (via V3) to the muscles of mastication, mylohyoid, anterior belly of the digastric, tensor tympani, and tensor veli palatini.

V1: Ophthalmic Division

V1 is the smallest division and is purely sensory. It enters the orbit through the superior orbital fissure and supplies:

• Sensation to the upper eyelid, cornea, conjunctiva
• Skin of the forehead, anterior scalp, bridge of the nose
• Mucous membranes of the frontal and ethmoid sinuses
• Part of the nasal cavity

Key branches of V1:

• Lacrimal nerve
• Frontal nerve (supraorbital and supratrochlear branches)
• Nasociliary nerve

NBDHE relevance: V1 has NO dental branches. If a question asks about innervation of the upper teeth or gingiva, V1 is the distractor answer. V1 is clinically relevant for the corneal reflex (afferent limb, tested during neurological examination) and herpes zoster ophthalmicus (shingles in the V1 distribution, which can threaten vision).

V2: Maxillary Division

V2 is purely sensory. It exits the skull through the foramen rotundum, crosses the pterygopalatine fossa, enters the orbit through the inferior orbital fissure, and emerges on the face through the infraorbital foramen as the infraorbital nerve.

Major branches of V2 (in clinical order of significance):

1. Posterior superior alveolar (PSA) nerve: Branches from V2 in the pterygopalatine fossa, enters the maxilla through the posterior superior alveolar foramina on the maxillary tuberosity. Supplies:
   • Maxillary third, second, and first molars (EXCEPT the mesiobuccal root of the first molar in most patients — see MSA)
   • Buccal gingiva and periodontium of maxillary molars
   • Part of the maxillary sinus lining

Clinical injection: PSA nerve block, deposited at the maxillary tuberosity behind the zygomatic process.

1. Middle superior alveolar (MSA) nerve: Present in approximately 70-80% of individuals (absent in the remainder, where the PSA and ASA cover its distribution). Branches from the infraorbital nerve within the infraorbital canal. Supplies:
   • Maxillary premolars
   • Mesiobuccal root of the maxillary first molar
   • Buccal gingiva and periodontium of maxillary premolars

Clinical injection: MSA nerve block; also anesthetized by the infraorbital block since the MSA branches from the infraorbital nerve.

1. Anterior superior alveolar (ASA) nerve: Branches from the infraorbital nerve within the infraorbital canal. Supplies:
   • Maxillary central and lateral incisors, and canine
   • Buccal gingiva and periodontium of the anterior maxilla

Clinical injection: Infraorbital nerve block or ASA nerve block.

1. Greater (anterior) palatine nerve: Descends through the pterygopalatine fossa and exits through the greater palatine foramen. Supplies:
   • Palatal gingiva of the maxillary posterior teeth (premolars and molars)
   • Hard palate mucous membrane and glands

Clinical injection: Greater palatine nerve block at the greater palatine foramen.

1. Nasopalatine nerve: Enters the nasal cavity and descends through the incisive foramen. Supplies:
   • Palatal gingiva of the maxillary anterior teeth (canine to canine)
   • Anterior hard palate mucous membrane

Clinical injection: Nasopalatine nerve block into the incisive papilla.

1. Infraorbital nerve: The terminal branch of V2, emerges on the face through the infraorbital foramen. Supplies:
   • Skin of the lower eyelid, side of nose, upper lip
   • (Also gives off MSA and ASA before exiting)

Clinical injection: Infraorbital nerve block.

V3: Mandibular Division

V3 is the largest division and the only one carrying motor fibers. It exits the skull through the foramen ovale. V3 has both sensory and motor components.

Major sensory branches of V3:

1. Inferior alveolar nerve (IAN): The largest branch of V3. Enters the mandibular foramen, travels through the mandibular canal, and divides at the mental foramen into the mental nerve (exiting) and incisive nerve (continuing anteriorly). Supplies:
   • All mandibular teeth (via inferior dental plexus)
   • Mandibular periodontium and alveolar bone
   • Mental nerve supplies skin of the chin and lower lip, and labial gingiva of mandibular anterior teeth and premolars

Clinical injection: Inferior alveolar nerve block (most commonly administered dental nerve block).

1. Lingual nerve: Descends anterior and medial to the IAN. In its course, it is joined by the chorda tympani (CN VII). Supplies:
   • General sensation to the anterior two-thirds of the tongue
   • Sensation to the lingual gingiva of the mandibular teeth
   • Floor of the mouth mucous membrane
   • Sublingual gland sensation

Clinical injection: Lingual nerve block (usually anesthetized simultaneously with the IAN block).

1. Buccal nerve (long buccal nerve): NOT to be confused with the buccal branch of CN VII (motor to facial muscles). The buccal nerve of V3 is purely sensory. Supplies:
   • Buccal gingiva of the mandibular molars
   • Mucous membrane of the cheek
   • Buccal vestibule

Clinical injection: Buccal nerve block.

1. Auriculotemporal nerve: Sensory to the TMJ, external ear, and temporal scalp. Carries postganglionic parasympathetic fibers from the otic ganglion (CN IX) to the parotid gland.

Motor branches of V3 (all branchial motor):

• Masseteric nerve → masseter
• Deep temporal nerves → temporalis
• Medial pterygoid nerve → medial pterygoid
• Lateral pterygoid nerve → lateral pterygoid
• Mylohyoid nerve → mylohyoid and anterior belly of digastric
• Nerves to tensor tympani and tensor veli palatini

Summary: Dental Innervation Chart

Clinical Application

The entire practice of dental local anesthesia is built on trigeminal nerve anatomy. The success of the inferior alveolar nerve block depends on depositing anesthetic near the mandibular foramen. Failure of a PSA block to anesthetize the mesiobuccal root of the first molar indicates the MSA is supplying that root — requiring a supplemental injection. Understanding these pathways allows clinicians to troubleshoot inadequate anesthesia. Additionally, trigeminal neuralgia (tic douloureux) is a severe pain disorder affecting V2 and/or V3, characterized by brief, electric shock-like pain triggered by light touch to trigger zones.

Common Traps

• Confusing the buccal nerve (V3 sensory) with the buccal branch of CN VII (motor)
• Thinking V2 has motor fibers — it doesn't
• Thinking the MSA is always present — in ~20-30% of people, the PSA or ASA covers the premolar region
• Confusing the mental foramen (exit of mental nerve, premolar region) with the infraorbital foramen (exit of infraorbital nerve, below orbit)
• Assuming the PSA block always anesthetizes all maxillary molars — the MB root of the first molar may need separate MSA coverage