Trigeminal Nerve & Facial Skin Innervation

• The facial skin is supplied by branches of the trigeminal nerve’s three divisions, meeting at the eyelids' margins and mouth angles.
• The junction lines of these zones curve upwards due to developmental changes.
• Initially, the trigeminal nerve branches met along horizontal lines from the eye and mouth angles.
• The skin innervated by the mandibular nerve (lowest trigeminal branch) was originally over the mandible.
• The auriculotemporal branch's area was first over the mandible's angle.
• As the cranial cavity expands, facial skin is drawn over it, moving beard skin over the temple and neck skin upwards.
• The temple is named for being the first area to show aging via graying hairs.
• The great auricular nerve supplies the skin over the mandibular angle and above.
• Facial muscles receive proprioceptive fibers from the cutaneous nerves, which connect with the facial nerve branches.
• The trigeminal nerve's divisions (ophthalmic, maxillary, and mandibular) give off cutaneous branches.
• Ophthalmic nerve gives off five cutaneous branches.
• Maxillary nerve gives off three cutaneous branches.
• Mandibular nerve gives off three cutaneous branches.
• Facial haemangiomas or herpes zoster rashes often demonstrate the skin area supplied by the trigeminal nerve's main branches.
• Herpes zoster less commonly affects the mandibular nerve, unlike the ophthalmic and maxillary nerves.
• Pain fibers in the trigeminal nerve are arranged in an 'onion-skin' pattern as they synapse with the spinal nucleus of the trigeminal nerve.
• The mouth area and lower nose are supplied by fibers connecting to the spinal nucleus' highest (pontine) part, followed by concentric areas connecting to progressively lower levels.
• The onion-skin arrangement for pain does not precisely match the sensory distribution of the main nerves.

Ophthalmic Nerve

• Five cutaneous branches exist.
• The lacrimal nerve supplies skin over the upper lid's lateral part.
• The supraorbital nerve passes up through a notch or foramen, branching to supply the forehead and scalp up to the vertex.
• The supratrochlear nerve ascends medially to the supraorbital nerve, supplying the mid-forehead up to the hairline.
• Above the hairline, the supraorbital nerves meet in the midline up to the vertex.
• The infratrochlear nerve supplies skin on the upper lid's medial part, descending along the external nose.
• It supplies skin over the nose's "bridge" but not the lower lid.
• The external nasal nerve supplies the middle of the external nose down to the tip.
• Involvement of this nerve in herpes zoster suggests potential corneal involvement.
• The cornea is supplied by ciliary branches of the same nerve which may lead to ulceration.
• The four branches also supply the upper lid conjunctiva.

Maxillary Nerve

• Composed of three cutaneous branches.
• The infraorbital nerve emerges through its foramen, branching to supply the lower eyelid, cheek, nose side and ala, upper lip, and labial gum up to the second premolar.
• It is located between the levator labii superioris and levator anguli oris.
• Many twigs connect to facial nerve branches for proprioceptive supply of face muscles.
• The zygomaticofacial nerve emerges from a foramen on the zygomatic bone's outer surface, supplying the overlying skin.
• The zygomaticotemporal nerve emerges in the temporal fossa through a foramen in the zygomatic bone's posterior surface.
• It supplies skin over the front of the temple level with the upper eyelid.

Mandibular Nerve

• Composed of three cutaneous branches.
• The auriculotemporal nerve passes around the mandible's neck, ascending behind the superficial temporal vessels.
• Its auricular part supplies the external acoustic meatus, tympanic membrane surface, and auricle skin above this level.
• Its temporal part supplies the hairy skin over the temple, where grey hairs commonly appear first.
• The buccal nerve gives off cutaneous twigs that supply a "thumb print" area over the cheek below the zygomatic bone.
• This is between the areas of the infraorbital nerve and the great auricular nerve.
• The mental nerve is a cutaneous branch of the inferior alveolar nerve, supplying the skin and mucous membrane of the lower lip and labial gum up to the second premolar tooth.

The facial skin is innervated by the trigeminal nerve's three divisions. These divisions meet at the eyelids and mouth. Cranial cavity expansion draws facial skin, moving beard skin and neck skin upwards.