V3 dermatome: the mandibular division of the trigeminal nerve

Mandibular division of the trigeminal nerve

Where is the V3 dermatome?

V3, the mandibular division of the trigeminal nerve, supplies the lower lip, the chin, the jaw except its angle, the temple and the front of the ear. The spinal standard gives the face no key sensory point; the chin is the usual test point. No disc compresses V3: it is a cranial nerve, not a spinal root.

V3 at a glance
Type	Mandibular division of the trigeminal nerve
Key sensory point	Not part of the spinal standard: the chin is the usual test point
Key movement	None in the spinal standard: V3’s chewing muscles are felt as the teeth clench
Tendon reflex	The jaw jerk
Usual disc	None: a cranial nerve, not a spinal root

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V3 motor supply: the muscles of chewing

V3 is the only trigeminal division with motor fibers, and it supplies the muscles of chewing.

The spinal standard grades no movement for V3; V3’s chewing muscles are felt as the teeth clench.

V3 and the tendon reflexes

No limb reflex tests V3. Its own is the jaw jerk, the stretch reflex of the chewing muscles, tested by tapping the chin with the mouth a little open; its way in and its way out both run through V3.

Why no disc compresses V3

The face is supplied by the trigeminal nerve, a cranial nerve whose three divisions leave the skull through separate openings, so a lesion of one division numbs its own area of the face. It is not a spinal root, so no disc presses on it.

What a lesion of V3 does

Sensation. Numbness over the V3 area of the face.
Strength. The muscles of chewing, which only V3 supplies.
Reflexes. The jaw jerk runs in and out through V3, so it may be reduced or lost.

A teaching map. Root territories overlap and vary between people, and a root lesion is diagnosed by examination and imaging, not by a map. Numbness between the legs or around the anus, new bladder or bowel problems, or quickly spreading weakness need emergency care.

Neighboring dermatomes

V1: the forehead and scalp to the crown, the upper eyelid and the bridge and tip of the nose
V2: the lower eyelid, the cheek, the side of the nose and the upper lip

Sources. Key sensory points and key movements: International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), American Spinal Injury Association, 2019 revision. Muscles: Aids to the Examination of the Peripheral Nervous System, Medical Research Council and the Guarantors of Brain, 5th ed. (2010), cross-checked with Kendall et al. (2005). Reflex roots: Walker (1990), Clinical Methods, 3rd ed., Table 72.1, and StatPearls, “Physiology, Deep Tendon Reflexes”; the main root as the MSD Manual’s “How to Assess Reflexes” names it. Overlap: Sherrington (1893), Phil Trans R Soc Lond B 184:641-763.