🦷 Muscular System Lesson

"My Teeth Make People Chew": muscles of mastication

Four muscles that all share the same nerve supply as the muscles of facial expression share CN VII — but this time it's a different cranial nerve entirely.

Mass

Temp

Elevate

MPter

LPter

OPEN

📖 Full Breakdown

Four chewing muscles, all sharing CN V3 — and one that stands alone

Three muscles close the jaw; only one opens it, making that muscle functionally unique within the group.

Masseter

The strongest muscle in the body relative to its size

Closes the jaw — its remarkable strength-to-size ratio makes it a frequently cited anatomical fact.

Temporalis

Elevates and retracts the mandible

Its fan-shaped structure is a distinctive visible feature on the side of the skull.

Medial Pterygoid

Closes the jaw

Works alongside masseter to close the jaw, though from a different structural position.

Lateral Pterygoid

The ONLY muscle that opens the jaw

Also protrudes the mandible — its unique opening function makes it functionally distinct from the other three muscles of mastication, which all close the jaw.

🩺 Clinical / Exam Application

A dentist evaluating jaw function tests a patient's ability to open their mouth against resistance. Because lateral pterygoid is the ONLY one of the four mastication muscles that opens the jaw — masseter, temporalis, and medial pterygoid all close it — weakness specifically in jaw opening (rather than jaw closing) points directly toward a problem with lateral pterygoid or its nerve supply (CN V3), rather than the other three muscles.

⚠️ Exam Alert

A frequently tested detail: lateral pterygoid is the single exception in this muscle group — while the other three muscles of mastication all CLOSE the jaw, lateral pterygoid OPENS it. Exam questions often test this specific muscle as the odd one out.

🚧 Common Trap

Don't assume all four muscles of mastication perform the same action just because they share the same nerve (CN V3) and the same general job category (chewing). Three close the jaw and only one opens it — shared innervation doesn't mean shared function.

✅ Quick Check

Which of the four muscles of mastication opens the jaw, and how does this differ from the other three?

📝 Exam Prep

Common Exam Questions

❓ What nerve innervates all the muscles of mastication?

✅ CN V3, the mandibular branch of the trigeminal nerve (CN V), innervates all four muscles of mastication: masseter, temporalis, medial pterygoid, and lateral pterygoid.

❓ Which muscle of mastication is functionally different from the others, and how?

✅ Lateral pterygoid is the only muscle of mastication that opens the jaw (it also protrudes the mandible) — the other three (masseter, temporalis, medial pterygoid) all close the jaw.