Which muscle is the primary retrusor of the mandible?
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NBDHE Review · Anatomy and Physiology (Scientific Basis)
Muscles of Mastication
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The NBDHE tests muscles of mastication primarily through their innervation (all by V3 — mandibular division of trigeminal nerve), their specific actions, and their origin/insertion relationships. Expect questions asking which muscle performs a specific jaw movement, which muscle is responsible for a particular deviation pattern, or what nerve damage would cause specific functional deficits.
The college version
Core Review
The Four Muscles of Mastication
All muscles of mastication are derived from the first pharyngeal (mandibular) arch and are therefore innervated by the mandibular division of the trigeminal nerve (CN V3). The four paired muscles are: masseter, temporalis, medial pterygoid, and lateral pterygoid.
Masseter
The most superficial and powerful muscle of mastication, the masseter is a thick quadrilateral muscle readily palpable at the angle of the mandible.
- Origin: Zygomatic arch (superficial head: anterior two-thirds of the lower border of the zygomatic arch; deep head: posterior one-third and medial surface of the zygomatic arch)
- Insertion: Lateral surface of the mandibular ramus, angle, and coronoid process
- Action: Elevation of the mandible (both heads); the superficial fibers also contribute to protrusion
- Innervation: Masseteric nerve, a branch of V3
- Clinical note: Masseter hypertrophy can occur with chronic bruxism. The masseter is palpable during the TMJ clinical examination — tenderness may indicate myofascial pain. Intramuscular injection into the masseter with botulinum toxin is sometimes used for treatment of bruxism and TMD.
Temporalis
A large fan-shaped muscle covering the temporal fossa, the temporalis is a powerful elevator and the primary retrusor of the mandible.
- Origin: Temporal fossa (the entire surface of the temporal bone including the inferior temporal line) and the deep surface of the temporal fascia
- Insertion: Coronoid process of the mandible and the anterior border of the mandibular ramus (via a strong tendon that passes deep to the zygomatic arch)
- Action: Elevation of the mandible (anterior and middle fibers); retrusion of the mandible (posterior fibers, which run nearly horizontally). The temporalis is the primary retrusor.
- Innervation: Deep temporal nerves (anterior and posterior), branches of V3
- Clinical note: The temporalis tendon can be palpated intraorally along the anterior border of the ramus. Temporal pain on palpation is common in TMD myofascial pain. The temporalis also plays a role in maintaining mandibular posture (rest position).
Medial Pterygoid (Internal Pterygoid)
A thick rectangular muscle on the medial surface of the mandibular ramus, the medial pterygoid forms a muscular sling with the masseter.
- Origin:
- Deep head (major portion): Medial surface of the lateral pterygoid plate of the sphenoid bone
- Superficial head (smaller): Maxillary tuberosity and pyramidal process of the palatine bone
- Insertion: Medial surface of the mandibular ramus and angle (pterygoid tuberosity)
- Action: Elevation of the mandible (synergist with masseter and temporalis); protrusion; assists in lateral excursion (contralateral movement)
- Innervation: Medial pterygoid nerve (nerve to the medial pterygoid), a branch of V3
- Clinical note: The medial pterygoid lies medial to the mandibular ramus, and the inferior alveolar nerve and lingual nerve pass between the medial pterygoid and the ramus. This relationship is important during the inferior alveolar nerve block. Trismus following an inferior alveolar injection may result from hemorrhage or anesthetic infiltration into the medial pterygoid muscle.
Lateral Pterygoid (External Pterygoid)
The lateral pterygoid is the key muscle for jaw opening and protrusive movements. It is the only muscle of mastication that primarily depresses (opens) the jaw.
- Origin:
- Superior head: Infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone
- Inferior head: Lateral surface of the lateral pterygoid plate of the sphenoid bone
- Insertion:
- Superior head: Anterior band of the articular disc and capsule of the TMJ (primarily into the disc)
- Inferior head: Pterygoid fovea on the anterior surface of the condylar neck (primarily into the condyle)
- Action: Depression of the mandible with the suprahyoid muscles (inferior head); protrusion (both heads); lateral excursion (contralateral movement — the right lateral pterygoid moves the mandible to the left, and vice versa)
- Innervation: Lateral pterygoid nerve, a branch of V3 (sometimes from the buccal nerve)
- Clinical note: The lateral pterygoid is the muscle most commonly implicated in disc displacement disorders. When the superior head of the lateral pterygoid contracts during closing, it helps stabilize the disc. Hyperactivity or spasm of the lateral pterygoid can contribute to anterior disc displacement. The lateral pterygoid is the only muscle of mastication that is difficult or impossible to palpate directly due to its deep location.
Jaw Movements: A Summary
Understanding which muscles perform which movements is critical for the NBDHE:
| Movement | Primary Muscles |
|---|---|
| Elevation (closing) | Temporalis, masseter, medial pterygoid |
| Depression (opening) | Lateral pterygoid (inferior head), suprahyoids (digastric, geniohyoid, mylohyoid) |
| Protrusion | Lateral pterygoid (both heads), medial pterygoid |
| Retrusion | Temporalis (posterior fibers), deep masseter |
| Lateral excursion (right) | Left lateral pterygoid, left medial pterygoid |
Note: The suprahyoid muscles (digastric, geniohyoid, mylohyoid, stylohyoid) are accessory muscles of mastication. They depress the mandible when the hyoid bone is stabilized by the infrahyoid muscles. They are NOT innervated by V3 (digastric anterior belly by V3, digastric posterior belly by CN VII; mylohyoid by V3; geniohyoid by C1 via CN XII; stylohyoid by CN VII).
Clinical Correlation: Deviation on Opening
Deviation on opening is a classic NBDHE concept:
- Deviation: The mandible shifts to one side during opening but returns to the midline at full opening. This suggests a muscular problem (muscle spasm or guarding on the side to which the jaw deviates).
- Deflection: The mandible shifts to one side and stays there throughout opening. This suggests a mechanical problem within the TMJ (disc displacement without reduction, or other internal derangement on the side to which the jaw deflects).
Clinical Application
Understanding muscle actions helps in diagnosing TMD. If a patient has pain on closing, suspect the elevators (masseter, temporalis, medial pterygoid). Pain on opening suggests the depressors (lateral pterygoid, suprahyoids). Deviation patterns help differentiate myofascial pain from internal derangement. The NBDHE may also ask about the effect of muscle damage — for example, damage to the right lateral pterygoid would impair contralateral excursion (moving the jaw to the left).
Common Traps
- Thinking the masseter is the only elevator — the temporalis and medial pterygoid are equally important elevators
- Confusing the lateral pterygoid's two heads: superior head → disc; inferior head → condylar neck
- Assuming suprahyoids are muscles of mastication — they are accessory muscles, and not all are V3-innervated
- Thinking the facial nerve (CN VII) innervates any muscle of mastication — it doesn't; it innervates muscles of facial expression
- Forgetting that lateral excursion is a contralateral movement (right lateral pterygoid moves jaw left)
Eli explains
The same idea, in plain words
Explain it like I’m 10
You have four pairs of chewing muscles. The big one on the side of your jaw (masseter) and the fan-shaped one on your temple (temporalis) close your mouth with power. Inside your mouth, two deeper muscles (pterygoids) help with side-to-side grinding and forward-backward movements. The real star is the lateral pterygoid — it's the only one that actually opens your mouth. When you chew, your jaw moves in a complex pattern: it drops open (lateral pterygoid), slides forward (lateral pterygoid again), then clamps shut (temporalis, masseter, medial pterygoid all firing together). All these muscles get their marching orders from the same nerve — the mandibular branch of your trigeminal nerve.
Key takeaways
- ALL muscles of mastication are innervated by V3 (mandibular division of CN V)
- The temporalis is the primary retrusor; its posterior fibers are key
- The lateral pterygoid is the primary protrusor and the only muscle of mastication that depresses the jaw
- The superior head of the lateral pterygoid inserts into the TMJ disc
- The medial pterygoid and masseter form a muscular sling around the mandibular angle
- Deviation ≠ Deflection: deviation returns to midline, deflection does not
- Which muscle is the primary retrusor of the mandible?
- A) Masseter
- B) Medial pterygoid
- C) Lateral pterygoid
Check yourself
1 review question from the chapter. Try each one, then open the answer.
D) Temporalis (posterior fibers)
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D.** The posterior fibers of the temporalis run nearly horizontally and are the primary retrusors.
Quick check
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The superior head of the lateral pterygoid muscle inserts into which structure?
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A patient's mandible deviates to the right during opening but returns to midline at full opening. This suggests:
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Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Name the four paired muscles of mastication and their V3 innervation
- Describe the origin, insertion, and action of each muscle of mastication
- Differentiate between elevation, depression, protrusion, retrusion, and lateral excursion
- Correlate muscle dysfunction with clinical signs of TMD
- Distinguish muscles of mastication from accessory muscles of the jaw
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