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axial section infratemporal fossa lateral pterygoid muscle at level of pons TMJ

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Answer: Lateral Pterygoid muscle — main action on the TMJ is protrusion (protraction) of the mandible, along with depression (helps open the jaw) and side-to-side (grinding) movement when acting unilaterally.

Identification of the marked structure: This is an axial cross-section at the level of the pons/cerebellum, cut through the base of skull and the ramus/condylar region of the mandible. The arrow points to a muscle sitting medial to the ramus, running toward the mandibular condyle/TMJ region — this is the lateral pterygoid muscle, the muscle most intimately related to the TMJ capsule (its superior head partially blends with the joint capsule and articular disc).
Anatomy recap (Lateral Pterygoid):
  • Origin: greater wing of sphenoid (superior head) + lateral pterygoid plate (inferior head)
  • Insertion: neck/anterior surface of the mandibular condyle (superior head) and pterygoid fovea (inferior head)
  • Innervation: nerve to lateral pterygoid, branch of mandibular nerve (CN V3)
  • Action on the TMJ: protrudes the mandible (pulls the condyle and articular disc forward onto the articular eminence), assists in depression/opening of the jaw, and produces contralateral deviation/grinding movement when contracting on one side.
  • K J Lee's Essential Otolaryngology, p. 664
This contrasts with the other muscles of mastication visible at this level:
  • Medial pterygoid — elevates the mandible + side-to-side movement (no translation at TMJ)
  • Masseter/Temporalis — primarily elevate the mandible (closing), with temporalis also retracting
So among the muscles crossing the TMJ, only the lateral pterygoid actively protrudes/translates the condyle anteriorly — this is why it is singled out as the muscle marked at this level (just deep/medial to the ramus, near the condylar neck).
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Isnt this answer wrong then

You're right to flag it — the answer key marks Depression as correct, not protrusion. Let me correct that.
Corrected answer: Depression (jaw opening)
The muscle marked is the lateral pterygoid, and while it does have both actions (protrusion and depression), most anatomy/exam sources classify its main action on the TMJ as depression of the mandible — i.e., it is the principal muscle responsible for opening the jaw.
Why depression is the "main" action rather than protrusion:
  • Mouth opening (depression) at the TMJ is a two-phase movement: initial rotation of the condyle-disc complex, followed by anterior translation (gliding) of the condyle down the articular eminence to achieve a wide opening.
  • The lateral pterygoid is the only muscle of mastication that actively pulls the condyle and articular disc forward, and this anterior pull is exactly what drives the translation phase of opening. Without it, the mouth could only open a small amount (rotation alone).
  • The suprahyoid muscles (digastric, mylohyoid, geniohyoid) assist by fixing/depressing the hyoid and mandible, but the lateral pterygoid is the prime mover initiating and sustaining the translational component of depression at the joint.
  • Protrusion is really a byproduct/secondary description of the same forward-pulling action — when the mouth is closed and both lateral pterygoids contract, the net visible effect is protrusion; but functionally at the TMJ during normal jaw function, this same forward pull is what's described as producing depression/opening.
So: masseter, temporalis, medial pterygoid = elevators (close jaw); lateral pterygoid = the main depressor (opener) at the TMJ, working with the suprahyoid muscles.
Apologies for the earlier imprecision — thank you for catching it. The correct answer is C) Depression.
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