Muscles of Mastication & Facial Expression

Muscles of mastication (all first arch, all V3)

Muscle	Origin → insertion	Main action
Masseter	Zygomatic arch → lateral ramus and angle	Elevates (closes); superficial part helps protrude
Temporalis	Temporal fossa → coronoid process and anterior ramus	Elevates; posterior (horizontal) fibers retract
Medial pterygoid	Medial surface of lateral pterygoid plate (deep head) and maxillary tuberosity → medial ramus and angle	Elevates; helps protrude and move the jaw to the opposite side
Lateral pterygoid	Inferior head: lateral surface of lateral pterygoid plate; superior head: infratemporal surface of greater wing → condylar neck (pterygoid fovea), disc and capsule	Protrudes, depresses (the only muscle of mastication that helps open) and moves the jaw to the opposite side

The masseter and medial pterygoid form a sling around the angle of the mandible.
Other jaw openers are suprahyoid muscles: digastric, mylohyoid, geniohyoid (C1 via the hypoglossal nerve).

Muscle	Why it matters
Buccinator	Keeps food on the occlusal table; pierced by the parotid duct opposite the maxillary second molar; its attachments decide where odontogenic pus spreads
Orbicularis oris	Lip seal; part of the 'buccinator mechanism' with the buccinator and superior constrictor
Mentalis	Elevates and protrudes the lower lip; a strong mentalis can lift a lower denture or complicate impressions
Risorius, zygomaticus major	Smile; insert at the modiolus at the corner of the mouth
Platysma	Tenses neck skin; supplied by the cervical branch of VII

Lower motor neuron lesion of VII (Bell palsy, or anesthetic reaching the parotid during an IANB): the whole half of the face is weak, including the forehead.
Upper motor neuron lesion (stroke): the forehead is spared because its motor neurons receive input from both hemispheres.
Trismus after an IANB usually comes from trauma or bleeding in the medial pterygoid.
With one lateral pterygoid not working (or a unilateral condylar fracture), the jaw deviates TOWARD the affected side on opening.

Calling the buccinator a muscle of mastication: it is a facial expression muscle (VII), though it helps chewing.
Saying the temporalis protrudes: its posterior fibers retract.
Deviating the jaw away from a nonfunctioning lateral pterygoid: it deviates toward the weak side.
Expecting forehead sparing in Bell palsy: Bell palsy is a lower motor neuron lesion, so the forehead is involved.

Pick an answer to see right away whether it is correct and why.

Which muscle of mastication helps open the mouth?
The parotid duct pierces which muscle?
After a right condylar neck fracture, the patient's mandible on opening deviates:
A patient cannot wrinkle the forehead or close the eye on one side and the mouth droops on that side. The lesion is most consistent with:
Which nerve supplies the geniohyoid muscle?

0 of 5 answered

Answer key: 1 D · 2 A · 3 D · 4 C · 5 D

Dental School Map · Original study material, written and checked by our team; always confirm clinical decisions against current guidelines.