Muscles of Mastication – Comprehensive Bullet-Point Notes

Learning Objectives

• Categorize head & neck muscles into major groupings (e.g., mastication, facial expression, etc.)
• Recall the general function performed by EACH group
• For EVERY individual muscle be able to state:
• Origin (anchored tendon-end)
• Insertion (movable tendon-end)
• Action (movement that occurs when the muscle contracts)
• Innervation (nerve supply)
• Any clinically significant information
• Describe muscle actions using precise anatomical directions (e.g., anterior, medial)
• Correctly label muscles on diagrams, photos, radiographs, or skulls
• Determine which specific muscles are active in common, real-world oral-health scenarios (chewing, clenching, lateral excursion, etc.)

Core Principle of Skeletal Muscle Function

• Muscles ONLY contract → they shorten along the direction of their fibers
• When a muscle shortens, its insertion is drawn TOWARD its origin
• Tendons (dense regular connective tissue) link muscle to bone, skin, or other soft tissue
• Action is ALWAYS described from the standpoint of the insertion moving toward the origin

Key Terminology (review & remember)

• Origin – stationary, proximal, or less-movable tendon attachment
• Insertion – movable, distal, or more-mobile tendon attachment
• Action – the specific motion produced when the muscle fibers contract
• Innervation – the cranial nerve or peripheral nerve that supplies motor fibers to the muscle
• Clinical Significance – any dental, radiographic, or pathological consideration relevant to a practicing hygienist or dentist

Muscles of Mastication – Purpose & Scope

• “Mastication” = the biomechanical act of chewing solid food to begin mechanical digestion
• Primary goals of the group:
• Elevate the mandible (close the jaw)
• Depress the mandible (open the jaw)
• Protrude / Retract the mandible (anterior-posterior translation)
• Excursion (side-to-side lateral movements)
• TOTAL of $4$ paired muscles (left & right sides):
• Masseter
• Medial pterygoid
• Lateral pterygoid
• Temporalis (sometimes called the temporal muscle)
• Main skeletal elements involved: mandible, zygomatic arch, temporal bone, sphenoid, palatine, maxilla, TMJ disc & capsule
• ALL four are innervated by the mandibular division of the trigeminal nerve $\left(CN\;V_{3}\right)$

Shared Characteristics of the Mastication Muscles

• Innervation → $CN\,V_{3}$ (motor branch)
• Embryologic origin → 1st branchial (mandibular) pharyngeal arch musculature
• All insert on some portion of the mandible and directly influence TMJ motion

Individual Muscle Break-Down

Masseter

• Location: superficial lateral face, over buccal region of mandible
• Heads & Origins:
• Superficial head – anterior $\tfrac{2}{3}$ of the zygomatic arch
• Deep head – posterior $\tfrac{1}{3}$ of the zygomatic arch
• Insertion: angle & buccal surface of mandibular ramus
• Actions:
• Primary elevator of the mandible (power clenching)
• Secondary protrusion (especially the superficial head)
• Innervation: $CN\,V_{3}$ (masseteric branch)
• Clinical notes:
• Palpated externally during extra-oral exam (EOE) while patient clenches
• Hypertrophy seen in bruxism / chronic clenching → square jaw appearance
• Landmark for delivery of inferior alveolar nerve block depth check

Medial Pterygoid

• Location: mirror-image of the masseter on the lingual side of the ramus
• Heads & Origins:
• Superficial head – medial surface of lateral pterygoid plate of the sphenoid
• Deep head – maxillary tuberosity & lateral pyramidal process of palatine bone
• Insertion: medial (lingual) aspect of mandibular angle & ramus
• Action: elevates the mandible (synergistic with masseter)
• Innervation: $CN\,V_{3}$ (medial pterygoid branch)
• Clinical notes:
• Less bulk than masseter; contributes to sling that closes jaw
• Infection in pterygomandibular space may affect this muscle → trismus

Lateral Pterygoid

• Location: deep, horizontal fibers within infratemporal fossa
• Heads & Origins:
• Superior head – infratemporal crest of greater wing of sphenoid
• Inferior head – lateral surface of lateral pterygoid plate
• Insertions:
• Superior head → pterygoid fovea on the mandibular condyle & capsule/disc of TMJ
• Inferior head → neck of the mandibular condyle
• Actions:
• Bilateral contraction → depresses (opens) & protrudes mandible
• Unilateral contraction → lateral shift (excursion) to opposite side
• Innervation: $CN\,V_{3}$ (lateral pterygoid branch)
• Clinical notes:
• Palpated intra-orally (IOE) for tenderness or spasm
• Range-of-motion check: “$3$-finger stack” should fit between incisors (normal opening)
• Hyperactivity may displace TMJ disc anteriorly → click/lock

Temporalis (Temporal Muscle)

• Location: fan-shaped over temporal fossa → deep to zygomatic arch
• Origin: entire temporal fossa (temporal lines to infratemporal crest)
• Insertion: coronoid process & anterior border of mandibular ramus
• Actions:
• Anterior vertical fibers → powerful elevation of mandible
• Posterior horizontal fibers → retrude (pull back) mandible
• Tonically active even at rest → maintains slight occlusal clearance, “suspending” mandible between teeth
• Innervation: $CN\,V_{3}$ (deep temporal branches)
• Clinical notes:
• Easily palpated above zygomatic arch during clench-release cycles
• Tenderness common in temporomandibular disorders (TMD)

Clinical Connections & Examination Techniques

• Palpation sequence (EOE & IOE):

1. Masseter (external)

2. Temporalis (external, superior to arch)

3. Medial pterygoid (intra-orally behind last molar, medial to ramus)

4. Lateral pterygoid (intra-orally ~ through vestibule superior-posterior to maxillary tuberosity)
   • Functional tests:
   • Ask patient to clench → evaluate masseter/temporalis
   • Ask patient to open widely → lengthening of masseter & activation of lateral pterygoid
   • Evaluate protrusion & retrusion for smoothness (lateral pterygoid vs. temporalis posterior fibers)
   • Lateral excursion against resistance (L → R; R → L) to isolate opposite lateral pterygoid
   • Pathologies / presentations:
   • Bruxism → hypertrophic masseter, wear facets, myalgia
   • Trismus (limited opening) → medial pterygoid spasm or infection
   • TMJ internal derangement → lateral pterygoid over-pulling disc
   • Temporal headaches → referred pain from temporalis overuse

Real-World Functional Examples

• Biting into a hard apple → bilateral masseter & temporalis elevate mandible powerfully
• Grinding peanuts side-to-side → alternating unilateral activation of lateral & medial pterygoids produces excursion
• Opening for dental mirror insertion → lateral pterygoids contract bilaterally while suprahyoids stabilize
• Resting posture while waiting for the dentist → posterior temporalis fibers hold mandible in slight retrusion, maintaining $\approx2\,mm$ freeway space (occlusal clearance)

Quick Comparison Snapshot (Mnemonic: "Ma-Me-La-Tem")

• Masseter – Power elevator & slight protruder (Think “Massive Clencher”)
• Medial Pterygoid – Inside twin of masseter, elevates from lingual side (“Mirror Masseter”)
• Lateral Pterygoid – Only depressor/protruder of the group (“Lateral Lowers”)
• Temporalis – Fan-shaped elevator & retruder (“Temporal Tug-back”)

Review / Self-Test Prompts

• List the origin and insertion of the lateral pterygoid superior head.
• Which muscle(s) maintain the mandibular rest position during speech pauses?
• A patient cannot laterally deviate mandible to the left; which mastication muscle is likely impaired?
• Explain why both masseter and medial pterygoid must be palpated in a full EOE/IOE.