Head & Neck Muscles – Comprehensive Study Notes

Muscles of Mastication (Chewing)

• Core role → generate bite force, guide mandibular movement, grind food for swallowing.

• Temporalis
• Origin → temporal fossa (fossa temporalis) on the temporal bone.
• Insertion → coronoid process + anterior border of ramus of mandible.
• Fiber orientation: fan-shaped, converging under the zygomatic arch.
• Action → powerful elevation of mandible; posterior fibers assist slight retraction, stabilizing TMJ during closing.
• Significance → synergist with masseter; maintains resting tonus that keeps teeth in light contact ("inter-cuspal position").

• Masseter (superficial + deep heads)
• Origin (superficial head) → zygomatic bone + inferior border of zygomatic arch.
• Insertion → angle + lateral surface of mandibular ramus.
• Action → strongest elevator of mandible; deep fibers aid protrusion; superficial fibers give forceful clench.
• Clinical → hypertrophy common in chronic gum-chewers; palpated at outer cheek.

• Medial Pterygoid (not visible on model)
• Origin → medial surface of lateral pterygoid plate + pyramidal process of palatine bone.
• Insertion → medial surface of mandibular angle.
• Action → elevation and small grinding (alternate unilateral contraction).

• Lateral Pterygoid (not visible on model)
• Origin (superior head) → infratemporal surface of greater wing of sphenoid; (inferior head) → lateral surface of lateral pterygoid plate.
• Insertion → neck of mandible + articular disc of TMJ.
• Action → protrusion, depression (opening), side-to-side translation.

• Buccinator (accessory to mastication)
• Location → muscular wall of cheek, deep to risorius.
• Action → compresses cheek, pushing food onto occlusal surfaces; prevents cheek biting during chewing.
• Innervation → facial nerve, unlike true muscles of mastication (trigeminal V3).

Muscles of Facial Expression

(ALL are innervated by the facial nerve – cranial nerve VII – and insert into skin, producing subtle emotional cues.)

• Frontal Belly of Occipitofrontalis
• Connected to occipital belly via galea (epicranial aponeurosis).
• Action → elevates eyebrows, wrinkles forehead (surprise).

• Orbicularis Oculi (sphincter of palpebral fissure)
• Palpebral part → gentle blinking; orbital part → forceful squinting.
• Action → closes eyelids; assists lacrimal fluid drainage.

• Levator Labii Superioris
• Action → elevates upper lip ("Elvis snarl").

• Zygomaticus Minor
• Action → lifts upper lip upward/backward – aids sadistic or smug expressions.

• Zygomaticus Major
• Action → draws mouth angle upward/backward → primary "smile" muscle.

• Risorius
• Action → retracts mouth corners laterally (wide grin or grimace).

• Depressor Anguli Oris (Triangularis)
• Action → pulls oral commissure downward (frown).

• Depressor Labii Inferioris
• Action → pulls lower lip downward (pout).

• Mentalis
• Action → protrudes lower lip & wrinkles chin (doubt/sulk).

• Orbicularis Oris (oral sphincter)
• Action → closes/purses lips (kissing, speech articulation).

• Transverse Nasalis
• Location → across cartilaginous dorsum of nose.
• Action → compresses nasal aperture.

Extra Auricular Muscles

(vestigial in humans; vary in activity among individuals)

• Auricularis Anterior → draws ear forward.
• Auricularis Superior → elevates ear.
• Auricularis Posterior → retracts ear.

Lateral Neck Muscles

• Sternocleidomastoid (SCM)
• Origins → manubrium sterni + medial clavicle.
• Insertion → mastoid process.
• Bilateral contraction → neck flexion; unilateral → ipsilateral lateral flexion + contralateral rotation.
• Landmark → divides neck into anterior/posterior triangles.

• Scalenes (Anterior 44, Middle 42, Posterior 43)
• Origins → transverse processes of cervical vertebrae.
• Insertions → first two ribs.
• Actions → lateral neck flexion; elevate ribs 1 and 2 during forced inspiration (accessory breathing).
• Clinical → compression space for brachial plexus → thoracic outlet syndrome.

Posterior Neck Muscles

• Levator Scapulae 45 → elevates scapula, assists neck extension.
• Splenius Capitis 19 → extension, rotation, and lateral flexion of head.
• Semispinalis Capitis → powerful neck extensor, maintains posture.
• Trapezius
• Descending fibers oriented inferior→superior.
• Superficial to semispinalis & splenius.
• Actions → extend neck, elevate scapula (shrugging), upward rotation of scapula.

Platysma

• Thin, sheet-like, subcutaneous.
• Origin → pectoral fascia over clavicle & upper chest.
• Insertion → inferior border of mandible, lower face skin.
• Actions → tenses skin of neck, depresses mandible, draws mouth corners downward ("fright" grimace).

Suprahyoid Muscles (Above Hyoid – elevators)

• Common function → elevate hyoid, floor of mouth; critical for swallowing & speech.

• Digastric
• Anterior belly (innerv. CN V3) → may depress mandible when hyoid fixed.
• Posterior belly 47 (innerv. CN VII).
• Linked by intermediate tendon to hyoid.

• Mylohyoid 49 → forms oral diaphragm; elevates hyoid & tongue in initial swallow.
• Geniohyoid 50 → pulls hyoid anteriorly & superiorly, shortening floor of mouth.
• Stylohyoid 48 → slender muscle, splits around digastric tendon; elevates/retracts hyoid.

Infrahyoid Muscles (Strap Muscles – depressors)

• Omohyoid (superior 53 & inferior bellies linked via intermediate tendon) → depresses/retracts hyoid.
• Sternohyoid 55 → depresses hyoid after elevation.
• Sternothyroid → depresses thyroid cartilage (+ indirectly hyoid).
• Thyrohyoid (implied) → elevates larynx or depresses hyoid.
• Collective Role → stabilize/depress hyoid during phonation & late swallowing phases.
• Voice Production → elevation \uparrow & depression \downarrow of hyoid modify tension of vocal cords and pitch.

Integrated Functional Notes

• Chewing Cycle → mandible depression (lateral pterygoid/platysma), elevation (masseter/temporalis/medial pterygoid), medial-lateral grinding (pterygoids + masseter asymmetry), buccinator keeps bolus centered.
• Facial Expression → muscles pull on skin not bone; combinations produce infinite micro-expressions essential to non-verbal communication.
• Swallowing (deglutition)
\text{Stage 1:} suprahyoids raise hyoid/tongue → bolus to oropharynx.
\text{Stage 2:} pharyngeal constrictors propel bolus.
\text{Stage 3:} infrahyoids reposition hyoid/larynx.
• Breathing Assistance → scalenes + SCM contract during labored inspiration (e.g., COPD posture).
• Posture → posterior neck muscles counterbalance forward head; prolonged desk work weakens them ("text-neck").

Clinical / Practical Implications

• TMJ Disorders → hyperactivity of lateral pterygoid or malocclusion strains joint.
• Bell’s Palsy → facial nerve paralysis → unilateral facial expression loss, orbicularis oculi weakness causes tear overflow.
• Torticollis (wry neck) → SCM spasm/contracture tilts head.
• Dysphagia → suprahyoid/infrahyoid dysfunction impairs swallowing; common post-stroke.
• Trigger Points → masseter & temporalis referral patterns cause tension headaches.

Quick Review / Checklist

☐ List four elevators of mandible (temporalis, masseter, medial pterygoid, anterior suprahyoids when hyoid fixed).
☐ Name eye sphincter muscle (orbicularis oculi).
☐ Main smile elevator (zygomaticus major).
☐ Accessory inspiration muscles in neck (SCM, scalenes).
☐ Strap muscles function (depress hyoid, modulate voice).
☐ Digastric innervation split (CN V3 vs VII).
☐ Buccinator innervation (facial nerve, not trigeminal).