Muscles of Facial Expression, Mastication, Neck, Vertebral Column & Eye

Muscles of Facial Expression

• General overview
  • All are superficial (except a few deep fibers such as the buccinator) and insert into the skin, allowing visible facial movements rather than large-scale limb motions.
  • Innervated by the facial nerve (cranial nerve VII) unless otherwise noted.
  • The laboratory list limits what must be identified on exams; several muscles visible in commercial cartoons/models are not on the required list.

Epicranius / Occipitofrontalis (three-part complex)

• Frontalis (frontal belly)
  • Broad, thin sheet covering the forehead.
  • Raises eyebrows; wrinkles skin of forehead.
  • Origin ≈ galea aponeurotica, insertion into skin above supra-orbital margin.
• Galea aponeurotica (epicranial aponeurosis = gallia aponeurotica)
  • Dense connective tissue sheet spanning the cranial vault.
  • Serves as intermediate tendon between frontal and occipital bellies.
• Occipitalis (occipital belly)
  • Covers superior nuchal line on occipital bone.
  • Stabilises galea and retracts scalp slightly.

Circular (sphincter-type) muscles

• Orbicularis oculi
  • Encircles palpebral fissure.
  • Two functional portions (palpebral & orbital) but subdivision not tested.
  • Closes eye firmly (orbital) or blinks softly (palpebral).
• Orbicularis oris
  • Encircles mouth opening.
  • Purses lips (“kissing” or “whistling” muscle).

Muscles of the cheek & mouth corner

• Buccinator
  • Deep to zygomatic muscles; forms muscular base of cheek.
  • Presses cheek against molars; important in chewing, sucking, blowing.
  • Landmark: found best once superficial muscles are peeled away.
• Zygomaticus major & minor
  • Both originate from zygomatic arch.
  • Major (inferior, slightly larger): pulls mouth角 supero-lateral (“smile”).
  • Minor (superior): elevates upper lip.
• Risor­ius
  • Horizontal, thin strip pulling oral commissure laterally (grin, widen mouth).

Lip-elevators/depressors (vertical fibres)

• Levator labii superioris (long version: levator labii superioris alaeque nasi – longest anatomical muscle name; “alae nasi” portion flanks nose)
  • Raises majority of upper lip; assists in snarling.
• Depressor labii inferioris
  • Draws lower lip downward & laterally.
• Levator anguli oris
  • Deep, approaches corner from superior/medial side; raises oral commissure at an angle.
• Depressor anguli oris
  • Triangular; pulls corner downward (frown) at an angle.

Chin, nose, scalp–wrinkling extras

• Mentalis
  • Central chin; protrudes lower lip (“pout”).
• Nasalis
  • Transverse fibres across nasal bridge; flares/compresses nostrils.
• Corrugator supercilii
  • Deep, small muscle medial to eyebrow.
  • Draws brows medially & inferiorly, creating vertical forehead wrinkles ("corrugated cardboard" analogy).
• Platysma
  • Extremely thin, paper-like sheet covering anterolateral neck & lower face.
  • Tenses skin of neck; depresses mandible slightly; produces broad “grimace.”
  • Clinical note: In ageing, loose areolar tissue allows platysma margin to sag → visible neck folds. Exercises (forceful grimacing; neck extension stretches) may reinforce fascia.
  • Not molded on most plastic skulls → may be indicated with paper overlay or in textbook images.

Muscles of Mastication (exam section D includes only two)

• Temporalis
  • Fan-shaped; arises from temporal fossa & deep temporal fascia → passes under zygomatic arch → inserts on coronoid process of mandible.
  • Elevates & retracts mandible; powerful jaw closer.
• Masseter
  • Quadrangular; superficial belly from zygomatic arch → lateral ramus/angle of mandible.
  • Elevates mandible, adds bite force.
  • Superficial and deep parts visible on many head models; sometimes partly obscured by parotid gland replica.

(Note: Pterygoids are deeper; omitted from the current lab list.)

Neck & Vertebral Column Muscles (selected subset, list-driven)

Superficial landmark

• Trapezius
  • Large kite-shaped back muscle; retained on some models, removed on others to expose deeper neck layers.

Sternocleidomastoid (SCM)

• Origins: manubrium (sternum) & medial clavicle → insert on mastoid process & superior nuchal line.
• Bilateral action: neck flexion; unilateral: ipsilateral lateral flexion + contralateral rotation ("turn head opposite direction").

Splenius capitis & Semispinalis capitis

• Splenius capitis (“Band-Aid” shape)
  • Superficial to semispinalis; extends head & rotates toward same side.
• Semispinalis capitis
  • Deep, long muscle from upper thoracic transverse processes → occipital bone; powerful extensor.

Erector spinae group

• Collective function: maintain erect posture; extension/lateral flexion of spine.
• Organized medial → lateral: spinalis, longissimus, iliocostalis.
  • In thoracic region (only level visible on lab torsos):
  • Spinalis thoracis (spinous → spinous processes)
  • Longissimus thoracis (transverse → transverse)
  • Iliocostalis thoracis (ilium → ribs)

Scalenes

• Three paired muscles bridging cervical transverse processes to first two ribs.
  • Anterior scalene (anterior to subclavian artery)
  • Middle scalene (posterior to artery, anterior to brachial plexus)
  • Posterior scalene (smallest, most posterior)
• Act on rib cage (elevate during forced inspiration) & assist in neck flexion.
• Anatomical significance: neurovascular structures (subclavian a., brachial plexus) pass between anterior & middle scalenes – landmark for future clinical discussions.

Levator scapulae

• From upper cervical transverse processes to superior angle of scapula.
• Elevates scapula; included here because it sits immediately posterior to scalenes on lab models.

Suprahyoid group (three required)

• Digastric (“two-bellied”)
  • Anterior belly (mandible) → central tendon via fibrous loop (trochlea) on hyoid → posterior belly (mastoid notch).
  • Main mouth-opening muscle (depresses mandible when hyoid fixed).
• Stylohyoid
  • Runs parallel/superficial to posterior digastric; from styloid process → hyoid.
  • Elevates & retracts hyoid during swallowing.
• Mylohyoid
  • Flat, muscular floor of mouth; two halves meet at midline raphe.
  • Elevates hyoid & tongue while speaking/swallowing.

(Other small infrahyoids, geniohyoid, etc., are excluded from the present checklist.)

Extra-ocular (Oculomotor) Muscles

• Six intrinsic eye movers; named by fiber direction & position relative to eyeball.
• All originate from the fibrous annulus in orbit apex (except inferior oblique) and insert on sclera.
• Cranial nerve innervation mnemonic: “LR6 SO4 AO3” (lateral rectus – CN VI; superior oblique – CN IV; all others – CN III)

Four recti (straight fibers)

• Superior rectus – elevates & adducts eye; slight medial rotation.
• Inferior rectus – depresses & adducts; slight lateral rotation.
• Lateral rectus – pure abduction ( ext{CN VI}).
• Medial rectus – pure adduction.

Two obliques (angled fibers)

• Superior oblique
  • Passes through fibro-cartilaginous pulley (trochlea) on medial orbital roof → inserts posterolaterally.
  • Depresses & abducts eye; intorts (medial rotation). Innervation = trochlear nerve (CN IV).
• Inferior oblique
  • Only extra-ocular muscle originating from anterior orbit (maxilla just lateral to nasolacrimal canal) → inserts posterolateral inferior sclera.
  • Elevates & abducts eye; extorts (lateral rotation).

Orientation clues on models

• Lacrimal gland (tear gland) = superior-lateral landmark; helps determine “top” & “lateral” when eyeball removed.
• Optic nerve stump visible exiting posterior globe; recti radiate around it.

Practical & Clinical Connections

• Ageing skin changes around platysma → “turkey neck”; strengthening & stretching can mitigate.
• Corrugator supercilii injections often targeted in cosmetic botulinum toxin treatments to reduce “frown lines.”
• Masseter hypertrophy can indicate bruxism (tooth grinding) or be selectively reduced for facial slimming.
• Scalene space syndrome (thoracic outlet) involves compression of subclavian vessels/plexus between anterior & middle scalene.
• Superior oblique dysfunction (trochlear nerve palsy) produces vertical diplopia relieved by head tilt to opposite side – leveraged in clinical tests.

Study / Lab Strategy Reminders

• “Paper-thin” or missing on plastic models: platysma, levator labii superioris alaeque nasi extension, some deep corrugator fibres.
• Best buccinator & corrugator views often require removal of overlying fat pads or superficial muscles (special dissections or textbook plates).
• Know model views: front, side (lateral), deep dissections; repeatedly identify each muscle to reinforce spatial memory.
• For erector spinae, identify relative position (medial → lateral) rather than memorising every minor subdivision.
• Practice palpating live landmarks (e.g., temporalis during clench, SCM when rotating head, platysma while grimacing) for kinaesthetic reinforcement.