Muscles of Facial Expression and Mastication: Structure and Function

Muscles of Facial Expression

Overview

The muscles of facial expression are a group of skeletal muscles situated in the subcutaneous tissue of the scalp and face. They are responsible for various facial movements, including expressions such as smiling, frowning, and pouting. Most of these muscles are innervated by the facial nerve (cranial nerve VII).

• Epicranius (Occipitofrontalis): Composed of two parts—the frontalis and occipitalis—connected by the galea aponeurotica.

  • Frontalis: Located over the forehead; function: draws the scalp anteriorly and wrinkles the skin of the forehead.

  • Occipitalis: Superficial to the occipital bone; function: draws the scalp posteriorly.

• Orbicularis oculi: Circular muscle surrounding the rim of the eyelid; function: closes the eye tightly (as in blinking or squinting).

• Orbicularis oris: Circular muscle around the mouth; function: closes, shapes, and protrudes the lips (as in whistling or kissing).

• Zygomaticus major: Extends from the cheekbone to the corner of the mouth; function: draws the angle of the mouth superiorly and laterally (as in smiling).

• Levator labii superioris: Thin muscle from the upper lip and lower eye margin; function: elevates the upper lip and opens the lips (as in showing the upper teeth or pouting).

• Depressor labii inferioris: Small muscle from the mandible to the lower lip; function: depresses the lower lip (as in pouting).

• Depressor anguli oris: Small muscle at the lateral aspect of the mouth; function: draws the corner of the mouth downward and laterally (as in frowning or expressing sadness; antagonist to zygomaticus major).

• Platysma: Broad, flat, superficial muscle of the neck; function: draws the outer part of the lower lip inferiorly and posteriorly (as in pouting), depresses the mandible, and tenses the skin of the neck (as during shaving).

• Buccinator: Thin muscle forming the wall of the cheek; function: compresses the cheek (as in blowing, sucking, or whistling), keeps food between the teeth during chewing; well developed in infants for suckling.

Table: Muscles of Facial Expression and Their Functions

Muscle	Location/Origin	Function
Frontalis	Forehead (part of epicranius)	Draws scalp anteriorly, wrinkles forehead
Occipitalis	Occipital bone (part of epicranius)	Draws scalp posteriorly
Orbicularis oculi	Surrounds eyelids	Closes eyes
Orbicularis oris	Encircles mouth	Closes, shapes, protrudes lips
Zygomaticus major	Cheekbone to mouth corner	Elevates mouth corners (smiling)
Levator labii superioris	Upper lip, lower eye margin	Elevates upper lip
Depressor labii inferioris	Mandible to lower lip	Depresses lower lip
Depressor anguli oris	Lateral mouth	Depresses mouth corners (frowning)
Platysma	Superficial neck	Depresses mandible, tenses neck skin
Buccinator	Cheek	Compresses cheek (blowing, sucking)

Muscles of Mastication (Chewing)

Overview

The muscles of mastication are responsible for the movements of the jaw required for chewing. These muscles are primarily innervated by the mandibular branch of the trigeminal nerve (cranial nerve V3).

• Masseter: The strongest muscle of mastication, located laterally on the mandibular ramus; function: elevates and retracts the mandible, primary jaw closer.

• Temporalis: Fan-shaped muscle over the temporal bone; function: elevates and retracts the mandible, closes the jaw, and maintains the position of the mandible at rest; synergist with pterygoids for side-to-side movement.

• Medial pterygoid: Two-headed muscle deep to the mandible; function: elevates and protracts the mandible, assists in side-to-side (lateral) movements.

• Lateral pterygoid: (Not clearly listed but commonly paired with medial pterygoid); function: protracts the mandible and produces side-to-side grinding movements.

Table: Muscles of Mastication and Their Functions

Muscle	Location/Origin	Function
Masseter	Lateral mandibular ramus	Elevates and retracts mandible
Temporalis	Temporal fossa	Elevates and retracts mandible, closes jaw
Medial pterygoid	Deep to mandible	Elevates, protracts mandible; side-to-side movement
Lateral pterygoid	Deep to mandible	Protracts mandible; side-to-side movement

Key Concepts

• Synergists and Antagonists: Muscles often work in pairs; for example, the zygomaticus major (smiling) is antagonized by the depressor anguli oris (frowning).

• Functional Importance: Muscles of facial expression are essential for nonverbal communication, while muscles of mastication are critical for mechanical digestion.

Additional info: The lateral pterygoid, though not explicitly listed in the original notes, is commonly included in the group of muscles of mastication and is important for jaw protrusion and side-to-side movements.