Facial Nerve and Mastication Muscles: Anatomy & Function

Facial Nerve Branches and Course

Facial Nerve: Innervated Muscles

The facial nerve (Cranial Nerve VII) provides motor innervation to the muscles of facial expression. Its terminal branches and the muscles they innervate include:

• Temporal Branches: Frontal belly of occipitofrontalis, orbicularis oculi, corrugator supercilii.
• Zygomatic Branches: Zygomaticus major and minor, levator labii superioris alaeque nasi, levator anguli oris, depressor septi nasi, orbicularis oris (upper part).
• Buccal Branches: Buccinator, orbicularis oris (lower part), depressor anguli oris.
• Marginal Mandibular Branch: Mentalis, depressor labii inferioris.
• Cervical Branch: Platysma.

Facial Nerve: Intracranial Branches and Terminal Division

Before exiting the skull, the facial nerve gives off several important branches:

• Greater Petrosal Nerve: Carries parasympathetic fibers to the lacrimal gland and glands of the nasal cavity and palate.
• Nerve to Stapedius: Innervates the stapedius muscle in the middle ear.
• Chorda Tympani Nerve: Carries taste sensation from the anterior two-thirds of the tongue and parasympathetic fibers to the submandibular and sublingual glands.

After exiting the stylomastoid foramen, the facial nerve penetrates the parotid gland, where it typically divides into two main trunks, which then further branch:

• Temporofacial Branch: Gives rise to the temporal and zygomatic branches.
• Cervicofacial Branch: Gives rise to the buccal, marginal mandibular, and cervical branches.

Facial Nerve: Anatomical Course

The facial nerve enters the internal auditory canal along with the vestibulocochlear nerve (Cranial Nerve VIII) and the labyrinthine artery. It then enters the facial canal (Fallopian aqueduct), traversing its labyrinthine, tympanic, and mastoid segments. The nerve exits the skull via the stylomastoid foramen, located near the mastoid process, and subsequently enters the parotid gland where it divides into its terminal branches.

Muscles of Mastication: Anatomy and Function

Temporal Muscle: Anatomy, Function, and Innervation

The Temporal Muscle is a large, fan-shaped muscle of mastication. It is characterized by:

• Origin: Superior and inferior temporal lines, and the entire temporal fossa.
• Insertion: Via a strong tendon into the coronoid process of the mandible and the anterior border of the ramus.
• Function: Primarily elevates the mandible (closing the jaw). Its posterior fibers also retract the mandible.
• Innervation: Deep temporal nerves (anterior, middle, and posterior), which are branches of the mandibular nerve (V3).

Masseter Muscle: Anatomy and Function

The Masseter Muscle is a powerful, quadrilateral muscle of mastication, composed of two fascicles (superficial and deep).

• Origin: Inferior border and medial surface of the zygomatic arch.
• Insertion: Lateral surface of the ramus and angle of the mandible.
• Function: Elevates the mandible (closing the jaw) and aids in protraction.
• Antagonist: Muscles that depress the mandible (e.g., suprahyoid and infrahyoid muscles, platysma).

Lateral Pterygoid Muscle: Anatomy and Function

The Lateral Pterygoid Muscle is located in the infratemporal fossa.

• Origin:
  • Superior head: Infratemporal surface of the greater wing of the sphenoid bone.
  • Inferior head: Lateral surface of the lateral pterygoid plate.
• Insertion: Pterygoid fovea on the neck of the mandibular condyle and the articular disc of the temporomandibular joint (TMJ).
• Function: Protracts the mandible, depresses the mandible (opening the jaw), and moves it laterally (side-to-side grinding movements).
• Innervation: Lateral pterygoid nerve (branch of the mandibular nerve, V3).

Medial Pterygoid Muscle: Anatomy and Function

The Medial Pterygoid Muscle is situated medial to the ramus of the mandible.

• Origin: Medial surface of the lateral pterygoid plate and the pyramidal process of the palatine bone.
• Insertion: Medial surface of the angle and ramus of the mandible.
• Function: Elevates the mandible (closing the jaw) and aids in protraction and lateral movements.
• Innervation: Medial pterygoid nerve (branch of the mandibular nerve, V3).