Muscles of Facial Expression and the Facial Nerve

Overview

The muscles of facial expression are a sheet of subcutaneous muscles that move the skin of the face, the scalp, and the external ear. They are unique in inserting into skin rather than bone, allowing the fine gradations of expression. All are derived from the second pharyngeal arch and are therefore supplied by its nerve, the facial nerve (CN VII).

Common Features

• Develop from the mesoderm of the second pharyngeal (hyoid) arch.
• Lie within the superficial fascia; most attach to skin.
• Grouped around the orifices of the face — the orbit, nose, mouth, and ear — acting as sphincters and dilators of these openings.

Key Muscles by Region

Around the mouth (most important for dentistry):

• Orbicularis oris: the sphincter of the mouth; closes and purses the lips, essential for a lip seal and denture retention.
• Buccinator: forms the muscular cheek, keeps food between the teeth during mastication, and is pierced by the parotid duct.
• Levator labii superioris, zygomaticus major and minor, risorius, levator anguli oris: elevate and retract the lips (smiling).
• Depressor anguli oris, depressor labii inferioris, mentalis: depress and protrude the lower lip and chin.

Around the eye: orbicularis oculi (closes the eyelids; its palpebral part blinks, its orbital part screws the eyes shut).

Around the nose: procerus, nasalis, depressor septi.

Scalp and ear: occipitofrontalis (the epicranius), auricular muscles.

Facial Nerve Motor Supply

After leaving the stylomastoid foramen and traversing the parotid, the facial nerve fans out as five terminal branches to supply all these muscles: temporal, zygomatic, buccal, marginal mandibular, and cervical. The buccal branches to buccinator and orbicularis oris are of daily relevance to the dentist.

Upper vs Lower Motor Neuron Lesions

This is a classic and high-yield distinction:

• Lower motor neuron (LMN) lesion (e.g. Bell's palsy, at or distal to the nucleus/nerve): paralysis of the entire ipsilateral half of the face — forehead cannot wrinkle, eye cannot close, corner of the mouth droops.
• Upper motor neuron (UMN) lesion (e.g. stroke, above the nucleus): the forehead is spared because the part of the facial nucleus supplying the upper face receives bilateral cortical input. The lower face on the opposite side is paralysed.

Clinical Relevance

• Bell's palsy: idiopathic LMN facial palsy; the inability to close the eye risks corneal exposure, and loss of the lip seal and buccinator tone causes food to pool in the vestibule — patients are often first noticed in the dental chair.
• Denture retention depends on balanced tone of orbicularis oris and buccinator; over-extended flanges are displaced by these muscles.
• Facial nerve injury during surgery: extraoral incisions and parotid surgery may damage individual branches; the marginal mandibular branch is most exposed.
• Botulinum toxin is injected into these muscles for both cosmetic and therapeutic (e.g. bruxism-related masseteric hypertrophy is a masticatory muscle, but perioral dynamics matter) indications; a working knowledge of muscle position prevents unwanted asymmetry.

Study Points

• State the common embryological origin and nerve of all facial muscles.
• Contrast the facial findings of UMN and LMN facial nerve lesions and explain forehead sparing.
• Explain how buccinator and orbicularis oris contribute to mastication and denture retention.