Muscles of Facial Expression – Anatomy and Function

What Are the Muscles of Facial Expression?

The muscles of facial expression (also called mimetic muscles or facial muscles) are a group of skeletal muscles located in the face and scalp. Unlike most skeletal muscles, they do not attach to bones at both ends - instead, they insert directly into the skin or oral mucosa. This allows them to create subtle movements of the facial skin that are essential for expressing emotions and for performing vital everyday functions. All muscles of facial expression are innervated by the facial nerve (cranial nerve VII).

Anatomy and Classification

The facial muscles can be grouped by their anatomical location on the face:

Muscles Around the Eye

• Orbicularis oculi: Closes the eyelid, enables blinking and tight eye closure.
• Corrugator supercilii: Draws the eyebrows together, creating vertical frown lines between the brows.
• Procerus: Pulls the skin between the eyebrows downward, forming horizontal wrinkles at the nasal root.

Muscles Around the Mouth

• Orbicularis oris: The circular muscle of the mouth; closes, purses, and compresses the lips - essential for speaking, whistling, and kissing.
• Zygomaticus major: Pulls the corners of the mouth upward and laterally - often called the smiling muscle.
• Zygomaticus minor: Elevates the upper lip and deepens the nasolabial fold during smiling.
• Levator labii superioris: Raises the upper lip and dilates the nostrils.
• Depressor labii inferioris: Pulls the lower lip downward and laterally.
• Depressor anguli oris: Draws the corners of the mouth downward (expressing sadness or disapproval).
• Risorius: Pulls the mouth corners laterally and creates dimples when smiling.
• Buccinator: Compresses the cheeks, prevents the cheek mucosa from being caught between the teeth during chewing, and is important for sucking.
• Mentalis: Elevates and protrudes the lower lip, wrinkles the chin skin.

Muscles of the Nose

• Nasalis: Dilates or constricts the nostrils (e.g., during deep inhalation or emotional expression).

Muscles of the Forehead and Scalp

• Frontalis (part of the occipitofrontalis): Raises the eyebrows and creates horizontal forehead wrinkles (expression of surprise or attention).
• Occipitalis: Draws the scalp backward.
• Auricularis (anterior, superior, posterior): Small muscles that move the outer ear - largely vestigial in humans.
• Platysma: A broad, thin neck muscle that pulls the corners of the mouth downward and tenses the skin of the neck.

Functions of the Facial Muscles

The functions of the muscles of facial expression can be divided into two main areas:

• Emotional expression: The coordinated action of different facial muscles produces the universal basic emotions - joy, sadness, surprise, disgust, anger, and fear. This is a central element of nonverbal communication.
• Functional tasks: Many facial muscles are involved in essential activities including speaking, chewing, swallowing, sucking, and closing the eyes to protect the cornea.

Clinical Relevance

Conditions or injuries affecting the facial nerve can cause facial palsy (facial nerve paralysis) - a unilateral or complete paralysis of the muscles of facial expression. This presents as a drooping corner of the mouth, inability to fully close the eye, and loss of facial expression on the affected side. Common causes include:

• Idiopathic facial palsy (Bell's palsy): The most common form, thought to be caused by viral inflammation of the facial nerve.
• Herpes zoster oticus (Ramsay Hunt syndrome): Viral infection involving the geniculate ganglion.
• Stroke (central facial palsy): Usually affects only the lower half of the face, since the forehead receives bilateral cortical innervation.
• Tumors, inflammation, or trauma along the course of the facial nerve.

The muscles of facial expression are also of great importance in aesthetic medicine: Botulinum toxin (Botox) is selectively injected to temporarily paralyze specific facial muscles and thereby reduce wrinkles. The corrugator supercilii, frontalis, and orbicularis oculi are among the most commonly treated muscles.

Diagnosis

The function of the facial muscles is assessed clinically through a targeted neurological examination. The physician asks the patient to perform specific facial movements (e.g., raising the eyebrows, closing the eyes, smiling, whistling). Additional diagnostic methods include:

• Electromyography (EMG): Measures the electrical activity of muscles to assess nerve and muscle damage.
• Imaging (MRI/CT): Used to exclude structural causes such as tumors or inflammation along the facial nerve pathway.

References

1. Standring, S. (Ed.) (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd edition. Elsevier.
2. Drake, R.L., Vogl, A.W. & Mitchell, A.W.M. (2019). Gray's Atlas of Anatomy. 3rd edition. Elsevier.
3. Rinn, W.E. (1984). The neuropsychology of facial expression: A review of the neurological and psychological mechanisms for producing facial expressions. Psychological Bulletin, 95(1), 52-77. PubMed PMID: 6701898.