The facial muscles are a group of about 20 flat skeletal muscles lying under the facial skin. Most of them originate from the skull or fibrous structures and radiate to the skin through an elastic tendon. Contrary to the other skeletal muscles they are not surrounded by a fascia (except the buccinator). The facial muscles are positioned around facial openings (mouth, eye, nose and ear) or stretch across the skull and neck.
Muscles of the mouth
- Orbicularis oris muscle: encircles the mouth;
- Buccinator muscle: maxilla and mandible → angle of the mouth. It forms the muscular base of the cheek;
- Levator labii superioris muscle: above the infraorbital foramen → upper lip;
- Depressor labii inferioris muscle: mandible underneath the mental foramen → lower lip;
- Levator labii superioris alaeque nasi muscle: medial orbital wall → upper lip and and nostrils;
- Mentalis muscle: forms the furrow between chin and lip;
- Risorius muscle ("laughing muscle"): cheek → angle of the mouth;
- Levator anguli oris muscle: underneath the infraorbital foramen → angle of the mouth;
- Depressor anguli oris muscle: lower border of mandible → angle of the mouth;
- Zygomaticus major and minor muscles: zygomatic arch → angle of the mouth;
Muscles of the nose
- Nasalis muscle: maxilla → nasal cartilages;
- Procerus muscle: nasal bridge → skin between eyebrows;
Muscles of the eyelid
- Orbicularis oculi muscle: encircles the eye;
- Depressor supercilii muscle: medial angle of eye → skin of the eyebrow;
- Corrugator supercilii muscle: above the root of the nose → skin of the eyebrow;
Muscles of the cranium and neck
- Epicranius muscle: forehead, outer surface of the back of the head (occipitofrontalis muscle), above the ear (temporoparietal muscle) → fibrous layer on the scalp (galea aponeurotica);
- Platysma muscle: above the mandible (parotid fascia) → at the height of the 2nd rib (pectoral fascia);
Muscles of the external ear
- Auricular muscles: Temporal fascia (anterior auricular muscle), galea aponeurotica (superior auricular muscle), mastoid process (posterior auricular muscle) → pinna;
All facial muscles derive from the 2nd pharyngeal arch and are therefore supplied by the facial nerve. Originally they formed one single muscle plate but during the course of embryological development they split apart.
The facial muscles differ widely from the other skeletal muscles because they do not move joints but instead the skin. The muscles of the mouth are mainly responsible for the elevation and depression of the angle of the mouth, upper and lower lips and the opening and closure of the mouth.
Additionally the levator labii superioris alaeque nasi pulls the nostrils upward and opens the nose.
The risorius and both the zygomaticus major and minor are the most important laughing muscles. The muscles of the nose wrinkle up the nose and narrow the nostrils.
The contraction of the orbicularis oculi closes the eyelids, allows squinting of the eyes and supports the flow of tear fluid. The other two muscles of the eyelid depress the eyebrow and allow wrinkling of the forehead. In contrast the epicranius elevates the eyebrows and the ear. The platysma pulls the corner of the mouth laterally and downwards and tightens the skin. The auricular muscles are quite variable which is why only some people are able to voluntarily move their ears.
The facial muscles do not only regulate the position and width of facial openings but also make them more expressive. By those means, the face is able to convey emotions and the present psychological state of a person, which plays an extraordinary role in the nonverbal communication between people.
The inability to move facial muscles is a classic symptom of facial nerve paralysis. Hereby, one can clinically differentiate between a peripheral lesion and central lesion.
- In peripheral facial lesions the facial muscles are completely paralyzed on the affected side. When trying to close the lids the eye rotates upwards exposing the sclera (Bell’s phenomenon).
- In central facial lesions one can still wrinkle the forehead on both sides.
The reason behind this is that the motor branches of the forehead muscles derive from both the ipsi- and contralateral facial nuclei. In both types of facial nerve paralysis speech, chewing and facial expression are severely impaired. Depending on the location of the lesion, the affected patients suffer from additional disturbances of tear and saliva secretion, hearing or taste.
There is a variety of causes for facial nerve paralysis including inflammation (e.g. herpes zoster infection), stroke, petrous bone fracture and tumors (e.g. vestibular schwannoma) but in most cases a definite cause cannot be found (idiopathic facial nerve paralysis, also known as Bell’s palsy). Current studies suggest that infections with the Herpes simplex virus type 1 and other less harmful viruses are behind Bell’s palsy.