The human face possesses over two dozen individual muscles on each side - upwards of 30, depending on how they are counted. The facial muscles are striated muscles that link the skin of the face to the bone of the skull to perform important functions for daily life, including mastication and expression of emotion. While the individual movements these muscles produce are varied and diverse, it may be helpful to categorize them into two rough groups: the muscles of facial expression, also known as the mimetic muscles, and the muscles of mastication. The muscles of facial expression are innervated by the facial nerve (cranial nerve VII), and the muscles of mastication are innervated by the mandibular division of the trigeminal nerve (cranial nerve V3).[1] Using innervation to distinguish the two groups makes categorization largely dependent on embryonic origin, which does not always precisely correlate with function; this is nevertheless the most common method of describing the muscles.
Despite different innervations and functions, the facial muscles, by and large, act in synchrony. For example, during chewing, the orbicularis oris and buccinator muscles act to retain the food inside the mouth while the masseter and temporalis muscles move the mandible to chew the food. If anteroposterior or lateral and medial adjustment of the mandible is required, the pterygoid muscles provide that movement. The facial and mandibular nerves also both control muscles that participate in neither facial expression nor mastication; however, dysfunction of either of these nerves can be debilitating, as few other motor nerves control so many individual muscles that are so critical to the human experience. When facial and trigeminal nerve function is impaired, patients may suffer massive quality of life sequelae and have great difficulty remaining comfortable as members of society.
The facial muscles serve two major functions for the body: mastication and facial expressions. The primary muscles of mastication include the temporalis, medial pterygoid, lateral pterygoid, and the masseter, although the mylohyoid and anterior digastric muscles also move the mandible and assist with the acts of chewing and swallowing. The masseter originates on the undersurface of the zygomatic arch and wraps around the angle of the mandible to provide forceful closure of the jaw. The temporalis has a similar function but originates as a large fan shape in the temporal fossa across the frontal, temporal, parietal, and sphenoid bones, then travels deep to the zygomatic arch and inserts on the coronoid process and ramus of the mandible. The medial pterygoid muscle originates on the lateral pterygoid plate of the sphenoid bone and inserts on the medial aspect of the mandibular ramus and angle, completing a sling with the masseter, which inserts on the lateral surface of the mandibular angle; the medial pterygoid aids closure of the jaw and protrusion of the chin. The lateral pterygoid muscle originates on the lateral pterygoid plate as well but inserts on the condyle of the mandible to assist with protrusion of the chin and opening of the jaw. The mylohyoid and anterior digastric muscles also serve to open the jaw, running from the hyoid bone to the inner aspect of the mandible.
There are far more muscles of facial expression than there are muscles of mastication. The mimetic muscles are considered to be an extension of the superficial musculoaponeurotic system (SMAS) of the face, which is a fascial plane deep to the subcutaneous tissue but superficial to the muscles of mastication, running from the platysma in the neck up to the galea aponeurotica and the temporoparietal fascia under the scalp.[5] In the upper face, there are several muscles, including the frontalis (connected to the occipitalis via the galea aponeurotica), which raises the eyebrows and produces transverse forehead rhytides, the more superficial corrugator supercilii, which depresses the eyebrows and causes vertical glabellar rhytides ("number 11s" or "frown lines"), and the procerus, which is an even more superficial, midline muscle that causes horizontal rhytides at the root of the nose. In the midface, there are the orbicularis oculi sphincter muscles, which close the eyes, depress the brows, and cause crow's feet wrinkles, and there are the muscles of the nose as well. The orbicularis oculi muscles are exceptionally close to the skin surface, with essentially no subcutaneous fat between its fibers and the dermis. Hypertrophy of the orbicularis oculi may produce bags under the lower eyelids known as festoons, although this is not the most common cause of baggy eyelids.
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