15.6: Muscles for Facial Expressions
The craniofacial muscles are a collection of approximately 20 thin skeletal muscles situated beneath the skin of the face and scalp. These muscles, primarily responsible for the vast array of human facial expressions, originate from the bones or fibrous structures of the skull and extend outwards to connect with the skin. While most skeletal muscles in the body are enveloped in thick fascia, facial muscles generally have a more delicate fascial covering, with the buccinator muscle being a notable exception.
These facial muscles are situated around the facial openings (mouth, eyes, nose, and ears) or span across the skull and neck and are crucial for non-verbal communication. They are aptly named muscles of facial expression or mimetic muscles due to their specific role in reflecting emotions and intentions. All facial muscles receive signals from the facial nerve, ensuring precise and coordinated movements, while the facial artery provides the necessary blood supply. However, the innervation and vascularization patterns can be complex and vary slightly between individuals.
For instance, the occipitofrontalis muscle, which plays a key role in lifting the eyebrows, is a two-part scalp muscle composed of frontal and occipital bellies. These parts are united by the epicranial aponeurosis, a tough layer of connective tissue. Another crucial muscle is the orbicularis oris, a circular muscle that encircles the mouth. It originates from adjacent muscles attached to the maxillae and mandible and inserts around the lips. It is also the insertion point for the buccinator muscle, which originates from the maxillae and mandible and controls mouth movements like pouting and puckering.
Additionally, muscles such as the zygomaticus major, responsible for pulling the corners of the mouth upward into a smile, and the corrugator supercilii, involved in frowning by drawing the eyebrows together, further exemplify the diversity and specificity of facial muscles. Along with others controlling the eyes, nose, and cheeks, these muscles orchestrate a symphony of expressions ranging from joy to sorrow.
