The ability of humans to form a variety of facial expressions allows them to convey many emotions wordlessly. Facial expressions depend on the contraction of tiny muscles in the skin of the face and scalp. Branches of cranial nerve VII, or the facial nerve, innervate nearly all these muscles. Damage to this nerve can paralyze facial muscles.
The occipitofrontalis (also called epicranius) muscle controls movement of the eyebrows, scalp, and skin of the forehead. It includes the forehead's frontalis muscle and the occipitalis muscle at the back of the head. The frontalis draws the scalp forward, raises eyebrows, and wrinkles the forehead. The occipitalis retracts the scalp. The parts are linked by the galea aponeurotica (also called the epicranial aponeurosis), a sheet-like tendon that is a type of connective tissue, or a tissue composed of cells and an extracellular matrix that binds the cells and the organs of the body together.The circular orbicularis oculi is a sphincter muscle that surrounds the eye and closes it. It attaches to the orbit and the eyelid and is innervated, or receives nerve impulses from, the facial nerve. This is why in conditions where the facial nerve is paralyzed, such as in Bell's palsy, the patient is unable to close the eye. The levator palpebrae superioris, behind the orbicularis oculi and between the orbit and upper eyelid, opens the eye and raises the upper eyelid. It is innervated by cranial nerve III, or the oculomotor nerve. Patients who suffer strokes affecting the oculomotor nerve are unable to open the eye. A levator is any muscle whose contraction raises a body part to which that muscle is attached. Other muscles that cause movements of the ocular and nasal regions are the procerus (pulls the skin between the eyelids down), the corrugator supercilii (known as the "frowning" muscle, it pulls the eyebrows down), and the nasalis (allows for flaring of the nostrils by depressing the nose tip and elevating the nostril corners).
Muscles Responsible for Facial Expression
The buccinator is the muscle that forms the cheek and through compression and contraction contributes to sucking, blowing, and chewing. This sucking motion allows for successful nursing in infants. The platysma muscle attaches from the upper chest and shoulder to the mandible. It lowers the mandible, aids in opening and widening the mouth, and tenses neck skin.
Muscles Responsible for Chewing and Swallowing
Several head and neck muscles enable the varied motions involved in chewing and swallowing food. The tongue is an extremely flexible organ controlled by two muscle groups: intrinsic and extrinsic tongue muscles. Intrinsic tongue muscles are a part of the tongue and without attachments elsewhere. These are arranged vertically and include (from superior to inferior): the superior longitudinal muscle (elevates the tip and sides of the tongue), the vertical muscle (widens and flattens the tongue), the transverse muscle (lengthens and "sticks out" the tongue), and the inferior longitudinal muscle (depresses the tip and sides of the tongue). Extrinsic tongue muscles connect to other head and neck structures. The genioglossus connects to the mandible, the hyoglossus to the hyoid bone, the styloglossus to the styloid process, and the palatoglossus to the soft palate. Together these muscles depress, retract, protrude, lift, and alter the shape of the tongue. The tongue and buccinator muscle jointly move food into position for chewing. Once food is chewed, the tongue pushes it into the pharynx, where it is swallowed. Cranial nerve XII, the hypoglossal nerve, innervates the genioglossus, hyoglossus, and styloglossus muscles; the accessory nerve, cranial nerve XI, innervates the palatoglossus.Chewing, or mastication, depends on four paired muscles innervated by cranial nerve V, the trigeminal nerve. The temporalis extends broadly from the skull's temporal lines to the mandible's coronoid process, passing behind the zygomatic arch. The thick, short masseter muscle, prominent on the side of the jaw, attaches from the zygomatic arch to the mandible's ramus and angle. The temporalis and masseter are extremely powerful muscles that work together to elevate the mandible for biting and chewing. The second pair of mastication muscles, the medial and lateral pterygoids, together elevate and protract, or pull back, the mandible and provide the lateral motion necessary for grinding food between molars. Both pterygoid muscles originate at the pterygoid process of the sphenoid bone and extend to locations along the mandible. Eight pairs of hyoid muscles enable chewing and swallowing. All attach at the U-shaped hyoid bone that anchors the tongue. Four of these, the suprahyoid group, are superior to or above the hyoid bone: the digastric, geniohyoid, mylohyoid, and stylohyoid muscles. The infrahyoid group includes four muscles inferior to or below the hyoid bone: the omohyoid, sternohyoid, thyrohyoid, and sternothyroid. Most hyoid muscles are innervated by the ansa cervicalis nerve. The suprahyoid muscles elevate the hyoid bone, and the infrahyoid muscles depress it. The geniohyoid muscle also protracts the hyoid, assisting swallowing by widening the top part of the throat, or pharynx. Mylohyoid muscles make up the mouth floor and combine with the digastric to open the mouth. As food is moved into the pharynx, pharyngeal muscles take action. The superior, middle, and inferior pharyngeal constrictors contract in sequence to pass food downward. Infrahyoid muscles prevent choking as food is swallowed. The thyrohyoid elevates the hyoid and larynx, also called the voice box, into a position where the larynx is blocked by the epiglottis. The epiglottis is a flap of tissue that covers the trachea, or windpipe, during eating. The sternohyoid muscles lower the larynx.
Muscles That Move the Head and Neck
Many head and neck muscles originate at the pectoral girdle (the region formed by the collarbone, or clavicle, and the shoulder blades, or scapulae) and the vertebral column, and insert on the cranial bones. Some of these include the trapezius and the semispinalis capitis, respectively. The sternocleidomastoid and three scalene muscles on each side of the neck are the major neck flexors. A flexor is any muscle whose contraction bends a body part by decreasing the angle between bones on two sides of a joint. The thick, cord-like sternocleidomastoid muscle is innervated by the accessory nerve and is the prime mover. A prime mover is a muscle that produces most of the force to accomplish a specific movement. The sternocleidomastoid attaches from the sternum and clavicle to behind the ear at the mastoid process. This oblique, or angular, attachment divides the neck into regions called the anterior and posterior triangles. Contraction of both sternocleidomastoid muscles flexes the neck forward so the eyes face down. Contraction of a single sternocleidomastoid muscle tilts the head downward and in the opposite direction. The scalene muscles flex the neck laterally.
Extensor muscles are at the back of the neck. An extensor is any muscle whose contraction straightens or extends a body part by increasing the angle between the sides of a joint. Extensor muscles in the neck extend the neck, as in looking up and backward, and they rotate and extend the head. There are three primary extensors: the trapezius, splenius capitis, and semispinalis capitis. The trapezius is a large triangular muscle named for the trapezoid shape formed by the combined right and left trapezius muscles. The trapezius is innervated by the accessory nerve. It inserts at the shoulder, narrowing after its broad origin between the skull's occipital protuberance and thoracic vertebra 12. The splenius lies just beneath, or deep to, the trapezius and extends between the vertebral column toward the ears. Its fascicles, a type of connective tissue, tightly compress underlying neck muscles. The semispinalis capitis lies a bit deeper than the splenius and inserts at the occipital bone.