Identifying Muscles of the Trunk, Core, and Head - Human Anatomy and Physiology

The zygomaticus major is a facial expression muscle that can raise the corners of the mouth during a smile. The muscle originates from the zygomatic arch and inserts in the fibrous tissue of the modiolus.

The frontalis is located within the forehead and helps raise the eyebrows. The orbicularis oris circles the mouth and helps to pucker the lips. The masseter is used to raise the mandible during mastication.

The upper fibers of the trapezius muscle elevate the scapula and raise the upper limb above the head. The trapezius muscle is innervated by the accessory nerve. Damage to the accessory nerve results in drooping of the shoulder and inability to raise the arm above the head. The deltoid muscle is also used to abduct the arm at the shoulder.

The rhomboid major serves to connect the scapula and the vertebrae, pulling the shoulders posteriorly. The levator scapulae elevates the medial angle of the scapula. The latissimus dorsi helps to adduct and extend the arm at the shoulder.

There are many structures that aid in the process of inspiration and expiration. Inspiration involves enlarging the thoracic cavity for the lungs to expand, while expiration compresses the cavity to allow carbon dioxide to leave. Inspiration will always be active, while expiration can be either passive or active. One of the most important structures for inspiration is the diaphragm. The diaphragm will contract and move downwards into the abdominal cavity, allowing the lungs to expand and draw in air. Muscles of forced expiration are the anterior abdominals, the internal intercostals, and the serratus posterior inferior muscles. They help to relax the diaphragm, which increases the abdominal pressure and pushes the ribs inwards.

The ribs are elevated and depressed by the muscles of the thoracic wall and neck. The external intercostals originate at the lower border of the ribs. With the help of the internal intercostals, they raise the ribs when inspiring. The scalene muscles of the neck work to lift the first two ribs mainly. The sternocleidomastoid muscles are attached to the sternum, and also lift upwards.

The transversus thoracis originates on the posterior part of the sternum and xiphoid, inserting into the costal cartilages of ribs 2 to 6. They pull the ribs down when expiring air.

The geniohyoid muscle is associated with the chin, as it is a muscle that is the passageway from the chin to the hyoid bone. Contraction of the geniohyoid can move the hyoid bone to further widen the airway, aiding in respiration.

The nuchal ligament is located at the back of the neck and helps maintain the vertical position of the head. The sternocleidomastoid muscles are located in the lateral portions of the neck and helps to turn the head from side to side. The iliocostalis muscles run down the back, connecting the posterior portions of the ribs; this muscle is deep to most other muscles in the back. The popliteus muscle is located poterior to the knee and helps to move the tibia posteriorly, relative to the femur.

Intercostal muscles are responsible for expanding and contracting the thoracic cavity during forced breathing. During relaxed breathing, the diaphragm is responsible for changing the volume of the thoracic cavity, driving pulmonary ventilation. Internal intercostals depress the ribs during forced exhalation and the external intercostals elevate the ribs during forced inhalation. The subscapularis is one of the rotator cuff muscles, and its function is medial rotation of the humerus.

The transversospinal group is made up of the multifidi, the semispinalis group, and the rotatores group. The overall action of these muscles is extension and rotation of the vertebral column. The quadratus lumborum muscle laterally flexes the vertebral column, and, when bilaterally contracted, depresses the rib cage.

The iliofemoral ligament connects the ilium to the femur and is taut in end range hip extension (as in doing front splits) and external rotation.

The occiptofrontalis muscle is responsible for pulling the skin of the forehead up and down. It is part of a sheet of muscle the extends from just above the eyes to the back of the head. The orbicularis oris is responsible for puckering the lips such as kissing. The masseter is in the jaw and assists in chewing. The temporalis muscle is also involved in chewing, but is situated on either side of the skull under the area commonly referred to as the temple. The zygomatic oculi does not exist.

The platysma is a broad, flat muscle that lowers the jaw and stretches the lips down or to the sides. When the entire sheet is contracted it can increase the diameter of the neck. This is the muscle men flex when trying to shave under the chin and along the superior and anterior portion of the neck.

The masseter is in the jaw and is involved in the chewing motion. The mentalis muscle furrows the chin, while the risorius and orbicularis oris muscles pull the corners of the mouth out and in respectively.

The procerus is the muscle that wrinkles the nose. It originates from the fascia covering the inferior aspect of the nasal bone and inserts into the skin on the forehead, in between the eyebrows. The frontalis raises the eyebrows and wrinkles the forehead. The buccinator pulls in the cheeks against the teeth. The risorius pulls the mouth into a grimace.

The orbicularis oculi closes the eyes. It originates from the frontal and lacrimal bones, and from the medial palpebral ligament, which is attached to the frontal process of the maxilla and inserts into the lateral palpebral raphe. The buccinator pulls the cheeks into the teeth, The procerus wrinkles the nose. The depressor labii inferioris pulls the lower lip down.

The nasalis widens the opening of the nose by compressing the nasal cartilages, flaring out the nostrils. The frontalis wrinkles the forehead while the procerus wrinkles the nose. The levator anguli oris raises the corners of the mouth.

The platsyma muscle is responsible for both wrinkling the skin of the neck and pulling the lower lip down. The platysma is superficial to the sternocleidomastoid, originates from the clavicle, and inserts onto the base of the mandible and the skin of the cheek, lower lip and lower mouth. The depressor labii inferioris only pulls down the lower lip. The depressor anguli oris pulls the corners of the mouth down, while the risorius pulls the sides of the mouth into a grimace.

The masseter muscle, one of the strongest in the body, is the main muscle of chewing (mastication). The temporalis muscle moves the mandible up and backwards. The mandible is the jaw bone. The lateral pterygoid muscle is involved in mastication, but is not the strongest.

The sternocleidomastoid muscle is a powerful muscle that inserts at the mastoid process posterior to the ear. It originates down at the clavicle and manubrium and its contraction is the major muscle movement or protrusion one sees when the neck is turned to one side. This muscle also helps in respiration by raising the superior most rib via its attachment to the manubrium. The middle scalene originates from the transverse processes of the lower six cervical vertebrae, and inserts on the superior aspect of the first and second ribs; its function is to elevate the first two ribs, and contralateral rotation of the head. The anterior scalene has the same action as the middle scalene, but its origin is does not include C2, and it inserts only on the first rib. The sternohyoid, as its name suggests, originates on the manubrium of the sternum, and inserts on the hyoid. Its action is depression of the hyoid. Thee three groups of muscle fibers that make up the trapezius work in concert to control movements of the scapula.

Tee lateral pterygoid moves the jaw forward (protraction) and from side to side. The medial pterygoid is responsible for elevating the mandible and closing the mouth. The buccinator sucks the cheeks in towards the teeth and helps during suckling in neonates. Finally, the depressor anguli oris pulls the corners of the mouth down, as in frowning.

The inferior oblique rotates the eye to look up and to the side. The medial rectus moves the eye to look towards the nose. The lateral rectus moves the eye to look out to the side only. The superior oblique rotates the eye to look down and out towards the side.

There are 6 muscles that coordinate eyeball movement. They are the inferior oblique, lateral rectus, superior rectus, medial rectus, inferior rectus, and the superior oblique.