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While there are many cervical muscles this area is are not as important to dental professionals, for this reason the list has been narrowed to the two largest and most important muscles. The sternocleidomastoid and trapezius, both of which are innervated by nerve fibers originating in the spinal accessory nerve (cranial nerve XI).
Sternocleidomastoid
The sternocleidomastoid muscle has a belly that originates from the superior portion of the sternum known as the manubrium and another belly that originates on the medial portion of the clavicle. Both heads merge to insert on the posterior portion of the mastoid process and the adjacent superior nuchal line. This muscle is prominent in the neck and used as a landmark in examinations as it defines the anterior triangle and posterior triangles of the neck. It is a prime mover of the head as it is capable of rotating and flexing the head but also laterally flexing it. It is the muscle that allows us to indicate we agree by nodding by bringing our chin downwards, disagree by rotating it to say no or tilt the head which can mean anything from, “I don’t believe you,” to “I don’t understand.”
Trapezius
While the sternocleidomastoid angles towards the posterior and can bring the head forward it must be opposed to lift the chin superiorly. This is the job of the trapezius muscle along with other muscles we will not be discussing. The trapezius is a large muscle that can be seen in the back of the neck and is the muscle that people love to have massaged. The large base of the trapezoid is the origin which extends from the superior nuchal line on the occipital bone, down the nuchal line and then continues attaching to the spinous processes of the cervical and thoracic vertebrae. It inserts on the lateral end of the clavicle and along the scapular spine to the acromion process. As this muscle’s insertion is superior to the inferior border of the muscle but inferior to the superior border of the muscle it can have opposing actions depending on whether the superior or inferior section is contracted. The superior fibers can elevate and rotate the scapula to help abduct the arm and the inferior fibers can depress the scapula and rotate the scapula to help adduct the arm. The middle section of the muscle draws the scapula medially towards the spine although the upper and lower fibers can also perform this action if coordinated. So far, no mention is made of the head movement discussed at the beginning of this section. The trapezius can draw its origin on the occipital bone towards the scapula if it contracts while the scapula is fixed in place by other muscles. This will extend or hyperextend the head on the neck. Keeping the scapula fixed and contracting the portion of the muscle attached to the cervical vertebra will extend the neck. Using just one side of the muscle with the scapula fixed can, like the sternocleidomastoid tilt the head laterally. The innervation of this muscle while having components from the spinal accessory nerve (cranial nerve XI) also gets fibers that originate from the 2nd and 3rd cervical spinal nerves which join the spinal accessory nerve to form a continuous nerve loop in the neck.
