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6. What muscles are part of the superficial layer of infrahyoid muscles? Deep layer? What are the actions of these muscles? What innervates these muscles?
6. What muscles are part of the superficial layer of infrahyoid muscles? Deep layer? What are the actions of these muscles? What innervates these muscles?
Infrahyoid (strap) muscles largely occupy the area inferior to the hyoid, superior to the sternum, and medial to the carotid sheaths (excepting the inferior belly of omohyoid m.). To help in locating individual infrahyoid mm., understand they are named for their attachments. The infrahyoids may be classified into two groups:
Superficial (sternohyoid mm. & omohyoid mm.)
Deep (sternothyroid mm. & thyrohyoid mm.)
All infrahyoid mm. (excepting the sternothyroid mm.) attach to the hyoid bone, and therefore are important in both depression and stabilization of the hyoid. Any infrahyoid muscle attaching to the thyroid cartilage (sternothyroid mm. & thyrohyoid mm.) will act upon the entire larynx.
Sternohyoid mm. basics:
Omohyoid mm. basics:
The omohyoid m. consists of two bellies (superior & inferior) that are connected by an intermediate tendon. The superior belly connects the intermediate tendon to the inferior margin of the body of the hyoid (lateral to the attachment of the sternohyoid m.). The inferior belly connects the intermediate tendon to the upper border of the scapula, medial to the scapular notch. The intermediate tendon is anchored to the medial clavicle via a fascial sling.
Sternothyroid mm. basics:
Thyrohyoid mm. basics:
The ansa cervicalis is a delicate loop of nerve consisting of two limbs: a superior limb (C1) and an inferior limb (C2, C3). The superior limb is proximally associated with the hypoglossal n. (CN XII), as fibers of the ventral primary ramus (VPR) of C1 ‘hitchhike’ along the hypoglossal n., before rejoining the superior limb. Keep in mind that nerves are collections of axons in the peripheral nervous system, and that these axons may move among different nerves on their way to or from their target tissues. The ansa cervicalis innervates nearly all infrahyoid mm. (excepting the thyrohyoid m.). The thyrohyoid m. is innervated by fibers from spinal nerve C1 that also hitchhike along the hypoglossal n., and exit distal to the superior limb of ansa cervicalis. The limbs of the ansa cervicalis lie upon (and sometimes within) the carotid sheath.
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