Dominic Stone
The infrahyoid muscles, also known as strap muscles, are a group of four pairs of muscles in the anterior (frontal) part of the neck. They are located underneath the hyoid bone and are involved in movements of the hyoid bone and thyroid cartilage during vocalization, swallowing and mastication. The four infrahyoid muscles are the sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscles. In this article, we will focus on the innervation of these important muscles.
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What You'll Learn
The omohyoid muscle
The inferior belly of the omohyoid originates from the superior border of the scapula, near the scapular notch. It sometimes also arises from the superior transverse scapular ligament. The muscle is directed anteriorly and superiorly from its origin, extending across the inferior portion of the neck. It passes posterior to the sternocleidomastoid muscle to insert at the intermediate tendon. The inferior belly of the omohyoid divides the posterior triangle of the neck into an occipital triangle (above) and a subclavian triangle (below).
The superior belly arises from the intermediate tendon, near the internal jugular vein. Here, the muscle changes direction and courses almost vertically to insert at the lower border of the body of the hyoid bone, lateral to the insertion of the sternohyoid muscle. The superior belly divides the anterior triangle into a carotid triangle (above) and a muscular triangle (below). It serves as the most lateral member of the infrahyoid muscles, located lateral to both the sternothyroid and thyrohyoid muscles.
The omohyoid is innervated by the ansa cervicalis of the cervical plexus. The superior belly is innervated by the superior root of ansa cervicalis (C1), while the inferior belly receives fibres from spinal nerves C1, C2 and C3. The function of the omohyoid muscle is to depress the hyoid bone and larynx, and to re-establish breathing following the act of swallowing.
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The sternohyoid muscle
The primary function of the sternohyoid muscle is to depress the hyoid bone. This action helps to reopen the laryngeal inlet after it has been elevated during swallowing, thus reinstating the breathing process. The muscle also aids in speech, particularly in modulating speech volume. Additionally, the sternohyoid muscle contributes to the protection of the trachea, oesophagus, blood vessels, and thyroid gland.
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The sternothyroid muscle
The sternothyroid is one of the four infrahyoid muscles, located underneath the hyoid bone in the neck. It is a paired strap muscle, comparatively wider and shorter than the other infrahyoid muscles. The sternothyroid muscle originates from the posterior edge of the costal cartilage of the first rib, as well as the posterior surface of the manubrium of the sternum. At its origin, the muscle is in contact with the contralateral sternothyroid. The vertically oriented fibres then diverge as they travel superiorly to insert on the oblique line of the thyroid cartilage.
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The thyrohyoid muscle
The thyrohyoid has two primary functions. Firstly, it depresses the hyoid bone in conjunction with the other infrahyoid muscles. Secondly, when the hyoid bone is stabilized by the suprahyoid muscles, the thyrohyoid elevates the larynx. This feature is important for vocalists who try to hit high notes. By controlling the position and shape of the larynx, it aids in making sound.
Unlike the other infrahyoid muscles, the thyrohyoid is not innervated by the ansa cervicalis. Instead, nerve fibres from the anterior ramus of the first cervical spinal nerve (C1) reach the muscle via the hypoglossal nerve (CN XII). This nerve is often referred to simply as the nerve to thyrohyoid.
The thyrohyoid has several close relationships with surrounding structures in the anterior neck. It is crossed anteriorly by the oblique fibres of sternohyoid and the superior belly of omohyoid. The superior thyroid artery and vein are laterally related to the muscle. The thyrohyoid is provided with arterial blood by branches of the superior thyroid artery and of the lingual artery.
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Innervation and functions
The infrahyoid muscles are a group of four paired muscles located in the anterior part of the neck, specifically underneath the hyoid bone. They are also referred to as "strap muscles" due to their long and flat shape. The four infrahyoid muscles are:
- Sternohyoid
- Sternothyroid
- Thyrohyoid
- Omohyoid
The innervation and functions of the infrahyoid muscles are as follows:
Sternohyoid
The sternohyoid muscle is the most superficial of all the infrahyoid muscles. It is innervated by the ansa cervicalis (C1-C3), arising from the cervical plexus. It originates from the posterior surface of the manubrium of the sternum and the posterior surface of the medial end of the clavicle, and inserts on the inferior border of the body of the hyoid bone. The sternohyoid functions to depress the hyoid bone from an elevated position.
Sternothyroid
The sternothyroid muscle is innervated by the ansa cervicalis (C1-C3), arising from the cervical plexus. It functions to depress the larynx.
Thyrohyoid
Unlike the other infrahyoid muscles, the thyrohyoid muscle is innervated by nerve fibres from the anterior ramus of spinal nerve C1, which reaches the muscle via the hypoglossal nerve. The thyrohyoid has two functions. Firstly, it can depress the hyoid bone in conjunction with the other infrahyoid muscles. Secondly, it can elevate the larynx if the hyoid bone is stabilized.
Omohyoid
The omohyoid muscle is innervated by the superior ramus of the ansa cervicalis and the hypoglossal nerve. It consists of two bellies (superior and inferior) that are connected by a muscular tendon. The superior belly arises from the hyoid bone, while the inferior belly arises from the scapula. The omohyoid functions to depress the hyoid bone.
Overall, the infrahyoid muscles, together with the suprahyoid muscle group, are important for several functions related to speech, swallowing, and the movement of the larynx, by controlling the positioning of the hyoid bone.
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Frequently asked questions
The infrahyoid muscles are a group of four pairs of muscles in the anterior (frontal) part of the neck. They are also known as "strap muscles".
The infrahyoid muscles are located underneath the hyoid bone in the neck. They connect the hyoid bone to the sternum, larynx and scapula.
The four infrahyoid muscles are the sternohyoid, sternothyroid, thyrohyoid and omohyoid muscles.
The sternohyoid, sternothyroid, and omohyoid muscles are innervated by the ansa cervicalis (C1-C3), arising from the cervical plexus.
Unlike the other infrahyoid muscles, the thyrohyoid muscle is innervated by nerve fibres from the anterior ramus of spinal nerve C1, which reaches the muscle via the hypoglossal nerve.
