The infrahyoid musculature is frequently given little attention in the course of medical education. While medical students get to hear this term at some point during a lecture or come across this musculature at least once during a dissection course, physiotherapists, for example, do not learn about these very important muscles at all and have to get up to speed on this subject via expensive training courses. To prevent misunderstanding it should be mentioned that this group of muscles is also referred to as hyoid musculature in the literature because of its direct osseous connection to the hyoid (tongue bone). Medical students will be pleased to hear all these muscles share the same innervation, which truncates the learning process.
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Anatomy head and neck

Image : “Head anatomy semi-profile throat” by medicalgraphics. License: CC BY-SA 4.0


Definition

The infrahyoid (or hyoid) musculature describes a group of skeletal muscles which originate caudally and insert on the os hyoideum (tongue bone), which means that they are located below (inferior to) the osseous structures. Muscles located above (superior to) the os hyoideum are referred to as suprahyoid musculature, which will be the topic of a separate article.

Muscles of the Anterior Neck

Picture: “Muscles of the Anterior Neck” by Phil Schatz. License: CC BY 4.0

Classification: Which muscles belong to this group?

Infrahyoid muscles include five muscles: m. omohyoideus, m. sternohyoideus, m. sternothyroideus, m. thyrohyoideus and m. levator glandulae thyroideae, which is, however, inconstant in humans.

In evolutionary terms, they are assigned to the large ventral longitudinal muscle system. In medical literature the m. omohyoideus is considered as part of the immigrated trunk musculature, which has insertion on the shoulder girdle – due to its osseous origin.

Musculus omohyoideus

Via the venter inferior the m. omohyoideus originates from the upper edge of the scapula, the margo superior scapulae, that is next to the incisura scapulae, and the respective venter superior inserts on the lateral third of the lower edge of the corpus ossis hyoidei. Its primary function is to stretch the fascia of the neck’s ventral chain and in a broader sense it is also the vascular muscle of the vena jugularis interna, which is located below the m. omohyoideus.

Clinically, the v. jugularis interna is infiltrated with a central venous catheter (CVC). Moreover, this muscle pulls the hyoid in a dorsocaudal direction and thus is involved in the process of swallowing. The muscle is innervated by the ansa cerviclis profunda of the plexus cervicalis (C1-C3).

Special Features: A peculiarity of this muscle is its origin since for some humans the origin is the clavicula and not the scapula. This variation is called musculus cleidohyoideus; the function and innervation remain the same.

The middle neck fascia, also referred to as lamina praetrachealis, encloses both muscle bellies (the venter superior and inferior). At the site of the intermediate tendon, the lamina grows together with the vagina carotica, which is a firm sheath of connective tissue surrounding a vessel-nerve-strand. The structures of this strand are a. carotis communis, v. jugularis interna and n. vagus.

Musculus sternohyoideus

This muscle originates at the posterior surface of the manubrium sterni, the medial end of the clavicula and partly from the sternoclavicular joint (SCJ). It inserts at the lower edge of the os hyoideum. Muscular sternohyoideus is innervated via ansa cervicalis profunda of the plexus cervicalis (C1-C3) and  pulls the hyoid bone in a caudal direction and fixates it if the suprahyoid muscles tenses.

Special Features: This muscle can flatten at the origin and becomes narrower and thicker in the cranial direction.

Musculus sternothyroideus

The m. sternothyroideus is thicker than the m. sternohyoideus that is located more superficially. The m. sternothyroideus originates from the rear of the manubrium sterni and its insertion site is at the linea obliqua of the cartilago thyroidea. This means that it is located directly next to the thyroid capsule. If the m. sternothyroideus tightens, the thyroid capsule and the larynx are moved in a caudal direction. The innervation is the same as for all other infrahyoid muscle-s via the ansa cervicalis profunda of the plexus cervicalis (C1-C3).

Special Features:  It connects to the glandula thyroidea with loose connective tissue.

Musculus thyrohyoideus

This muscle is the continuation of the m. sternothyroideus and thus originates at its insertion, the linea obliqua of the cartilago thyroidea. The insertion of the m. thyrohyoideus is at the inner side of the lateral third of the corpus ossis hyoidei and at the lower edge of the medial surface of the cornu majus. This means that its insertion is lateral and distal to the m. omohyoideus. If the m. thyrohyoideus tenses, it closes the larynx by pulling the m. hyoid and the cartilago thyroidea close to each other. It is also innervated by the ansa cervicalis profunda of the plexus cervicalis (C1-C3).

Special Features: Morphologically, the m. thyrohyoideus represents a wide muscle plate covering and adhering to the membrana thyrohyoidea.

Musculus levator glandulae thyroideae

As already mentioned above, the m. levator glandulae thyroideae is not found in every human. It is a separated part of the m. thyrohyoideus. Its origin is at the corpus ossis hyoidei and its insertion site is at the isthmus glandulae thyroidea. Innervated by  ansa cervicalis profunda of the plexus cervicalis (C1-C3), it raises the thyroid gland.

Function and pathophysiology of infrahyoid musculature

Muscles That Move the Lower Jaw

Picture: “Muscles That Move the Lower Jaw” by Phil Schatz. License: CC BY 4.0

The infrahyoid muscles pull the os hyoideum and the larynx in a caudal direction. Besides the stabilization of the larynx via the sternothyroidal muscle loop, the infrahyoid muscles have another function, they influence the phonation- formation of speech. For example, If the thyroid cartilage is tilted against the cricoid cartilage due to changes in the muscle tone, the vocal chord tension also changes.

This leads to the formation of a hyperfunctional dysphonia, that represents itself as asynchronous oscillations of the vocal chord or even a unilateral halt of the vocal chord. Changes of the muscle tone cranially of the vocal chord plane influence the resonance chamber as well as the vocal sound. Furthermore, the supra- and infrahyoid musculature is a functional muscle loop with the os hyoideum as punctum fixum.

If the masticatory muscles of the jaw joint are stabilized, supra- and infrahyoid muscles act as flexors on the cervical spine and reduce its lordosis. This means that these structures also play an important role regarding to the statics of cervical spine.

Because of their connection to the hyoid, all the muscles described above have an influence on the hyoid, that results in the motion of osseous structures. If soft tissues are damaged (e.g. neuronal lesions) this can lead to a position and movement disorder of the os hyoideum, which can result in speech disorders. In case of such symptoms an examination of the supra and infrahyoid muscles is required.

Because of the direct connection to the jaw muscles and the cranium or the cranial base, it may also be necessary to examine the atlantooccipital joint. Treatment may be provided in the form of physiotherapy or speech therapy.

Ansa cervicalis profunda

Non-tunneled central venous catheter.

Picture: “Non-tunneled central venous catheter.” by Bruce Blausen. License: CC BY 3.0

Since the infrahyoid musculature is usually affected by secondary pathologies, largely as a result of neuronal lesions, it makes sense to take a closer look at the motor nerve branches innervating these muscles.

The ansa cervicalis profunda is a nerve loop of segments C1-C3 which is responsible for the motor innervation of the infrahyoid muscles. The sensory innervation takes place via other structures. The ansa cervicalis profunda is formed by two stems: the radix superior and radix inferior.

The motor fibers from segment C1 form the radix superior, which runs alongside the n. hypoglossus (cranial nerve XII) and then separates into the trigonum caroticum. There it fuses with the radix inferior, which proceeds diagonally above the v. jugularis interna, connection of it  to the infrahyoid muscle should already be familiar to the reader. The radix inferior contains fibers of vertebral segments C2 and C3.

Vena jugularis interna

The sinus sigmoideus fuses with the v. jugularis interna shortly before the foramen jugulare. Blood from the sinus transversus and sinus cavernosus is transported in this vein. Basically, the entire blood supply of the inner skull is drained via the v. jugularis interna. It is clinically important structure as central venous catheter (CVC) is inserted into this vein.

Review Questions

The answers can be found below the references.

1. Which segments innervate the infrayoid musculature?

  1. C1-C2
  2. C1-C3
  3. C1-C4
  4. C4-C5
  5. C5-C7

2. Which of the following infrahyoid muscles is not found in every human?

  1. omohyoideus
  2. sternohyoideus
  3. sternothyroideus
  4. thyrohyoideus
  5. levator glandulae thyroideae

3. The m. omohyoideus is a vascular muscle for which vessel?

  1. jugularis anterior
  2. jugularis externa
  3. jugularis interna
  4. subclavia
  5. vertebralis
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