- a.k.a. Infrahyoid Muscles
- Consist of Sternohyoid, Omohyoid, Sternothyroid, Thyrohyoid
| Muscle | Origin | Insertion | Actions | Nerve Supply |
|---|---|---|---|---|
| Omohyoid (superior and inferior bellies connected by a tendon) | Superior border of the scapula | Body of the hyoid | Depresses the hyoid | Ansa cervicalis |
| Sternohyoid | Manubrium | Body of the hyoid | Depresses the hyoid | |
| Sternothyroid | Manubrium | Oblique line of the thyroid cartilage | Depresses the larynx | |
| Thyrohyoid | Oblique line of the thyroid cartilage | Greater cornu (horn) of the hyoid | Depresses the hyoid | C1 (ventral ramus, which follows the hypoglossal n.) |
Sternohyoid
- Attachments:
- Back of upper part of Manubrium and adjoining Sternoclavicular Joint and Clavicle
- Lower border of Body of Hyoid
- Lie edge-to-edge at Hyoid bone and diverge from each other below
- Nerve supply:
- Branch from Ansa Cervicalis
- Enters lower part of muscle
Omohyoid
- Attachments:
- Lateral part of inferior border of Hyoid
- Complex course as detailed below
- Upper border of Scapula and Transverse Scapular Ligament
- Course:
- Lies edge-to-edge with Sternohyoid at its attachment
- Diverges away from Sternohyoid as it descends
- Passes deep to Sternocleidomastoid
- Lie over Carotid Sheath
- Muscle fibres replaced by flat tendon where it lies over the Internal Jugular Vein
- Inferior Belly runs almost horizontally above the Clavicle
- Intermediate Tendon and Supraclavicular portion of muscle bound down close to Clavicle by fascial sling derived from deep layer of the Investing Layer of the Deep Cervical Fascia
- Nerve supply:
- Superior Belly: Superior Root of the Ansa Cervicalis
- Inferior Belly: Ansa Cervicalis
Thyrohyoid
- Lies under cover of upper ends of Sternohyoid and Omohyoid
- Attachments:
- Greater Horn of Hyoid
- Oblique Line of Thyroid Cartilage (alongside Sternothyroid)
- Nerve supply:
- Branch of Hypoglossal Nerve – but fibres are all ‘hitch-hiking’ from C1
Sternothyroid
- Lies deep to Sternohyoid
- Attachments:
- Posterior surface of Manubrium (lower down than Sternohyoid) and adjacent 1st Costal Cartilage
- Oblique Line of Thyroid Cartilage
- Nerve supply:
- Ansa Cervicalis – gives a branch to lower part of muscle
Actions of Infrahyoid Muscles
- Depress Larynx
- Sternothyroid acts directly on Thyroid Cartilage
- Others act indirectly via Hyoid
- Increases volume of resonating chambers during phonation and affects quality of voice
- Oppose Larynx elevators
- I.e. Mylohyoid, Palatopharyngeus, Stylopharyngeus, Salpingopharyngeus
- Enables them to act progressively and gradually
- Prevent ascent of Hyoid Bone when Digastric and Geniohyoid lower Mandible