Infrahyoid Muscles: Anatomy and Function

• The anterior triangle of the neck, bordered by the mandible's lower edge and the sternocleidomastoid muscles, includes the neck's viscera.
• Beneath the investing layer of deep cervical fascia lie two groups of longitudinal muscles extending from the mandible to the sternum, segmentally supplied by the anterior rami of the upper three cervical nerves.
• The hyoid bone, at the C3 vertebral level, is a key landmark in the front and side of the neck, serving as attachment for muscles including suprahyoid and infrahyoid groups.

Suprahyoid Muscles

• There are four suprahyoid muscles: digastric, stylohyoid, mylohyoid, and geniohyoid.
• The mylohyoid muscles of each side combine to create the oral cavity floor, with the digastric and stylohyoid muscles being more superficial and the geniohyoid muscles lying deeper.

Digastric

• The digastric muscle has a posterior belly originating from the digastric notch on the mastoid process's medial surface.
• The posterior belly tapers to the intermediate tendon, held by a fibrous sling on the hyoid bone's lesser horn, with a synovial sheath for lubrication.
• The stylohyoid's bifurcated tendon embraces the digastric tendon but doesn't secure it.
• The digastric's anterior belly connects the intermediate tendon to the digastric fossa on the mandible's inner surface, lying on the mylohyoid's inferior surface.
• The posterior belly receives nerve supply from the facial nerve.
• The anterior belly receives nerve supply from the nerve to mylohyoid.
• The digastric muscle depresses and retracts the chin and aids the lateral pterygoid in mouth opening.

Stylohyoid

• The stylohyoid muscle originates from the styloid process near the skull base and runs along the digastric's upper border.
• The stylohyoid inserts by two slips into the base of the hyoid bone's greater horn, embracing the digastric tendon.
• Nerve supply to stylohyoid is from the facial nerve.
• Its action is to retract and elevate the hyoid bone during swallowing.

Mylohyoid

• The mylohyoid muscles unite to form the mouth floor.
• Each arises from the mylohyoid line on the mandible, extending back to the third molar tooth.
• The posterior quarter inserts into the hyoid bone's anterior surface while the front interdigitates in a midline raphe from chin to hyoid.
• Mylohyoid receives nerve supply from the nerve to mylohyoid, a branch of the inferior alveolar nerve from the trigeminal nerve's mandibular division.
• It forms a mobile floor of the mouth, supporting the tongue; contraction elevates the tongue and hyoid during swallowing or tongue protrusion.

Geniohyoid

• The geniohyoid is a slender muscle from the mandible's inferior mental spine to the hyoid bone's upper border.
• It lies between the mylohyoids and genioglossus in the mouth floor.
• It receives nerve supply from the hypoglossal nerve, consisting of C1 nerve fibers.
• It protracts and elevates the hyoid bone during swallowing, or it depresses the mandible if the hyoid is fixed.

Infrahyoid Muscles

• The infrahyoid muscles are sternohyoid, omohyoid, thyrohyoid, and sternothyroid, often called "strap muscles".
• These muscles are in the body wall musculature plane and the rectus abdominis.

Sternohyoid

• The sternohyoid is a flat muscle from the hyoid bone's lower border to the sternoclavicular joint and adjoining parts of the manubrium and clavicle.
• The muscles lie edge to edge at the hyoid bone but diverge below, with the Adam's apple visible between them.
• Nerve supply comes from the ansa cervicalis (C1, 2, 3).

Omohyoid

• The omohyoid runs edge to edge with the sternohyoid at the hyoid bone.
• It passes beneath the sternocleidomastoid over the carotid sheath.
• The intermediate tendon slides on the internal jugular vein, serving as a surgical landmark.
• The inferior belly runs horizontally above the clavicle, attaching to the transverse scapular ligament and scapula.
• The intermediate tendon is secured to the clavicle by a fascial sling.
• Nerve supply is from the ansa cervicalis (C1, 2, 3).

Thyrohyoid

• The broader, shorter thyrohyoid muscle lies under the sternohyoid and omohyoid.
• It arises from the hyoid bone's greater horn, inserting into the thyroid cartilage's oblique line with the sternothyroid.
• Nerve supply comes from the hypoglossal nerve, with fibers from C1.

Sternothyroid

• The sternothyroid lies deep to the sternohyoid, attaching lower on the manubrium, extending to the first costal cartilage, and inserts on the thyroid cartilage's oblique line.
• Nerve supply is from the ansa cervicalis (C2, 3).

Actions of Infrahyoid Muscles

• They all depress the larynx.
• The sternothyroid directly acts on the thyroid cartilage.
• Others act indirectly via the hyoid bone.
• Larynx depression affects voice quality.
• These muscles also oppose larynx elevators.
• They prevent hyoid bone ascent when the digastric muscle is active.
• The contracting thyrohyoid approximates the thyroid cartilage and hyoid bone.
• When the head rotates, the opposite omohyoid straightens, pulling the hyoid bone downward.

Explores the anatomy and function of the infrahyoid muscles, including their innervation and attachments. Covers the role of these muscles in controlling the position of the larynx and hyoid bone. Includes information on the anterior belly, stylohyoid, mylohyoid, and geniohyoid muscles.