Neck muscles: groups, triangles of the neck and what each muscle does

The muscles of the front of the neck are divided by the hyoid bone: the suprahyoid muscles lie above it and the infrahyoid muscles below it. Sternocleidomastoid, the main muscle that bends and turns the head, runs across the side of the neck, and the three scalene muscles (anterior, middle and posterior) lie deeper at the side. (OpenStax Anatomy and Physiology 2e, 11.3: Axial Muscles of the Head, Neck, and Back.)

Deeper still, in front of the vertebral column, are the prevertebral muscles: longus capitis, longus colli and the rectus capitis anterior and lateralis. The neck fascia is described in three layers: the investing layer around trapezius and sternocleidomastoid, the visceral (pretracheal) layer around the thyroid, infrahyoid muscles and the air and food passages, and the prevertebral layer over the prevertebral muscles, the scalenes and the phrenic nerve. (O'Rahilly R et al. Basic Human Anatomy, Chapter 50: The Neck (Dartmouth Medical School).)

At the back of the neck, under the skull, lie the four small suboccipital muscles. They are covered by semispinalis capitis, one of the deep back muscles, which forms the roof of the suboccipital triangle. (TeachMeAnatomy: The Suboccipital Muscles.)

Trapezius, splenius, erector spinae and the deep back muscles, layer by layer. Back muscles.

Platysma is a thin, four-sided sheet of muscle in the skin of the front of the neck. It arises from the skin over the deltoid and pectoralis major and inserts into the lower border of the mandible and the skin around the mouth. It is supplied by the cervical branch of the facial nerve (cranial nerve VII). (O'Rahilly R et al. Basic Human Anatomy, Chapter 50: The Neck (Dartmouth Medical School).)

Sternocleidomastoid arises by two heads, from the front of the manubrium of the sternum and from the upper surface of the medial third of the clavicle. It inserts on the lateral surface of the mastoid process and the lateral part of the superior nuchal line. It is supplied mainly by the accessory nerve (cranial nerve XI), with contributions from cervical nerves 2 to 4. (O'Rahilly R et al. Basic Human Anatomy, Chapter 50: The Neck (Dartmouth Medical School).)

One sternocleidomastoid bends the neck to its own side and turns the head to the opposite side. Acting together, the two muscles bend the lower neck forward while tilting the head back at the upper cervical joints. The muscle lies superficially, so it is easy to see and feel, and the carotid pulse can be felt along its front edge. Beneath it run the common carotid artery, the internal jugular vein, the vagus nerve and the ansa cervicalis. Spasm of the muscle causes wryneck (torticollis), with the head tilted towards the affected side and turned away from it. (Kenhub: Sternocleidomastoid muscle.)

Sternocleidomastoid divides each side of the neck into an anterior triangle in front of it and a posterior triangle behind it. (OpenStax Anatomy and Physiology 2e, 11.3: Axial Muscles of the Head, Neck, and Back.)

The anterior triangle is bounded by the lower border of the mandible above, the anterior border of sternocleidomastoid laterally and the midline of the neck medially. Its roof is the investing fascia. It is divided into four smaller triangles: submental, submandibular, carotid and muscular. (TeachMeAnatomy: The Anterior Triangle of the Neck.)

The posterior triangle is bounded by the posterior border of sternocleidomastoid in front, the anterior border of trapezius behind and the middle third of the clavicle below. Its roof is the investing fascia and its floor is the prevertebral fascia over splenius capitis, levator scapulae and the three scalene muscles. The inferior belly of omohyoid crosses it, dividing it into an occipital triangle above and a subclavian triangle below. (TeachMeAnatomy: The Posterior Triangle of the Neck.)

The posterior triangle contains the accessory nerve (cranial nerve XI), which runs quite superficially, branches of the cervical plexus including the phrenic nerve (C3 to C5), the external jugular vein, the last part of the subclavian artery, the trunks of the brachial plexus, and the transverse cervical and suprascapular vessels. (TeachMeAnatomy: The Posterior Triangle of the Neck.)

The submental triangle lies under the chin. It is bounded by the hyoid bone below, the midline medially and the anterior belly of digastric laterally. Its floor is the mylohyoid muscle and it contains the submental lymph nodes. The submandibular triangle lies between the body of the mandible and the anterior and posterior bellies of digastric and contains the submandibular gland, lymph nodes and the facial artery and vein. (TeachMeAnatomy: The Anterior Triangle of the Neck.)

The carotid triangle is bounded by the posterior belly of digastric above, the anterior border of sternocleidomastoid laterally and the superior belly of omohyoid below. It contains the common carotid artery, which divides into the internal and external carotid arteries here, the internal jugular vein and the hypoglossal and vagus nerves. The muscular triangle lies below the hyoid between the midline, the superior belly of omohyoid and the lower sternocleidomastoid; it contains the infrahyoid muscles, the pharynx and the thyroid and parathyroid glands. (TeachMeAnatomy: The Anterior Triangle of the Neck.)

The carotid sheath is a sleeve of deep cervical fascia on each side of the neck. It contains the common and internal carotid arteries, the internal jugular vein and the vagus nerve. (O'Rahilly R et al. Basic Human Anatomy, Chapter 50: The Neck (Dartmouth Medical School).)

The suprahyoid muscles raise the hyoid bone, the floor of the mouth and the larynx during swallowing (deglutition). Digastric has anterior and posterior bellies; stylohyoid moves the hyoid backwards and lifts the larynx; mylohyoid lifts the hyoid and helps press the tongue up against the roof of the mouth; geniohyoid pulls the hyoid forwards and can also lower (depress) the mandible. (OpenStax Anatomy and Physiology 2e, 11.3: Axial Muscles of the Head, Neck, and Back.)

Digastric has two bellies joined by an intermediate tendon, held to the hyoid by a fibrous sling. The anterior belly arises from the digastric fossa of the mandible and is supplied by a branch of the mandibular nerve (from the trigeminal nerve, cranial nerve V). The posterior belly arises from the mastoid process and is supplied by the facial nerve (cranial nerve VII). Digastric lowers the mandible and raises the hyoid. (TeachMeAnatomy: The Suprahyoid Muscles.)

Stylohyoid runs from the styloid process of the temporal bone to the side of the hyoid and is supplied by the facial nerve. Mylohyoid runs from the mylohyoid line of the mandible to the hyoid and is supplied by the mandibular branch of the trigeminal nerve. Geniohyoid runs from the inferior mental spine of the mandible to the hyoid and is supplied by fibres from C1 that travel with the hypoglossal nerve. (TeachMeAnatomy: The Suprahyoid Muscles.)

The four infrahyoid muscles, sternohyoid, omohyoid, sternothyroid and thyrohyoid, are flat strap-like muscles. Sternohyoid and omohyoid lie in a superficial plane, sternothyroid and thyrohyoid in a deeper one. Together they anchor the hyoid bone and control the position of the larynx. (O'Rahilly R et al. Basic Human Anatomy, Chapter 50: The Neck (Dartmouth Medical School).)

Sternohyoid runs from the manubrium and the back of the medial end of the clavicle to the body of the hyoid. Sternothyroid runs from the back of the manubrium and the first costal cartilage to the oblique line of the thyroid cartilage, and pulls the larynx down. Thyrohyoid runs from the oblique line of the thyroid cartilage up to the hyoid; it can raise the larynx when the hyoid is fixed. Omohyoid has an inferior belly from the upper border of the scapula and a superior belly to the hyoid, joined by an intermediate tendon connected to the carotid sheath. (Kenhub: Infrahyoid muscles.)

Sternohyoid, omohyoid and sternothyroid are supplied by the anterior rami of C1 to C3 through the ansa cervicalis, a nerve loop in the front of the neck. Thyrohyoid is the exception: it is supplied by fibres from C1 carried within the hypoglossal nerve. (TeachMeAnatomy: The Infrahyoid Muscles.)

There are three scalene muscles on each side. Anterior scalene arises from the anterior tubercles of the transverse processes of C3 to C6 and attaches to the scalene tubercle of the first rib. Middle scalene, the largest and longest, arises from the posterior tubercles of C2 to C7 and attaches to the upper surface of the first rib. Posterior scalene, the smallest and deepest, arises from the posterior tubercles of C5 to C7 and attaches to the second rib. All are supplied by anterior rami of cervical nerves. (TeachMeAnatomy: The Scalene Muscles.)

The scalenes lift the first and second ribs and bend the neck to the same side. They act as accessory muscles of breathing; their visible use in a person at rest is a sign of respiratory distress. The brachial plexus and the subclavian artery pass between the anterior and middle scalenes, while the subclavian vein crosses in front of the anterior scalene and the phrenic nerve runs down its front surface. (TeachMeAnatomy: The Scalene Muscles.)

Not all textbooks agree on whether the scalenes work in quiet breathing. One classic text states that the scalenes may act as muscles of inspiration even during quiet breathing. (O'Rahilly R et al. Basic Human Anatomy, Chapter 50: The Neck (Dartmouth Medical School).)

OpenStax describes the scalenes as working together to flex, laterally flex and rotate the head, and notes that they contribute to deep inhalation. Of the three, the middle scalene is the longest and the posterior scalene the smallest. (OpenStax Anatomy and Physiology 2e, 11.3: Axial Muscles of the Head, Neck, and Back.)

Longus colli lies directly on the front of the cervical and upper thoracic vertebrae and has three parts. The superior oblique part runs from the anterior tubercles of C3 to C5 to the anterior tubercle of the atlas (C1). The vertical intermediate part runs from the bodies of C5 to T3 to the bodies of C2 to C4. The inferior oblique part runs from the bodies of T1 to T3 to the anterior tubercles of C5 and C6. It is supplied by the anterior rami of C2 to C6; both muscles together flex the neck. (Kenhub: Prevertebral muscles.)

Longus capitis arises from the anterior tubercles of C3 to C6 and inserts on the basilar part of the occipital bone; it flexes the head and is supplied by C1 to C3. Rectus capitis anterior runs from the lateral mass of the atlas to the base of the occipital bone and flexes the head. Rectus capitis lateralis runs from the transverse process of the atlas to the jugular process of the occipital bone, bends the head to the side and helps stabilise the atlanto-occipital joint. (Kenhub: Prevertebral muscles.)

Longus colli is active in talking, coughing and swallowing. (O'Rahilly R et al. Basic Human Anatomy, Chapter 50: The Neck (Dartmouth Medical School).)

The suboccipital muscles are four small muscles under the back of the skull, all supplied by the suboccipital nerve (the posterior ramus of C1). Rectus capitis posterior major runs from the spinous process of C2 to the lateral part of the inferior nuchal line. Rectus capitis posterior minor runs from the posterior tubercle of C1 to the medial part of the inferior nuchal line. Both extend the head, and the major also rotates it. (TeachMeAnatomy: The Suboccipital Muscles.)

Obliquus capitis inferior runs from the spinous process of C2 to the transverse process of C1; it is the only suboccipital muscle with no attachment to the skull. It extends and rotates the head. Obliquus capitis superior runs from the transverse process of C1 to the occipital bone between the superior and inferior nuchal lines and extends the head. (TeachMeAnatomy: The Suboccipital Muscles.)

Three of these muscles outline the suboccipital triangle: rectus capitis posterior major above and medially, obliquus capitis superior above and laterally, and obliquus capitis inferior below. Its floor is the posterior atlanto-occipital membrane and the posterior arch of C1, and its roof is semispinalis capitis. It contains the vertebral artery, the suboccipital venous plexus and the suboccipital nerve. (TeachMeAnatomy: The Suboccipital Muscles.)

Digastric

A two-bellied muscle above the hyoid bone linking the mandible and the mastoid region.

Geniohyoid

A narrow muscle above the mylohyoid in the floor of the mouth.

Inferior oblique part of longus colli

The lower oblique fibres of longus colli on the front of the cervical spine.

Longus capitis

A prevertebral muscle in front of the upper cervical spine.

Mylohyoid

A flat muscle forming the muscular floor of the mouth.

Obliquus capitis inferior

A suboccipital muscle below the atlas.

Obliquus capitis superior

A small suboccipital muscle forming the superolateral border of the suboccipital triangle.

Omohyoid

A thin infrahyoid strap muscle with two bellies joined by an intermediate tendon.

Rectus capitis anterior

A small deep prevertebral muscle at the base of the skull.

Rectus capitis lateralis

A small deep muscle lateral to the atlanto-occipital joint.

Rectus capitis posterior major

A suboccipital muscle forming the superomedial border of the suboccipital triangle.

Rectus capitis posterior minor

A small suboccipital muscle lying medial to rectus capitis posterior major.

Scalenus anterior

A deep neck muscle and an important landmark in the root of the neck.

Scalenus medius

The largest and longest of the scalene muscles.

Scalenus posterior

The smallest and deepest of the scalene muscles.

Sternocleidomastoid

A thick superficial neck muscle forming the main landmark dividing the anterior and posterior triangles.

Sternohyoid

A thin infrahyoid strap muscle at the front of the neck.

Sternothyroid

An infrahyoid strap muscle lying deep to sternohyoid.

Stylohyoid

A slender suprahyoid muscle running from the styloid process to the hyoid bone.

Superior oblique part of longus colli

The upper oblique part of longus colli, a prevertebral neck muscle.

Thyrohyoid

A short infrahyoid muscle continuing upward from sternothyroid.

Vertical intermediate part of longus colli

The central vertical part of longus colli, a prevertebral neck muscle.