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Sternocleidomastoid Muscle Anatomy

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Origin
  • Sternal Head: Manubrium of the sternum (superior and anterior aspect)
  • Clavicular Head: Medial quarter of the superior surface of the clavicle
Insertion
  • Mastoid process of the temporal bone and anterior portion of the superior nuchal line
Action
  • Unilateral contraction: Ipsilateral flexion and contralateral rotation
  • Bilateral contraction: Neck flexion; may contribute to neck extension depending on position; assists with forced inspiration by elevating the clavicle and manubrium of the sternum
Innervation
  • Accessory nerve (CN XI)
  • Branches of the cervical plexus (C2-C3)
Blood Supply
  • Occipital artery
  • Superior thyroid artery
  • Suprascapular artery
Last updated: Estimated Reading Time: 8 min

Location & Overview

The sternocleidomastoid muscle (often abbreviated as SCM) is a large, palpable neck muscle with an easily recognizable shape. It is one of the largest and most superficial cervical muscles and plays an important role in movements of the neck, such as flexion and rotation. It can also assist with inspiration.[1,2]

The sternocleidomastoid muscle has two heads, the clavicular head and the sternal head, which are distinguished by their origins. The sternal head arises from the sternum (breastbone), while the clavicular head arises from the clavicle (collarbone). The clavicular head is the larger of the two heads, and the sternal head is the smaller of the two heads. The clavicular and sternal heads are often fused along the muscle’s length. Occasionally, some sources break these sternocleidomastoid divisions down further into four portions: the sterno-mastoid, sterno-occipital, cleido-mastoid, and cleido-occipital.[1,2]

The sternocleidomastoid muscle is usually stronger and thicker in men than in women.[2] Approximately 65% of the sternocleidomastoid’s muscle fibers are fast-twitch muscle fibers. The remaining fibers are slow-twitch. This means the sternocleidomastoid is best at expressing a large amount of strength quite quickly and is less adept at endurance-based activities.[3,1]

sternocleidomastoid muscle anterior view
Here, we can see the sternocleidomastoid muscle from an anterior view.
sternocleidomastoid muscle lateral view
Here, we can see the sternocleidomastoid muscle from a lateral view.
sternocleidomastoid muscle posterolateral view
Here, we can see the sternocleidomastoid muscle from a posterolateral view.
Sternocleidomastoid highlighted among the other muscles
Here, we can see the sternocleidomastoid muscle highlighted in red. Superficial muscles such as the platysma muscle have been removed to reveal the sternocleidomastoid.

Origin & Insertion

The sternocleidomastoid muscle originates from the superior edge of the manubrium and from the medial quarter of the superior surface of the clavicle. The two heads of this muscle (sternal and clavicular) merge to form a single muscle belly that continues up the neck in a superior and lateral direction. The sternocleidomastoid then inserts onto the mastoid process of the temporal bone and the anterior portion of the superior nuchal line (located on the occipital bone of the skull).[1,2,4] These origins and insertions are displayed in the images below.

sternocleidomastoid origin on manubrium and clavicle
Here, we can see the sternocleidomastoid muscle’s origin points on the superior and anterior aspects of the manubrium of the sternum (sternal head) and the medial quarter of the superior surface of the clavicle (clavicular head).
sternocleidomastoid insertion on mastoid process and superior nuchal line
Here, we can see the insertion of the sternocleidomastoid muscle on the mastoid process of the temporal bone and the anterior portion of the superior nuchal line. The mastoid process can be easily palpated; it is a bony bump behind the ear.
sternum sections: manubrium, body, and xiphoid process
Here, we can see the different sections of the sternum. The manubrium of the sternum is an origin point of the sternocleidomastoid’s sternal head.
superior nuchal line
Here, you can see the superior nuchal line, which is part of the insertion point of the sternocleidomastoid muscle. The sternocleidomastoid muscle’s insertion is on the lateral and superior aspects of the superior nuchal line. It does not span the entirety of the superior nuchal line.

Action & Function

The sternocleidomastoid muscle can assist in head and neck rotation when one side of the muscle contracts (unilateral contraction). This rotation causes the face to turn away from the contracting muscle (contralateral rotation). Similarly, a unilateral contraction can also cause the neck to bend toward the contracting muscle (ipsilateral flexion).[1,4]

When both sternocleidomastoid muscles contract together, their action depends on how the cervical spine is stabilized. If the cervical spine is held relatively straight and rigid by other neck muscles, bilateral contraction of the sternocleidomastoids flexes the cervical spine and brings the head and neck forward. If the cervical spine is free to move, bilateral contraction can instead contribute to head extension and increased cervical lordosis.[1,4]

The sternocleidomastoid muscle can also assist with forced inspiration. When the head and neck are fixed, bilateral contraction of the muscle helps lift the sternum and clavicles, allowing greater expansion of the chest during inhalation.[1,5]

Image of a person demonstrating the movement of neck extension.
This image demonstrates neck extension, which involves bending the neck backward. The opposite of neck extension is neck flexion.
Image of a person demonstrating the movement of neck flexion.
This image demonstrates neck flexion, which involves bending the neck forward. The opposite of neck flexion is neck extension.
Image of a person demonstrating the movement of neck lateral flexion.
This image shows neck lateral flexion, which involves bending the neck to one side. This action is caused by the contraction of the sternocleidomastoid muscle. Specifically, it is called ipsilateral flexion because the muscle on the same side as the direction of neck bending contracts to bring the neck toward that side. The opposite of ipsilateral flexion is contralateral flexion, which occurs when the neck bends away from the side of the contracting muscle.
Image of a person demonstrating the movement of neck rotation.
This image demonstrates neck rotation, which involves turning the neck and head to the side. This type of rotation can also be referred to as contralateral rotation: the sternocleidomastoid’s contraction causes the head to turn away from the side of the contracting muscle. When the movement occurs on the opposite side to the contracting muscle, this is referred to as contralateral. The opposite of contralateral is ipsilateral.

Innervation

The sternocleidomastoid muscle receives motor innervation from the accessory nerve (cranial nerve XI). After supplying the sternocleidomastoid, the accessory nerve passes across the posterior triangle of the neck toward the trapezius. Branches from the cervical plexus also contribute to the sternocleidomastoid, mainly for proprioceptive function.[1]

The sternocleidomastoid is also innervated by branches of the cervical plexus, commonly described as contributions from C2-C3. The cervical plexus lies deep to the sternocleidomastoid muscle on the anterolateral aspect of the neck.[4]

Blood Supply

Blood is supplied to the sternocleidomastoid via the occipital artery and superior thyroid artery, which are branches of the external carotid artery. Venous drainage from the sternocleidomastoid region is generally via tributaries of the jugular venous system.[5,1]

The middle third of the sternocleidomastoid muscle receives blood from branches of the superior thyroid artery, branches of the external carotid artery, or branches from both sources. In one anatomical study, the superior thyroid artery supplied the middle third in approximately 42% of cases, the external carotid artery in 23%, and branches from both vessels in 27%. In the majority of cases, the lower third of the sternocleidomastoid received blood from a branch arising from the suprascapular artery (>80%).[6]

References

  1. [1] Bordoni B, Varacallo M. Anatomy, Head and Neck, Sternocleidomastoid Muscle. (Updated 2022 Apr 5). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532881/
  2. [2] Kennedy E, Albert M, Nicholson H. The fascicular anatomy and peak force capabilities of the sternocleidomastoid muscle. Surg Radiol Anat. 2017 Jun;39(6):629-645. doi: 10.1007/s00276-016-1768-9. Epub 2016 Nov 2. PMID: 27807639.
  3. [3] Meznaric M, Eržen I, Karen P, Cvetko E. Effect of ageing on the myosin heavy chain composition of the human sternocleidomastoid muscle. Ann Anat. 2018 Mar;216:95-99. doi: 10.1016/j.aanat.2017.12.001. Epub 2017 Dec 28. PMID: 29289708.
  4. [4] Moore KL, Agur AMR, Dalley AF. Clinically Oriented Anatomy. 8th ed. Philadelphia: Lippincott Williams & Wilkins; 2017.
  5. [5] Kohan EJ, Wirth GA. Anatomy of the neck. Clin Plast Surg. 2014 Jan;41(1):1-6. doi: 10.1016/j.cps.2013.09.016. PMID: 24295343.
  6. [6] Kierner AC, Aigner M, Zelenka I, Riedl G, Burian M. The blood supply of the sternocleidomastoid muscle and its clinical implications. Arch Surg. 1999 Feb;134(2):144-7. doi: 10.1001/archsurg.134.2.144. PMID: 10025452.

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