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Sternocleidomastoid muscle
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<h2 id="etymology-and-location">Etymology and location</h2> <p>It is given the name <i>sternocleidomastoid</i> because it originates at the <a href="/facts/Manubrium/qIPMpyIz">manubrium</a> of the <a href="/facts/Human_sternum/qIPMpyIz">sternum</a> (<i>sterno-</i>) and the <a href="/facts/Clavicle/YWNjLv6s">clavicle</a> (<i>cleido-</i>) and has an <a href="/facts/Insertion_(anatomy)/jTk8Hjay">insertion</a> at the <a href="/facts/Mastoid_process/1bB4JRGK">mastoid process</a> of the <a href="/facts/Temporal_bone/H6dozthp">temporal bone</a> of the <a href="/facts/Human_skull/A9Wp63Np">skull</a>.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p> <h2 id="structure">Structure</h2> <p>The sternocleidomastoid muscle originates from two locations: the <a href="/facts/Manubrium/qIPMpyIz">manubrium</a> of the <a href="/facts/Human_sternum/qIPMpyIz">sternum</a> and the <a href="/facts/Clavicle/YWNjLv6s">clavicle</a>, hence it is said to have two heads: sternal head and clavicular head.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> It travels obliquely across the side of the neck and inserts at the <a href="/facts/Mastoid_process/1bB4JRGK">mastoid process</a> of the <a href="/facts/Temporal_bone/H6dozthp">temporal bone</a> of the <a href="/facts/Human_skull/A9Wp63Np">skull</a> by a thin <a href="/facts/Aponeurosis/Uhq7UcN4">aponeurosis</a>.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> The sternocleidomastoid is thick and narrow at its center, and broader and thinner at either end. </p><p>The sternal head is a round <a href="/facts/Muscle_fascicle/9m3dVs38">fasciculus</a>, tendinous in front, fleshy behind, arising from the upper part of the front of the <a href="/facts/Manubrium_sterni/qIPMpyIz">manubrium sterni</a>. It travels superiorly, laterally, and posteriorly. </p><p>The clavicular head is composed of fleshy and <a href="/facts/Aponeurotic/Uhq7UcN4">aponeurotic</a> fibers, arises from the upper, frontal surface of the medial third of the <a href="/facts/Clavicle/YWNjLv6s">clavicle</a>; it is directed almost vertically upward. </p> <p>The two heads are separated from one another at their origins by a triangular interval (<i>lesser supraclavicular fossa</i>) but gradually blend, below the middle of the neck, into a thick, rounded muscle which is inserted, by a strong tendon, into the lateral surface of the <a href="/facts/Mastoid_process/1bB4JRGK">mastoid process</a>, from its apex to its superior border, and by a thin <a href="/facts/Aponeurosis/Uhq7UcN4">aponeurosis</a> into the lateral half of the <a href="/facts/Superior_nuchal_line/K1BnLeSj">superior nuchal line</a> of the <a href="/facts/Occipital_bone/zsgqpNIa">occipital bone</a>. </p> <h3>Nerve supply</h3> <p>The sternocleidomastoid is innervated by <a href="/facts/Accessory_nerve/o1ytrt6V">accessory nerve</a> of the same side.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> It supplies only motor fibres. The cervical plexus supplies sensation, including <a href="/facts/Proprioception/Ik79bA2x">proprioception</a>, from the <a href="/facts/Ventral_ramus/cFDEvFyy">ventral primary rami</a> of <a href="/facts/Cervical_spinal_nerve_2/bbyvbsBT">C2</a> and <a href="/facts/Cervical_spinal_nerve_3/x9Ehrjdt">C3</a>.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h3>Variation</h3> <p>The clavicular origin of the sternocleidomastoid varies greatly: in some cases the clavicular head may be as narrow as the sternal; in others it may be as much as 7.5 millimetres (0.30 in) in breadth. </p><p>When the clavicular origin is broad, it is occasionally subdivided into several slips, separated by narrow intervals. More rarely, the adjoining margins of the sternocleidomastoid and <a href="/facts/Trapezius/67PxLb8e">trapezius</a> are in contact. This would leave no <a href="/facts/Posterior_triangle/hPUjTFla">posterior triangle</a>. </p><p>The supraclavicularis muscle arises from the manubrium behind the sternocleidomastoid and passes behind the sternocleidomastoid to the upper surface of the clavicle. </p> <h2 id="function">Function</h2> <p>The function of this muscle is to rotate the head to the opposite side or obliquely rotate the head.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> It also flexes the neck.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> When both sides of the muscle act together, it <a href="/facts/Anatomical_terms_of_motion/KmBVfcPk">flexes</a> the neck and extends the head. When one side acts alone, it causes the head to rotate to the opposite side and flexes laterally to the same side (ipsilaterally). </p><p>It also acts as an <a href="/facts/Muscles_of_respiration/SQ5fy9hy">accessory muscle of respiration</a>, along with the <a href="/facts/Scalene_muscles/gaKFtIWy">scalene muscles</a> of the neck. </p> <h3>Contraction</h3> <p>The signaling process to contract or relax the sternocleidomastoid begins in Cranial Nerve XI, the <a href="/facts/Accessory_nerve/o1ytrt6V">accessory nerve</a>. The accessory nerve nucleus is in the <a href="/facts/Anterior_grey_column/pddm07pl">anterior horn of the spinal cord</a> around C1-C3, where lower motor neuron fibers mark its origin. The fibers from the accessory nerve nucleus travel upward to enter the cranium via the foramen magnum. The internal carotid artery to reach both the sternocleidomastoid muscles and the <a href="/facts/Trapezius/67PxLb8e">trapezius</a>. After a signal reaches the accessory nerve nucleus in the anterior horn of the spinal cord, the signal is conveyed to <a href="/facts/Motor_endplate/Rkcqwqht">motor endplates</a> on the muscle fibers located at the clavicle. <a href="/facts/Acetylcholine/v1d3wRUd">Acetylcholine</a> (ACH) is released from vesicles and is sent over the synaptic cleft to receptors on the postsynaptic bulb. The ACH causes the resting potential to increase above -55mV, thus initiating an action potential which travels along the muscle fiber. Along the muscle fibers are t-tubule openings which facilitate the spread of the action potential into the muscle fibers. The t-tubule meets with the <a href="/facts/Sarcoplasmic_reticulum/DVbLA7GE">sarcoplasmic reticulum</a> at locations throughout the muscle fiber, at these locations the sarcoplasmic reticulum releases calcium ions that results in the movement of <a href="/facts/Troponin/gQdzvCCb">troponin</a> and <a href="/facts/Tropomyosin/pJwDRMXQ">tropomyosin</a> on thin filaments. The movement of troponin and tropomyosin is key in facilitating the myosin head to move along the thin filament, resulting in a contraction of the sternocleidomastoid muscle.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h3>Anatomical landmark</h3> <p>The sternocleidomastoid is within the <a href="/facts/Deep_fascia/aeFVf4Ed">deep fascia</a> of the neck, along with the <a href="/facts/Trapezius_muscle/67PxLb8e">trapezius muscle</a>, with which it shares its nerve supply (the <a href="/facts/Accessory_nerve/o1ytrt6V">accessory nerve</a>). It is thick and thus serves as a primary <a href="/facts/Anatomical_landmark/n3IASQlr">landmark</a> of the neck, as it divides the neck into anterior and posterior cervical triangles (in front and behind the muscle, respectively) which helps define the location of structures, such as the lymph nodes for the head and neck.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p><p>Many important structures relate to the sternocleidomastoid, including the <a href="/facts/Common_carotid_artery/lndR9CgD">common carotid artery</a>, accessory nerve, and <a href="/facts/Brachial_plexus/3kmp3Mwj">brachial plexus</a>. </p> <h2 id="clinical-significance">Clinical significance</h2> <p>Examination of the sternocleidomastoid muscle forms part of the <a href="/facts/Cranial_nerve_examination/rjbHNTL9">examination of the cranial nerves</a>. It can be felt on each side of the neck when a person moves their head to the opposite side.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p><p>The triangle formed by the clavicle and the sternal and clavicular heads of the sternocleidomastoid muscle is used as a landmark in identifying the correct location for <a href="/facts/Central_venous_catheter/DbU4ejKP">central venous catheterization</a>.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p><p>Contraction of the muscle gives rise to a condition called <a href="/facts/Torticollis/PpKAqgkM">torticollis</a> or <i>wry neck</i>, and this can have a number of causes, including rheumatic, reflex or congenital causes. Torticollis gives the appearance of a tilted head on the side involved. Treatment involves physiotherapy exercises to stretch the involved muscle and strengthen the muscle on the opposite side of the neck. <a href="/facts/Torticollis/PpKAqgkM">Congenital torticollis</a> can have an unknown cause or result from <a href="/facts/Birth_trauma_(physical)/WXzIo9VE">birth trauma</a> that gives rise to a mass or tumor that can be palpated within the muscle. </p> Wikimedia Commons has media related to Sternocleidomastoid muscles. <p> <i>This article incorporates text in the <a href="/facts/Public_domain/YWKMDKw4">public domain</a> from <a href="https://archive.org/stream/anatomyofhumanbo1918gray#page/390/mode/2up">page 390</a> of the 20th edition of</i> <a href="/facts/Gray%2527s_Anatomy/dLMyjr2p">Gray's Anatomy</a> <i>(1918)</i> </p> <p class="note">This article uses <a href="/facts/Anatomical_terminology/n3IASQlr">anatomical terminology</a>.</p> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Kohan, Emil J.; Wirth, Garrett A. (2014). "Anatomy of the neck". Clinics in Plastic Surgery. 41 (1): 1–6. doi:10.1016/j.cps.2013.09.016. ISSN 1558-0504. PMID 24295343. <a href="https://pubmed.ncbi.nlm.nih.gov/24295343/" target="_blank">https://pubmed.ncbi.nlm.nih.gov/24295343/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Kohan, Emil J.; Wirth, Garrett A. (2014). "Anatomy of the neck". Clinics in Plastic Surgery. 41 (1): 1–6. doi:10.1016/j.cps.2013.09.016. ISSN 1558-0504. PMID 24295343. <a href="https://pubmed.ncbi.nlm.nih.gov/24295343/" target="_blank">https://pubmed.ncbi.nlm.nih.gov/24295343/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Robinson, June K; Anderson, E Ratcliffe (2005-01-01), Robinson, June K; Sengelmann, Roberta D; Hanke, C William; Siegel, Daniel Mark (eds.), "1. Skin Structure and Surgical Anatomy", Surgery of the Skin, Edinburgh: Mosby, pp. 3–23, doi:10.1016/b978-0-323-02752-6.50006-7, ISBN 978-0-323-02752-6, retrieved 2020-11-06 <a href="978-0-323-02752-6" target="_blank">978-0-323-02752-6</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Robinson, June K; Anderson, E Ratcliffe (2005-01-01), Robinson, June K; Sengelmann, Roberta D; Hanke, C William; Siegel, Daniel Mark (eds.), "1. Skin Structure and Surgical Anatomy", Surgery of the Skin, Edinburgh: Mosby, pp. 3–23, doi:10.1016/b978-0-323-02752-6.50006-7, ISBN 978-0-323-02752-6, retrieved 2020-11-06 <a href="978-0-323-02752-6" target="_blank">978-0-323-02752-6</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Robinson, June K; Anderson, E Ratcliffe (2005-01-01), Robinson, June K; Sengelmann, Roberta D; Hanke, C William; Siegel, Daniel Mark (eds.), "1. 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Skin Structure and Surgical Anatomy", Surgery of the Skin, Edinburgh: Mosby, pp. 3–23, doi:10.1016/b978-0-323-02752-6.50006-7, ISBN 978-0-323-02752-6, retrieved 2020-11-06 <a href="978-0-323-02752-6" target="_blank">978-0-323-02752-6</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Walker HK (1990). "64 Cranial Nerve XI: The Spinal Accessory Nerve". In Walker HK, Hall WD, Hurst JW (eds.). Clinical Methods: The History, Physical, and Laboratory Examinations (3rd ed.). Butterworths. ISBN 0-409-90077-X. PMID 21250228. NBK387. <a href="0-409-90077-X" target="_blank">0-409-90077-X</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Fehrenbach, Margaret J.; Herring, Susan W. (2012). Illustrated Anatomy of the Head and Neck. Elsevier. p. 87. 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