The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the accessory nerve.
Neck muscles, with the sternocleidomastoid shaded.
The sternocleidomastoid (right muscle shown) can be clearly observed when rotating the head.
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|Manubrium and medial portion of the clavicle
|Mastoid process of the temporal bone, superior nuchal line
|Occipital artery and the superior thyroid artery
|Motor: spinal accessory nerve
sensory: cervical plexus
Proprioceptive: C2,C3 of ventral rami
|Unilaterally: contralateral cervical rotation, ipsilateral cervical flexion Bilaterally: cervical flexion, elevation of sternum and assists in forced inhalation.
|Anatomical terms of muscle
It is given the name sternocleidomastoid because it originates at the manubrium of the sternum (sterno-) and the clavicle (cleido-), and has an insertion at the mastoid process of the temporal bone of the skull.
The sternocleidomastoid muscle originates from two locations: the manubrium of the sternum and the clavicle. It travels obliquely across the side of the neck and inserts at the mastoid process of the temporal bone of the skull. The sternocleidomastoid is thick and narrow at its centre, and broader and thinner at either end.
The two heads are separated from one another at their origins by a triangular interval (supraclavicular fossa) 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 mastoid process, from its apex to its superior border, and by a thin aponeurosis into the lateral half of the superior nuchal line of the occipital bone.
The sternocleidomastoid is innervated by the accessory nerve of the same side. It supplies only motor fibres. The cervical plexus supplies sensation, including proprioception, from the ventral primary rami of C2 and C7.
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.
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 trapezius are in contact. This would leave no posterior triangle.
The supraclavicularis muscle arises from the manubrium behind the sternocleidomastoid and passes behind the sternocleidomastoid to the upper surface of the clavicle.
The function of this muscle is to rotate the head to the opposite side or obliquely rotate the head. It also flexes the neck. When both sides of the muscle act together, it flexes 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).
The signaling process to contract or relax the sternocleidomastoid begins in Cranial Nerve XI, the accessory nerve. The accessory nerve nucleus is in the anterior horn of the spinal cord 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 trapezius. After a signal reaches the accessory nerve nucleus in the anterior horn of the spinal cord, the signal is conveyed to motor endplates on the muscle fibers located at the clavicle. Acetylcholine (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 sarcoplasmic reticulum at locations throughout the muscle fiber, at these locations the sarcoplasmic reticulum releases calcium ions that results in the movement of troponin and tropomyosin 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.
The sternocleidomastoid is within the investing fascia of the neck, along with the trapezius muscle, with which it shares its nerve supply (the accessory nerve). It is thick and thus serves as a primary landmark 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.
Examination of the sternocleidomastoid muscle forms part of the examination of the cranial nerves. It can be felt on each side of the neck when a person moves their head to the opposite side.
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 central venous catheterization.
Contraction of the muscle gives rise to a condition called torticollis or wry neck, and this can have a number of 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. Congenital torticollis can have an unknown cause or result from birth trauma that gives rise to a mass or tumor that can be palpated within the muscle.
|Wikimedia Commons has media related to Sternocleidomastoid muscles.
- "Sternocleidomastoid". Merriam-Webster Dictionary. Retrieved 2016-01-21.
- "Sternocleidomastoid". Oxford Dictionaries. Oxford University Press. Retrieved 2016-01-21.
- "The Neurologic System, Cranial Nerve XI: The Spinal Accessory Nerve." accessed on 3 October 2016 https://www.ncbi.nlm.nih.gov/books/NBK387/
- Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, page 87