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 The accessory nerve, also known as the eleventh cranial nerve, cranial nerve XI, or simply CN XI, is a cranial nerve that supplies the sternocleidomastoid and trapezius muscles. It is classified as the eleventh of twelve pairs of cranial nerv ...

Etymology and location

It is given the name ''sternocleidomastoid'' because it originates at the

manubrium The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...

of the sternum (''sterno-'') and the

clavicle The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...

(''cleido-'') and has an

insertion Insertion may refer to: *Insertion (anatomy), the point of a tendon or ligament onto the skeleton or other part of the body *Insertion (genetics), the addition of DNA into a genetic sequence *Insertion, several meanings in medicine, see ICD-10-PCS ...

at the

mastoid process The mastoid part of the temporal bone is the posterior (back) part of the temporal bone, one of the bones of the skull. Its rough surface gives attachment to various muscles (via tendons) and it has openings for blood vessels. From its borders, t ...

of the

temporal bone The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temples, and house the structures of the ears. Th ...

of the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

Structure

The sternocleidomastoid muscle originates from two locations: the

of the sternum and the

. It travels obliquely across the side of the neck and inserts at the

of the

by a thin

aponeurosis An aponeurosis (; plural: ''aponeuroses'') is a type or a variant of the deep fascia, in the form of a sheet of pearly-white fibrous tissue that attaches sheet-like muscles needing a wide area of attachment. Their primary function is to join musc ...

. The sternocleidomastoid is thick and narrow at its centre, and broader and thinner at either end. The sternal head is a round fasciculus, tendinous in front, fleshy behind, arising from the upper part of the front of the

manubrium sterni The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...

. It travels superiorly, laterally, and posteriorly. The clavicular head is composed of fleshy and aponeurotic fibers, arises from the upper, frontal surface of the medial third of the

; it is directed almost vertically upward. The two heads are separated from one another at their origins by a triangular interval (''lesser 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

, from its apex to its superior border, and by a thin

into the lateral half of the

superior nuchal line The nuchal lines are four curved lines on the external surface of the occipital bone: * The upper, often faintly marked, is named the highest nuchal line, but is sometimes referred to as the Mempin line or linea suprema, and it attaches to the epi ...

of the

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobe ...

Nerve supply

The sternocleidomastoid is innervated by

of the same side. It supplies only motor fibres. The cervical plexus supplies sensation, including proprioception, from the ventral primary rami of C2 and C3.

Variation

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 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 The trapezius is a large paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula. It moves the scapula and supports the ...

are in contact. This would leave no

posterior triangle Posterior may refer to: * Posterior (anatomy), the end of an organism opposite to its head ** Buttocks, as a euphemism * Posterior horn (disambiguation) * Posterior probability The posterior probability is a type of conditional probability that r ...

. The supraclavicularis muscle arises from the manubrium behind the sternocleidomastoid and passes behind the sternocleidomastoid to the upper surface of the clavicle.

Function

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 Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

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). It also acts as an accessory muscle of respiration, along with the

scalene muscles The scalene muscles are a group of three pairs of muscles in the lateral neck, namely the anterior scalene, middle scalene, and posterior scalene. They are innervated by the third to the eight cervical spinal nerves (C3-C8). The anterior and m ...

of the neck.

Contraction

The signaling process to contract or relax the sternocleidomastoid begins in Cranial Nerve XI, the

. 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

. After a signal reaches the accessory nerve nucleus in the anterior horn of the spinal cord, the signal is conveyed to

motor endplate A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...

s on the muscle fibers located at the clavicle.

Acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Part ...

(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 Tropomyosin is a two-stranded alpha-helical, coiled coil protein found in actin-based cytoskeletons. Tropomyosin and the actin skeleton All organisms contain organelles that provide physical integrity to their cells. These type of organelles ar ...

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.

Anatomical landmark

The sternocleidomastoid is within the

investing fascia Deep fascia (or investing fascia) is a fascia, a layer of dense connective tissue that can surround individual muscles and groups of muscles to separate into fascial compartments. This fibrous connective tissue interpenetrates and surrounds the mu ...

of the neck, along with the trapezius muscle, with which it shares its nerve supply (the

). It is thick and thus serves as a primary

landmark A landmark is a recognizable natural or artificial feature used for navigation, a feature that stands out from its near environment and is often visible from long distances. In modern use, the term can also be applied to smaller structures or f ...

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. Many important structures relate to the sternocleidomastoid, including the common carotid artery, accessory nerve, and brachial plexus.

Clinical significance

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 catheter A central venous catheter (CVC), also known as a central line(c-line), central venous line, or central venous access catheter, is a catheter placed into a large vein. It is a form of venous access. Placement of larger catheters in more central ...

ization. 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 Birth trauma may refer to: * Childbirth-related posttraumatic stress disorder, psychological trauma to the mother following childbirth *Birth trauma (physical) Birth trauma refers to damage of the tissues and organs of a newly delivered child, of ...

that gives rise to a mass or tumor that can be palpated within the muscle.

References

{{Authority control Muscles of the head and neck

Etymology and location

Structure

Nerve supply

Variation

Function

Contraction

Anatomical landmark

Clinical significance

See also

References