Victor Forge
The sternocleidomastoid muscle, also known as the SCM, is a two-headed neck muscle that plays a crucial role in various movements of the head and neck. It gets its name from its attachments to the manubrium of the sternum (sterno-), the clavicle (-cleido-), and the mastoid process of the temporal bone (-mastoid). The SCM is responsible for rotating the head to the opposite side, flexing the neck, tilting the head, and assisting in breathing and maintaining neck posture. It is innervated by the accessory nerve, also known as Cranial Nerve XI, and is one of the largest and most superficial cervical muscles. The SCM has dual innervation and is an important anatomical landmark within the neck region.
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What You'll Learn
The sternocleidomastoid muscle's role in head rotation
The sternocleidomastoid (SCM) muscle is a two-headed neck muscle that plays a crucial role in head rotation and neck movement. It is a large, superficial muscle that is easily palpable and serves as an important anatomical landmark within the neck region. The SCM muscle gets its name from its attachments: "sterno-" from the manubrium of the sternum, "-cleido-" from the clavicle, and "-mastoid" from its insertion at the mastoid process of the temporal bone.
The SCM muscle has multiple functions, including head rotation, head tilt, neck flexion, and neck extension. When both SCM muscles contract, they work together to flex the neck and extend the head. This movement brings the chin down towards the breastbone. When acting alone, the SCM rotates the head to the opposite side and slightly flexes the neck to the same side. For example, when turning your head to the right, the left SCM contracts, resulting in a rotation to the opposite side. This action also involves a slight upward turn of the face, adding a bit of neck extension.
The SCM muscle is innervated by the accessory nerve, also known as Cranial Nerve XI. It receives signals from the accessory nerve nucleus in the anterior horn of the spinal cord, leading to muscle contraction or relaxation. The SCM is closely associated with certain neurovascular structures passing through the neck towards the head or the periphery of the body. It also plays a role in maintaining neck posture and facilitating correct temporomandibular joint (TMJ) function.
The SCM muscle is involved in pathological conditions such as torticollis, a tonic spasm of the SCM resulting in bending of the head to the affected muscle and rotation to the healthy side. This condition can cause difficulties in swallowing, extreme immobility of the throat, facial asymmetries, and scoliosis. Understanding the anatomy and functions of the SCM muscle is essential for clinical examinations and surgical procedures involving the neck and head region.
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The muscle's function in neck flexion
The sternocleidomastoid muscle (SCM) is a two-headed neck muscle that originates at the manubrium of the sternum and the clavicle. It has a cylindrical, strap-like shape and emerges from the side of the neck, tapering at the ends. The SCM is one of the largest and most superficial cervical muscles and is involved in various functions related to neck flexion and head movement.
The SCM plays a crucial role in neck flexion, which is the action of bending the neck forward. When both sides of the SCM contract simultaneously, they work together to flex the neck and extend the head. This movement brings the chin downwards towards the breastbone. The SCM also assists in neck posture and helps maintain correct head positioning.
Additionally, the SCM is involved in neck extension, which is the backward movement of the head. When one side of the SCM acts alone, it causes the head to rotate to the opposite side and flexes laterally to the same side. This isolated action of the SCM results in a slight upward turn of the face, contributing to neck extension.
The SCM is also associated with head rotation. It enables the rotation of the head to the opposite side of the contracting SCM. This movement is facilitated by the SCM on the opposite side of the rotation, which contracts to facilitate the turning of the head. The SCM also aids in head tilt, allowing the chin to point towards the breastbone.
Furthermore, the SCM has inspiratory muscle action. It can lift the sternum and clavicles, assisting in breathing and respiration. The SCM is closely related to neurovascular structures passing through the neck and plays a significant role in maintaining neck posture and facilitating complex physiological movements.
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The muscle's origin in the manubrium of the sternum
The sternocleidomastoid muscle (SCM) is one of over 20 pairs of muscles that act on the neck. It is a two-headed neck muscle that originates at the manubrium of the sternum (sterno-) and the clavicle (cleido). The SCM has dual innervation and multiple functions. It is a superficially palpable muscle that acts as an anatomical landmark within the neck region.
The SCM has its origin in the manubrium of the sternum, which is the upper part of the breastbone in the centre of the chest. The muscle's sternal head is a round fasciculus, tendinous in front, and fleshy behind. It arises from the upper part of the front of the manubrium sterni. The SCM's origin in the manubrium of the sternum gives it one of its names, with "sterno-" referring to its origin in the sternum.
The SCM plays an important role in the posture of the neck and the body. It helps with functions such as head rotation, head tilt, and pointing the chin towards the breastbone. The SCM can also have inspiratory muscle action by taking a fixed point on the temporal bone and then lifting the sternum and the clavicles. This muscle is closely related to certain neurovascular structures that pass through the neck on their way to the head or the periphery of the body.
The SCM is thick and narrow at its centre, and broader and thinner at either end. It runs diagonally from the collarbone and the breastbone to the back of the ear. On either side, the SCM divides the neck musculature into anterior (front) and posterior (back) triangles. The SCM is innervated by the spinal accessory nerve (cranial nerve XI).
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The muscle's insertion at the mastoid process of the temporal bone
The sternocleidomastoid muscle (SCM) is a two-headed neck muscle that originates at the manubrium of the sternum and the clavicle. It is named so because it bears attachments to the manubrium of the sternum ("sterno-"), the clavicle ("-cleido-"), and the mastoid process of the temporal bone ("-mastoid").
The SCM has multiple functions, including head rotation, head tilt, and pointing the chin toward the breastbone. It also assists in breathing and maintaining neck posture. The SCM is a superficially palpable muscle that serves as an anatomical landmark within the neck region. It divides the neck into anterior and posterior triangles, helping to define the location of structures such as the lymph nodes for the head and neck.
The SCM is closely related to certain neurovascular structures that pass through the neck, either towards the head or the periphery of the body. It is innervated by the accessory nerve (Cranial Nerve XI), which originates in the anterior horn of the spinal cord around C1-C3. The SCM has dual innervation and receives signals from both sides of the body to facilitate various complex physiological movements.
The muscle fibres from the accessory nerve nucleus travel upward to enter the cranium via the foramen magnum and reach the SCM. Acetylcholine (ACH) is released from vesicles and sent over the synaptic cleft to receptors on the postsynaptic bulb, initiating an action potential that travels along the muscle fibre. This action potential results in the contraction of the SCM.
The SCM is inserted, by a strong tendon, into the lateral surface of the mastoid process of the temporal bone, from its apex to its superior border. This insertion point is a key aspect of the SCM's structure and function, allowing it to facilitate movements such as head rotation and neck flexion. The mastoid process of the temporal bone is located behind the ear, and the SCM inserts into this process by a thin aponeurosis, a type of fibrous connective tissue.
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The muscle's dual innervation
The sternocleidomastoid muscle (SCM) is one of over 20 pairs of muscles acting on the neck. It is a two-headed neck muscle that bears attachments to the manubrium of the sternum, the clavicle, and the mastoid process of the temporal bone. The SCM has dual innervation and multiple functions. It acts as an accessory muscle of inspiration, and plays an important role in the posture of the neck and the body.
The SCM has a variable innervation arrangement, with the "classical anastomotic pattern" being observed in 50% of cases. The muscle is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C2-C3). The accessory nerve supplies motor fibres, while the cervical plexus supplies sensation, including proprioception. The cutaneous branches of the cervical plexus emerge from the posterior edge of the SCM, aiding the muscle in its proprioceptive functions.
The SCM is closely related to certain neurovascular structures that pass through the neck on their way to the head or the periphery of the body. It is supplied by the superior thyroid artery, a branch of the external carotid artery. The common carotid artery, accessory nerve, and brachial plexus are also related to the SCM.
The SCM has been shown to have a close connection with the vestibular area and the motoneurons of the SCM. Vestibular stimulation electrically activates the sternocleidomastoid. The SCM can also have inspiratory muscle action by taking a fixed point on the temporal bone and then lifting the sternum and the clavicles.
The SCM has been observed to fuse with the trapezius muscle in some cases, resulting in the absence of a posterior triangle. This phenomenon is described by Sinohara's law of separation, which states that two muscles with a common nerve supply are derived from a common muscle mass.
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Frequently asked questions
The sternocleidomastoid muscle is located in the neck.
The sternocleidomastoid muscle helps with head rotation, head tilt, neck flexion, neck extension, and pointing the chin towards the breastbone. It also assists in breathing and maintaining neck posture.
The sternocleidomastoid muscle has two heads that originate from the manubrium of the sternum and the clavicle. They join to form one muscle belly that inserts into the mastoid process of the temporal bone and the occipital bone.
The sternocleidomastoid muscle divides the neck into anterior and posterior triangles. It is closely related to neurovascular structures passing through the neck towards the head or periphery of the body. It also shares its nerve supply (the accessory nerve) with the trapezius muscle.
