Logan Steele
The sternocleidomastoid muscle is a two-headed neck muscle that is responsible for flexing and rotating the head. It is one of the largest and most superficial cervical muscles and is easily recognisable. The muscle's name comes from its points of origin and insertion: the manubrium of the sternum and clavicle, and the mastoid process of the temporal bone of the skull. The sternocleidomastoid muscle has several antagonists, which are muscles that produce opposite movements. The primary antagonists of the sternocleidomastoid muscle are the trapezius and splenius muscles, which work to extend the neck and oppose the flexion and rotation facilitated by the sternocleidomastoid.
| Characteristics | Values |
|---|---|
| Antagonists | Splenius muscles, Trapezius muscles |
| Antagonists' function | Extend the neck, straighten the neck, extend the head backward |
| Antagonists' location | Trapezius: back of the neck and upper back; Splenius: back of the neck |
| Antagonists' parts | Splenius Capitis, Splenius Cervicis, middle and superior parts of the trapezius |
| Sternocleidomastoid's function | Flex the neck, rotate the head |
| Sternocleidomastoid's location | Both sides of the neck |
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What You'll Learn
Sternocleidomastoid muscles rotate the head to the opposite side
The sternocleidomastoid muscle is a two-headed neck muscle that is one of the largest and most superficial cervical muscles. The muscle is thick and narrow at its centre, and broader and thinner at each end. It originates at the manubrium of the sternum and the clavicle, and has an insertion at the mastoid process of the temporal bone of the skull.
The primary action of the sternocleidomastoid muscle is to rotate the head to the opposite side. This muscle also facilitates the flexion of the neck, allowing for forward flexion and bringing the chin closer to the sternum. When acting unilaterally, the sternocleidomastoid muscle flexes the cervical vertebral column to the same side and rotates the head to the opposite side.
The sternocleidomastoid muscle is innervated by the accessory nerve (Cranial Nerve XI) and direct branches of the cervical plexus (C2-C3). The nerve supply to this muscle is shared with the trapezius muscle, with which it occasionally fuses. The blood supply to the sternocleidomastoid muscle is provided by the superior thyroid artery, a branch of the external carotid artery.
The antagonists to the sternocleidomastoid muscles are the muscles that extend the neck, such as the splenius muscles and the trapezius muscles. These antagonist muscles work to oppose the actions of the sternocleidomastoid muscles by extending or straightening the neck. For example, when looking up at the sky, the sternocleidomastoid muscles are relatively relaxed while the trapezius and splenius muscles are active, aiding in neck extension.
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The muscles also flex the neck
The sternocleidomastoid muscle is a two-headed neck muscle that plays a central role in the formation of the triangles of the neck. It divides the neck into anterior and posterior cervical triangles, which helps define the location of structures such as the lymph nodes for the head and neck. It is one of the largest and most superficial cervical muscles, and it can be felt on each side of the neck when a person moves their head to the opposite side.
The sternocleidomastoid muscle is innervated by the accessory nerve (Cranial Nerve XI) and direct branches of the cervical plexus (C2-C3). It originates at the manubrium of the sternum (sterno-) and the clavicle (cleido), with an insertion at the mastoid process of the temporal bone of the skull. The muscle is thick and narrow at its centre, and broader and thinner at either end.
The primary actions of the sternocleidomastoid muscle are flexion of the neck and rotation of the head to the opposite side. 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).
A unilateral contraction of the sternocleidomastoid muscle flexes the cervical vertebral column to the same side (lateral flexion) and rotates the head to the opposite side. A bilateral contraction elevates the head by dorsally extending the upper cervical joints. At the same time, it flexes the lower cervical column, causing an overall bending of the neck towards the chest. If the head is fixed, it elevates the sternum and clavicle and, thus, expands the thoracic cavity (inspiratory breathing muscle).
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Antagonists include the splenius muscles
The sternocleidomastoid muscle is a large and superficial cervical muscle that divides the neck into anterior and posterior triangles. It is responsible for rotating the head to the opposite side and flexing the neck. The muscle is supplied by the accessory nerve.
The antagonists to the sternocleidomastoid muscles are the muscles that produce the opposite movement, extending the neck. These include the splenius muscles and the trapezius muscles.
The splenius muscles are located on the back of the neck. They include the splenius capitis and splenius cervicis. These muscles assist in extending the neck and rotating the head. When contracted, they extend the head, opposing the action of the sternocleidomastoid.
The relationship between the sternocleidomastoid and its antagonists, including the splenius muscles, is essential for balanced movements and maintaining proper posture. For example, when looking up at the sky, the sternocleidomastoid muscles relax while the splenius muscles are active, helping to extend the neck. Similarly, when nodding forward, the sternocleidomastoid contracts while the splenius muscles relax to allow that movement.
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Antagonists also include the trapezius muscles
The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. It is responsible for rotating the head to the opposite side and flexing the neck. The sternocleidomastoid muscle is innervated by the accessory nerve. The antagonists to the sternocleidomastoid muscles are the muscles that produce the opposite movement, namely the muscles that extend the neck.
The trapezius muscle is one such antagonist. It 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 arm. The trapezius is located at the back of the neck and upper back. It is involved in movements of the shoulder girdle and is therefore considered a muscle of the upper limb rather than of the back. The main function of the trapezius is to stabilise the scapula in its anatomical place and control it during movements of the shoulder and upper limb.
The trapezius muscle has several origin points along the midline of the posterior neck and back. The superior fibres attach to the medial third of the superior nuchal line, the external occipital protuberance of the occipital bone, and the nuchal ligament. The trapezius is the only muscle of the upper limb that does not receive its innervation from the brachial plexus. Instead, motor innervation is conveyed by the accessory nerve (CN XI) and the anterior rami of the C3 and C4 spinal nerves.
The sternocleidomastoid and trapezius muscles share their nerve supply (the accessory nerve). The trapezius muscle is also involved in the movements of the head and neck. For example, when looking up at the sky, the sternocleidomastoid muscles are relatively relaxed while the trapezius and splenius muscles are active, helping to extend the neck.
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The SCM is an important anatomical landmark
The sternocleidomastoid muscle (SCM) is an important anatomical landmark in the neck region. It is a large, easily recognisable, and palpable muscle, and its importance as an anatomical landmark is evident in several ways.
Firstly, the SCM serves as a primary landmark of the neck by dividing it into anterior and posterior cervical triangles, or anterior and posterior triangles. These triangles help define the location of structures such as the lymph nodes of the head and neck. The SCM is also a key reference point for identifying the correct location for central venous catheterisation.
Secondly, the SCM is closely associated with several other important anatomical structures. It shares its nerve supply, the accessory nerve, with the trapezius muscle, and is innervated by the accessory nerve of the same side. The SCM is also related to the common carotid artery and the brachial plexus.
Thirdly, the SCM plays a significant role in maintaining posture and facilitating various complex physiological movements. It has inspiratory muscle action and helps in the correct temporomandibular joint (TMJ) function. The SCM is also involved in neuromuscular pathologies such as torticollis.
Lastly, the SCM's unique anatomical characteristics, such as its variable origin and innervation, are crucial considerations in surgical planning, particularly in reconstructive surgeries and muscle flap harvesting procedures. Its organisational pattern can be classified into five distinct topographical parts, which influence its function and structure.
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Frequently asked questions
The sternocleidomastoid muscles are a pair of two-headed neck muscles that originate from the upper edge of the sternal manubrium and the medial quarter of the upper face of the clavicle. They are responsible for flexing and rotating the head.
The antagonists of the sternocleidomastoid muscles are the muscles that extend the neck, such as the splenius muscles and the trapezius muscles. These include the splenius capitis, splenius cervicis, and the middle and superior parts of the trapezius muscle.
The antagonists of the sternocleidomastoid muscles work to oppose the actions of the sternocleidomastoid muscles by extending or straightening the neck. For example, when you look up at the sky, the sternocleidomastoid muscles are relaxed while their antagonists are active, helping to extend the neck.
