Dominic Stone
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend your neck and turn or tilt your head. It is the largest muscle in the front of your neck and is easily visible and palpable. The SCM muscle is a paired muscle, with two parts that begin at the breastbone (sternum) and collarbones (clavicles) and end at the base of the skull (mastoid process). The SCM muscle is important as it helps to stabilise the neck and protect the neurovascular bundle of the neck, including nerves, blood vessels, lymph nodes and glands. SCM syndrome can be caused by stress and anxiety, leading to pain and stiffness, and can be treated with stress-relieving techniques, physical therapy and osteopathic manipulation.
| Characteristics | Values |
|---|---|
| Full Form | Sternocleidomastoid |
| Synonyms | SCM, Sternomastoid, Sterno, Musculus Sternocleidomastoideus |
| Location | The SCM muscle is located in the neck |
| Function | It allows you to bend your neck, turn and tilt your head |
| Composition | The SCM muscle is made up of two parts, the sternal head and the clavicular head |
| Innervation | The SCM muscle is innervated by the accessory nerve |
| Blood Supply | The SCM muscle receives blood supply from the internal carotid artery |
| Conditions | SCM syndrome, which can be caused by stress, anxiety, or poor posture |
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What You'll Learn
SCM Muscle Anatomy
The sternocleidomastoid (SCM) muscle is a two-part, long, bilateral muscle in the anterior portion of the neck. It is the largest muscle in the front of the neck, extending from the mastoid process at the base of the skull to the collarbones (clavicles) and breastbone (sternum). The SCM muscle is easily visible and palpable, and you can feel your pulse through it.
The SCM muscle gets its name from its location and parts. 'Sterno' refers to the sternum or breastbone, where one part of the muscle, the sternal head, originates. 'Cleido' refers to the clavicle or collarbone, which is where the other part of the muscle, the clavicular head, originates. The two parts join into one muscle belly, which inserts on the lateral surface of the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone. The clavicular head is composed of fleshy and aponeurotic fibres and is directed almost vertically upward. The SCM muscle ends at the mastoid process, a section of bone located at the base of the skull behind the ears.
The SCM muscle is closely related to certain neurovascular structures that pass through the neck towards the head or the periphery of the body. It protects the vertical neurovascular bundle of the neck, branches of the cervical plexus, deep cervical lymph nodes, and soft tissues of the neck from damage. The SCM muscle also acts as an accessory muscle of inspiration and respiration. It helps to stabilise the neck and aids in breathing by working with other neck muscles to lift the breastbone and collarbone when inhaling.
The SCM muscle is involved in various neck movements, including flexion, lateral flexion, and rotation. It allows for bending the neck and turning or tilting the head. The muscle is wider and thinner at its starting and ending points, and narrower and thicker in the middle where the sternal and clavicular heads merge. The SCM muscle borders two neck regions known as triangles: the anterior triangle and the posterior triangle. These triangles contain important structures such as nerves, blood vessels, lymph nodes, and glands.
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SCM Muscle Functions
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend, turn and tilt your head. SCM stands for sternocleidomastoid, which refers to the muscle's points of attachment: the sternum, the clavicle and the mastoid process.
The SCM is a two-headed muscle, with one part (the sternal head) originating from the breastbone (sternum) and the other (the clavicular head) originating from the collarbone (clavicle). These two heads merge in the middle of the neck, where the muscle is at its thickest. From here, the muscle continues up both sides of the neck and inserts at the mastoid process, a section of bone at the base of the skull behind the ears.
The SCM muscle is an important landmark in the neck, dividing it into an anterior and a posterior triangle. These triangles contain important nerves, blood vessels, lymph nodes and glands, which healthcare providers may use the SCM muscle to locate during surgery.
The SCM muscle helps to stabilise the neck, even when you are not moving. It also works with other neck muscles to lift the breastbone and collarbone when you inhale, creating space for your lungs to take in air. The SCM muscle supports the temporomandibular joint (TMJ), which connects the jaw to the skull and allows you to open and close your mouth.
In terms of movement, the SCM muscle helps to flex the neck and extend the head. When acting alone, it rotates to the opposite side (contralaterally) and slightly to the same side (laterally).
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SCM Muscle Attachments
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend your neck and turn or tilt your head. It is a paired muscle, with two parts. The SCM muscle borders two neck regions known as triangles: the anterior triangle and the posterior triangle.
The SCM muscle gets its name from its location and its parts. The sternocleidomastoid muscle extends from the mastoid process at the base of the skull to the collarbones (clavicles) and breastbone (sternum). The muscle is wider and thinner where it starts and ends, and narrow and thicker in the middle where the two parts merge.
The first part of the SCM muscle is called the sternal head, which begins at the breastbone and travels up both sides of the neck. The second part is called the clavicular head, which begins in the centre of the collarbones and travels up both sides of the neck, merging with the sternal head.
The SCM muscle can be divided into four portions, owing to its two sites of origin (clavicle and sternum) and two sites of attachment (occiput and mastoid process): sternomastoid, sterno-occipital, cleidomastoid, and cleido-occipital. The sternomastoid portion of the SCM is the muscle area that can develop a more significant percentage of contractile strength than the other portions during muscle action.
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SCM Muscle Innervation
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend your neck and turn or tilt your head. It is a paired muscle, with two parts that start at the bottom of your neck, wrap around both sides, and end at the base of your skull. The SCM muscle borders two neck regions known as triangles: the anterior triangle and the posterior triangle.
The SCM has dual innervation and multiple functions. It is innervated by the accessory nerve of the same side, which supplies only motor fibres. The SCM also has a variable innervation arrangement, with the "classical anastomotic pattern" being observed in 50% of cases. This arrangement is important for planning pedicle muscle flaps in reconstructive surgeries.
The SCM muscle is an important landmark in the neck, protecting the vertical neurovascular bundle of the neck, branches of the cervical plexus, deep cervical lymph nodes, and soft tissues of the neck from damage. The cervical plexus supplies sensation, including proprioception, from the ventral primary rami of C2 and C3.
The SCM muscle can be divided into four portions due to its two sites of origin (clavicle and sternum) and two sites of attachment (occiput and mastoid process): sternomastoid, sterno-occipital, cleidomastoid, and cleido-occipital. The sternomastoid portion can develop a more significant percentage of contractile strength than the other portions during muscle action.
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SCM Muscle Clinical Relations
The sternocleidomastoid (SCM) muscle is a prominent landmark in the neck, closely related to several important neurovascular structures. Its variable anatomy is clinically relevant, especially for neck surgeries.
A cadaveric study found variations in the attachment of the SCM muscle in 27.8% of specimens, with unilateral variations in 16.7% and bilateral variations in 11.1%. As such, detailed knowledge of the SCM muscle is crucial for head and neck surgeons and anaesthetists to avoid post-procedural complications.
The SCM muscle is also important in the reconstruction of the mandible and mandibular defects, as well as in the reconstruction of the oral floor. Furthermore, the SCM serves as a lead structure when placing a central venous catheter (CVC). The medial edge of the SCM muscle at Erb's point can be used as a puncture site for local anaesthesia.
The SCM muscle is associated with various clinical disorders. For instance, unilateral diffuse or localized enlargement of the SCM is a common occurrence in infancy, known as "sternocleidomastoid tumour". This condition usually resolves spontaneously or with physiotherapy. In some cases, it leads to fibromatous changes, resulting in torticollis. The SCM muscle is also implicated in muscular torticollis, a tonic spasm characterised by difficulties swallowing, extreme immobility of the throat, facial asymmetries, and scoliosis.
Additionally, SCM muscle pain can develop due to poor posture or prolonged static positions. Treatment options for SCM-related pain include stretching, physical therapy, and osteopathic manipulation. Maintaining good posture and managing stress can also help prevent SCM-related issues.
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Frequently asked questions
SCM stands for Sternocleidomastoid, which is a two-headed neck muscle. It is the largest muscle in the front of your neck.
The SCM muscle allows you to bend your neck, turn your head, tilt your head and helps you maintain your posture. It also supports the joint that connects your jaw to your skull, allowing you to open and close your mouth.
You can care for your SCM muscle by managing your stress levels, maintaining good posture and practising relaxation techniques such as meditation, yoga and breathing exercises.
