The sternocleidomastoid (SCM) muscle plays a critical role in the movement and function of the head and neck. As the largest and most superficial muscle in the anterior neck, the actions and capabilities of the SCM muscle enable us to fluidly move our heads in multiple directions.
Understanding what this important muscle does provides insight into the mechanics of the cervical spine and neck mobility. By evaluating the anatomy, actions, and role of the SCM muscle, we can appreciate how this muscle allows us to move our heads and hold proper neck and head posture.
Key Takeaways on the Sternocleidomastoid Muscle
- The SCM muscle allows bending, turning, and tilting of the head and neck.
- It stabilizes neck posture when the head is not moving.
- It separates the anterior and posterior neck triangles as a key landmark.
- It protects vital neck structures like the spine, arteries, and lymph nodes.
- The SCM has dual innervation from the spinal accessory nerve.
What Is the Sternocleidomastoid Muscle?
The sternocleidomastoid muscle, commonly referred to as the SCM, is a thick, strong muscle located in the front and side of the neck. The name “sternocleidomastoid” reflects its bony attachments on the sternum (sterno), clavicle (cleido), and mastoid process of the temporal bone behind the ear (mastoid).
The SCM originates from the manubrium of the sternum and medial clavicle and runs obliquely upward and lateralward to insert on the mastoid process of the temporal bone. It is thick and rope-like in appearance and the most prominent surface landmark in the neck area.
What Does the Sternocleidomastoid Muscle Do?
The primary functions of the sternocleidomastoid muscle include:
Bending and Rotating the Head and Neck
The SCM muscle is responsible for contralateral rotation and lateral flexion of the head and neck. When only one SCM contracts, it causes the head to turn toward the opposite side. For example, contracting the right SCM rotates the head left.
The SCM also tilts the head toward the shoulder on the same side as the contracted muscle. When both SCM muscles contract together, the neck flexes and the head moves forward and down.
Maintaining Proper Head and Neck Posture
Even when the head is not being actively rotated or tilted, the SCM muscles help stabilize and support proper head and neck posture. The tonic activity and slight tension in the SCMs at rest hold the head upright and balanced on the neck.
Separating the Anterior and Posterior Triangles of the Neck
The SCM muscle serves as an important landmark in the neck that divides it into a triangular anterior region and a triangular posterior region. The two triangles contain many vital structures including blood vessels, nerves, and lymph nodes.
Protecting Neck Structures
By its position and bulk, the SCM protects important underlying structures in the neck. It shields the vertical bundle of neck vessels and nerves along with branches of the cervical plexus. It also overlies the deep cervical lymph nodes, safeguarding these drainage hubs.
Key Functions of the Sternocleidomastoid Muscle
To summarize, the major functions carried out by the sternocleidomastoid muscle include:
- Turning the head side-to-side (contralateral rotation)
- Tilting the head side-to-side (lateral flexion)
- Flexing the neck to tilt the head forward
- Maintaining proper upright neck posture
- Separating the neck into triangular regions
- Protecting vital vessels, nerves, and lymphatics
How Does the Sternocleidomastoid Muscle Allow the Head to Turn and Tilt?
The SCM muscle enables the head to turn, tilt, and nod in different directions because of its diagonal orientation and attachments on the sternum/clavicle and behind the ear.
When only one SCM contracts, it pulls the mastoid process down toward the sternal and clavicular attachment site. This one-sided contraction causes the neck to laterally bend and the head to turn to the opposite side of the contracted SCM muscle.
Contracting both SCMs simultaneously flexes the cervical spine forward and down, bringing the chin toward the chest. This movement allows the head to nod up and down.
The SCM action of lateral flexion and rotation allows us to fluidly look and face to the left or right and tilt our head sideways toward the shoulder.
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How Does the SCM Help Maintain Proper Neck Posture?
The SCM passively contributes to proper upright neck posture due to its constant, low-level contraction and engagement. The tonic activity and tension of the bilateral SCM muscles act like guide wires keeping the head balanced and positioned over the neck.
When the head is upright and still, both SCM muscles are lightly contracted to stabilize the head in proper alignment without needing active effort. This postural function provides a foundation for fluid head movement in all directions.
Proper neck posture also reduces strain on delicate neck tissues including vertebrae, discs, ligaments, and nerves. The active support from the SCMs reduces the load on these structures that can be stressed from prolonged poor neck posture.
Why Is the SCM an Important Landmark That Separates the Neck?
The sternocleidomastoid muscle diagonally overlies and bifurcates the neck into an anterior/front triangle and a posterior/back triangle. This division allows important structures to be compartmentalized based on location and function.
The posterior triangle of the neck houses the cervical spine, overlying muscles, supporting ligaments and fascia, and the emerging spinal nerves of the cervical plexus.
The anterior triangle contains vital neurovascular structures including the common carotid artery, internal jugular vein, vagus nerve, lymphatic vessels, and lymph nodes. The SCM acts as a border and facilitates localization of anatomical structures during physical exams or procedures.
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How Does the SCM Help Protect Structures of the Neck?
The bulk and strategic location of the SCM provides a protective barrier for delicate structures running through the neck area.
On the posterior surface, the SCM shields the emerging spinal nerves and vessels of the cervical plexus. Along its anterior surface, it overlies lymph nodes that collect and filter lymph drainage from the head and neck.
The SCM also reinforces the lateral margins of the carotid sheath which houses the carotid artery, jugular vein, and vagus nerve. Together with surrounding fascia, it adds padding and protection as these vital channels run upwards through the neck.
What Nerve Innervates the Sternocleidomastoid Muscle?
The sternocleidomastoid muscle receives dual innervation from both a cranial nerve and a cervical nerve root:
- Spinal accessory nerve (CN XI) – Provides motor input to the sternocleidomastoid muscle. Damage to CN XI can weaken neck rotation and tilting.
- Ventral rami of C2 and C3 – Carries sensory and proprioceptive signals from the muscle to the spinal cord. Injury can impair position sense of the SCM muscle.
This dual innervation allows both the brainstem and cervical spine to control and coordinate the actions of the SCM muscle. The accessory nerve directly stimulates the muscle fibers to contract while the cervical nerves relay sensory feedback for modulating control.
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Clinical Significance of the Sternocleidomastoid Muscle
- Torticollis – Unilateral tightness or spasm of the SCM causing the head to tilt and turn to one side. Can arise from muscle trauma, infection, or nerve damage.
- SCM Reflex Testing – Stretching the SCM can test function of the spinal accessory nerve. Loss of the reflex may indicate injury along the nerve pathway.
- SCM Strengthening – Isometric exercises help fortify the SCM muscles and improve neck pain stemming from poor posture, muscle imbalance, or whiplash injury.
- SCM Stretching – Gentle stretching is commonly prescribed to alleviate pain and increase range of motion in individuals with chronic tightened SCM muscles.
- SCM Massage – Massage techniques applied to the SCM can help relieve muscle tension, spasm, and postural neck pain.
In summary, the sternocleidomastoid muscles play an integral role in the mobility and stability of the head and neck. This paired muscle allows us to fluidly rotate, tilt, and nod our heads by contracting unilaterally or bilaterally.
The SCM also facilitates proper upright neck posture and serves as an important landmark separating the neck. It additionally overlies and protects delicate structures traversing the vital neck region. An understanding of SCM anatomy and function provides insight into maintaining neck health.
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5 Key Facts About the Sternocleidomastoid Muscle
- Allows turning and tilting head side-to-side
- Helps maintain proper neck posture
- Divides the neck into anterior and posterior triangles
- Protects vital nerves, vessels, and tissues of the neck
- Receives dual innervation for movement and sensation
What is the function of the sternocleidomastoid muscle?
The main functions of the sternocleidomastoid (SCM) muscle are to turn and tilt the head side to side, flex the neck, maintain proper upright neck posture, separate the neck into regions, and protect underlying structures. The SCM acts to contralaterally rotate and laterally flex the head when contracted unilaterally. It tilts the head forward when both SCMs contract together. Even at rest, it helps support proper head alignment on the neck. The SCM also serves as a key landmark separating the anterior and posterior triangles of the neck. It overlies vital neurovascular structures, providing a protective barrier.
How does the SCM allow the head to turn left and right?
The SCM enables the head to turn left and right because of its diagonal orientation from the sternum and clavicle up to the mastoid process behind the ear. When the right SCM contracts, it pulls the mastoid process down and to the left, turning the head to the right. Left SCM contraction turns the head to the left in the same manner. This unilateral contraction results in contralateral rotation.
What happens if both SCM muscles contract together?
When both left and right SCM muscles contract simultaneously, they flex the neck forward and down, bringing the chin toward the chest in a nodding motion. Bilateral contraction allows the head to tilt forward on the neck. The SCM works unilaterally or bilaterally to produce the different directions of neck movement.
How does the SCM help hold the head upright?
The SCM muscles are lightly engaged even when the head is upright and motionless. This baseline tonic activity acts like guide wires that stabilize the head in proper alignment over the neck. The constant tension provides postural support and reduces strain on other neck tissues. Proper posture relies on balanced bilateral baseline SCM activity.
Why is the SCM an important dividing landmark in the neck?
The diagonal orientation of SCM separates the neck into an anterior triangle and a posterior triangle. This division compartmentalizes structures based on location and function. The posterior triangle contains the cervical spine and muscles. The anterior triangle houses blood vessels, nerves, and lymph nodes. The SCM border facilitates identification of anatomical structures in the neck.
How does the SCM protect the structures of the neck?
The bulk and position of the SCM provides padding and protection to delicate structures running through the neck. It overlies and reinforces the carotid sheath containing major blood vessels and nerves. The SCM also shields the cervical plexus nerves, lymph nodes, and soft tissues of the neck from potential damage. Its strategic placement safeguards vital neurovascular pathways.
Which nerve innervates the SCM muscle?
The SCM has dual innervation from the spinal accessory nerve (CN XI) and the ventral rami of cervical levels C2 and C3. The spinal accessory nerve provides direct motor input, stimulating the SCM to contract. The cervical nerve roots relay sensory signals and proprioception from the muscle back to the CNS. Dual innervation allows both brainstem and spinal cord to control the SCM.
How can the SCM contribute to neck pain and dysfunction?
Chronic tightness, spasm, or imbalance of the SCM muscles can lead to neck pain and reduced mobility. Torticollis results when one SCM is tight, pulling the head to one side. Injury, poor posture, or whiplash can cause abnormal SCM tension. Stretching, strengthening, massage, and physical therapy help rebalance the SCMs.
What is the SCM reflex test?
Stretching the SCM normally elicits a reflexive contraction, testing the function of the spinal accessory nerve pathway. Loss of the SCM reflex after stretching may indicate damage or disruption along the spinal accessory nerve or its connections in the neck. Checking this reflex helps diagnose neurologic causes of SCM weakness or dysfunction.
How is the SCM muscle assessed during a physical exam?
The SCM is visually inspected for symmetry and indirect tests check its strength and function. Resistance is applied against bending, rotation, and tilting of the head. The SCM reflex is tested by turning and tilting the head against resistance. Tenderness of the muscle may indicate strain or spasm. Range of motion and posture are evaluated for tightness or weakness.
Key Takeaways on the Sternocleidomastoid Muscle
- Enables turning, tilting, and nodding head movements
- Passively supports proper upright neck posture
- Serves as a key dividing landmark in the anterior neck
- Overlies and protects vital neurovascular structures
- Receives dual innervation from a cranial nerve and cervical nerve roots