Sternocleidomastoid

1. Overview

The sternocleidomastoid (SCM) is a prominent, paired muscle located in the anterolateral aspect of the neck. It plays a vital role in head and neck movement and serves as an important anatomical landmark for clinical examination and surgical procedures. The name of the muscle reflects its attachments: sternum (sterno), clavicle (cleido), and mastoid process (mastoid).

2. Location

The sternocleidomastoid is found in the superficial layer of the neck, spanning from the base of the skull to the thorax:

• Superiorly: Inserts on the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone.

• Inferiorly: Arises from two heads:

  • Sternal head: Manubrium of the sternum.

  • Clavicular head: Medial third of the clavicle.

• Deep to: External jugular vein and superficial cervical fascia.

3. Structure

The sternocleidomastoid is a thick, fusiform muscle with two heads that blend to form a single belly:

• Muscle type: Skeletal, voluntary muscle.

• Innervation: Spinal accessory nerve (cranial nerve XI) for motor function, with sensory input from C2 and C3 (cervical plexus).

• Blood supply: Branches of the occipital artery, superior thyroid artery, and suprascapular artery.

• Orientation: Oblique, running upward and laterally from the thoracic region to the base of the skull.

4. Function

The sternocleidomastoid muscle controls several important head and neck movements:

• Unilateral contraction:

  • Rotates the head to the opposite side.

  • Tilts the head to the same side (lateral flexion).

• Bilateral contraction:

  • Flexes the neck and brings the chin toward the chest.

  • Assists in elevation of the sternum and clavicle during forced inspiration.

5. Physiological role(s)

Beyond basic movement, the SCM contributes to several physiological and postural roles:

• Head and neck stabilization: Helps maintain balance and proper orientation of the head.

• Breathing assistance: Acts as an accessory muscle during labored or deep breathing.

• Proprioception: Contains receptors that relay information on head position, aiding in balance and spatial orientation.

• Surface landmark: Defines important triangles of the neck—anterior and posterior triangles—used in clinical and surgical assessments.

6. Clinical Significance

The sternocleidomastoid is involved in various clinical scenarios:

• Torticollis (wry neck):

  • Condition characterized by abnormal, involuntary contraction of the SCM, causing head tilt and rotation.

  • Can be congenital (due to fibrous shortening of SCM) or acquired (due to muscle spasm or nerve injury).

• Accessory nerve injury:

  • Damage to cranial nerve XI can impair SCM function, leading to weakness in head rotation and shoulder elevation (trapezius involvement).

• Muscle strain:

  • Common in whiplash injuries or poor posture, resulting in pain and stiffness along the side of the neck.

• Palpation landmark:

  • Useful in locating carotid pulse, internal jugular vein, and cervical lymph nodes during physical examination.

• Referred pain:

  • Trigger points in the SCM can refer pain to the jaw, ear, or around the eye, mimicking other conditions.