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What is sternocleidomastoid muscle?

The sternocleidomastoid muscle is the principal muscle of the neck region. It obliquely extends across the side of the neck region & dividing the anterior and posterior triangle of the neck.

Origin and insertion

Origin:

  • Sternal head: Upper part of the anterior surface of the manubrium of the sternum.
  • The clavicular head of SCM: Superior surface of medial 1/3rd of the clavicle.

Insertion

  • Sternal head: Lateral one-half of superior nuchal line.
  • The clavicular head of SCM: lateral surface of the mastoid process.

Sternocleidomastoid muscle

Nerve supply

  • Accessory nerve [ XI ]
  • Branches from anterior rami of C2 to C3 (C4).

Blood supply

Arterial supply

  • Upper part: Posterior auricular artery & occipital artery.
  • Middle part: Superior thyroid artery.
  • Lower part: Suprascapular artery.

Actions of the sternocleidomastoid muscle

  • Unilateral contraction
  • Bilateral contraction

Individually — will tilt head toward the shoulder area on the same side rotating head to turn face to the opposite side; acting together, draw head forward.

Relations of the sternocleidomastoid muscle

  • The investing layer of deep cervical fascia present outside the sternocleidomastoid muscle. 
  • Four sternomastoid arteries and spinal accessory nerve pierced this muscle. 
  • There are the following superficial & deep relations are below; 

Superficial Relations

  • Skin
  • Platysma
  • Three cutaneous nerves
    • Great auricular
    • Transverse cervical
    • Medial supraclavicular
    • Lesser occipital
  • External jugular vein
  • Superficial cervical lymph nodes
  • Parotid gland

Deep Relations

In the upper part

  • Muscle: 
    • Posterior belly of digastric
    • Longissimus capitis
    • Splenius capitis
  • Artery: Occipital artery

In the middle part

  • Muscles: 
    • Levator scapulae muscle
    • Scalenus anterior muscle
    • Scalenus medius muscle
    • Scalenus posterior muscle
    • Splenius capitis muscle
    • Inferior belly of the omohyoid muscle

Arteries: 

  • Common carotid artery
  • Internal carotid artery

Veins: 

  • Internal jugular vein
  • anterior jugular vein

Nerves: 

  • Vagus nerve
  • Spinal accessory nerve
  • Cervical plexus nerve 
  • Brachial plexus (upper part)
  • Ansa cervicalis (inferior root)

Glands: 

  • The thyroid gland
  • Lymph nodes

In the lower part

Muscles: 

  • Sternohyoid muscle
  • Sternothyroid muscle
  • Scalenus anterior muscle

Arteries: 

  • Suprascapular artery 
  • Transverse cervical artery

Veins: 

  • Anterior jugular veins

Nerves: 

  • Brachial plexus (lower part)
  • Phrenic nerve

Applied anatomy 

  • Torticollis or wry neck
  • Sternomastoid tumor

Torticollis or wry neck

  • It occurs due to a spasm of the trapezius muscle and sternocleidomastoid muscle in the neck.
  • Spasmodic torticollis:
    • Repeated painful contraction muscles
    • Caused by exposure to cold & maladjustment of pillow during sleep. 
  • Reflex torticollis:
    • It occurs due to irritation of the spinal accessory nerve caused by inflamed or suppurating lymph nodes.
  • Congenital torticollis: 
    • Due to birth injury to muscles. 

Sternomastoid tumor

  • The Middle Third of the sternocleidomastoid muscle swelling due to edema & ischemic necrosis caused by trauma at birth. 

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